Adiposis dolorosa, also known as Dercum disease, is characterized by generalized obesity and pronounced, disabling, and chronic pain in the adipose tissue of the proximal extremities, trunk, pelvic area, and buttocks; the face and hands are usually spared. There are a number of associated symptoms, including multiple lipomas, generalized weakness, fatigue, sleep disturbances, constipation, and psychiatric abnormalities. It is 5 to 30 times more common in women than men, and usually presents between 35 and 50 years of age (summary by Campen et al., 2001; review by Hansson et al., 2012). Based on a review of the literature and studies of 111 patients, Hansson et al. (2012) proposed a classification of Dercum disease into 4 types: (I) generalized diffuse form without clear lipomas, (II) generalized nodular form with multiple lipomas, (III) localized nodular form, and (IV) juxtaarticular form with solitary fatty deposits near joints.

Genetic prion disease generally manifests with cognitive difficulties, ataxia, and myoclonus (abrupt jerking movements of muscle groups and/or entire limbs). The order of appearance and/or predominance of these features and other associated neurologic and psychiatric findings vary. The three major phenotypes of genetic prion disease are genetic Creutzfeldt-Jakob disease (gCJD), fatal familial insomnia (FFI), and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Although these phenotypes display overlapping clinical and pathologic features, recognition of these phenotypes can be useful when providing affected individuals and their families with information about the expected clinical course. The age at onset typically ranges from 50 to 60 years. The disease course ranges from a few months in gCJD and FFI to a few (up to 4, and in rare cases up to 10) years in GSS syndrome.

Pantothenate kinase-associated neurodegeneration (PKAN) is a type of neurodegeneration with brain iron accumulation (NBIA). The phenotypic spectrum of PKAN includes classic PKAN and atypical PKAN. Classic PKAN is characterized by early-childhood onset of progressive dystonia, dysarthria, rigidity, and choreoathetosis. Pigmentary retinal degeneration is common. Atypical PKAN is characterized by later onset (age >10 years), prominent speech defects, psychiatric disturbances, and more gradual progression of disease.

Huntington disease (HD) is a progressive disorder of motor, cognitive, and psychiatric disturbances. The mean age of onset is 35 to 44 years, and the median survival time is 15 to 18 years after onset.

Obsessive-compulsive disorder (OCD) is characterized by recurring obsessions and/or compulsions and has been estimated to affect nearly 5 million people in the United States (Karno et al., 1988). Evidence for a strong genetic component in OCD comes from twin studies, family genetics studies, and segregation analyses, as reviewed by Alsobrook et al. (2002). Zhang et al. (2002) suggested that hoarding is likely to be an evolutionarily conserved trait that, in times of adversity, was associated with increased survival and reproductive fitness. However, extreme forms of this trait are associated with marked disability and poor response to treatment (Black et al., 1998; Mataix-Cols et al., 1999).

Phenylalanine hydroxylase (PAH) deficiency results in intolerance to the dietary intake of the essential amino acid phenylalanine and produces a spectrum of disorders. The risk of adverse outcome varies based on the degree of PAH deficiency. Without effective therapy, most individuals with severe PAH deficiency, known as classic PKU, develop profound and irreversible intellectual disability. Affected individuals on an unrestricted diet who have phenylalanine levels above normal but below 1,200 µmol/L (20 mg/dL) are at much lower risk for impaired cognitive development in the absence of treatment.

The stiff-person syndrome (SPS) is most often an adult-onset sporadic acquired disorder characterized by progressive muscle stiffness with superimposed painful muscle spasms accompanied by electromyographic evidence of continuous motor activity at rest. SPS has been associated with autoimmune disorders, diabetes mellitus, thyrotoxicosis, and hypopituitarism with adrenal insufficiency (George et al., 1984). Approximately 60% of patients with SPS have antibodies to glutamic acid decarboxylase (GAD2, or GAD65; 138275), the rate-limiting enzyme in the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), suggesting an immune-mediated pathogenesis (Folli et al., 1993). Approximately 10% of patients develop SPS as a paraneoplastic neurologic disorder associated with antibodies to amphiphysin (AMPH; 600418), an intracellular protein associated with neuronal synaptic vesicle endocytosis (Burns, 2005). See also congenital stiff-man syndrome, or hereditary hyperexplexia (149400), which is caused by mutations in subunits of the glycine receptor gene (GLRA1, 138491; GLRB, 138492). Meinck and Thompson (2002) provided a detailed review of stiff-person syndrome. They also discussed 2 possibly related conditions, progressive encephalomyelitis with rigidity (PERM), a more severe disorder with other neurologic features, and stiff-limb or stiff-leg syndrome, a focal disorder.

Hereditary coproporphyria (HCP) is an acute (hepatic) porphyria in which the acute symptoms are neurovisceral and occur in discrete episodes. Attacks typically start in the abdomen with low-grade pain that slowly increases over a period of days (not hours) with nausea progressing to vomiting. In some individuals, the pain is predominantly in the back or extremities. When an acute attack is untreated, a motor neuropathy may develop over a period of days or a few weeks. The neuropathy first appears as weakness proximally in the arms and legs, then progresses distally to involve the hands and feet. Some individuals experience respiratory insufficiency due to loss of innervation of the diaphragm and muscles of respiration. Acute attacks are associated commonly with use of certain medications, caloric deprivation, and changes in female reproductive hormones. About 20% of those with an acute attack also experience photosensitivity associated with bullae and skin fragility.

Acute intermittent porphyria (AIP), an autosomal dominant disorder, occurs in heterozygotes for an HMBS pathogenic variant that causes reduced activity of the enzyme porphobilinogen deaminase. AIP is considered "overt" in a heterozygote who was previously or is currently symptomatic; AIP is considered "latent" in a heterozygote who has never had symptoms, and typically has been identified during molecular genetic testing of at-risk family members. Note that GeneReviews does not use the term "carrier" for an individual who is heterozygous for an autosomal dominant pathogenic variant; GeneReviews reserves the term "carrier" for an individual who is heterozygous for an autosomal recessive disorder and thus is not expected to ever develop manifestations of the disorder. Overt AIP is characterized clinically by life-threatening acute neurovisceral attacks of severe abdominal pain without peritoneal signs, often accompanied by nausea, vomiting, tachycardia, and hypertension. Attacks may be complicated by neurologic findings (mental changes, convulsions, and peripheral neuropathy that may progress to respiratory paralysis), and hyponatremia. Acute attacks, which may be provoked by certain drugs, alcoholic beverages, endocrine factors, calorie restriction, stress, and infections, usually resolve within two weeks. Most individuals with AIP have one or a few attacks; about 3%-8% (mainly women) have recurrent attacks (defined as >3 attacks/year) that may persist for years. Other long-term complications are chronic renal failure, hepatocellular carcinoma (HCC), and hypertension. Attacks, which are very rare before puberty, are more common in women than men. Latent AIP. While all individuals heterozygous for an HMBS pathogenic variant that predisposes to AIP are at risk of developing overt AIP, most have latent AIP and never have symptoms.

PLP1 disorders of central nervous system myelin formation include a range of phenotypes from Pelizaeus-Merzbacher disease (PMD) to spastic paraplegia 2 (SPG2). PMD typically manifests in infancy or early childhood with nystagmus, hypotonia, and cognitive impairment; the findings progress to severe spasticity and ataxia. Life span is shortened. SPG2 manifests as spastic paraparesis with or without CNS involvement and usually normal life span. Intrafamilial variation of phenotypes can be observed, but the signs are usually fairly consistent within families. Heterozygous females may manifest mild-to-moderate signs of the disease.

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

The phenotypic spectrum of sepiapterin reductase deficiency (SRD), which ranges from significant motor and cognitive deficits to only minimal findings, has not been completely elucidated. Clinical features in the majority of affected individuals include motor and speech delay, axial hypotonia, dystonia, weakness, and oculogyric crises; symptoms show diurnal fluctuation and sleep benefit. Other common features include parkinsonian signs (tremor, bradykinesia, masked facies, rigidity), limb hypertonia, hyperreflexia, intellectual disability, psychiatric and/or behavioral abnormalities, autonomic dysfunction, and sleep disturbances (hypersomnolence, difficulty initiating or maintaining sleep, and drowsiness). Most affected individuals have nonspecific features in infancy including developmental delays and axial hypotonia; other features develop over time.

Primary trimethylaminuria is characterized by a fishy odor resembling that of rotten or decaying fish that results from excess excretion of trimethylamine in the urine, breath, sweat, and reproductive fluids. No physical symptoms are associated with trimethylaminuria. Affected individuals appear normal and healthy; however, the unpleasant odor often results in social and psychological problems. Symptoms are usually present from birth and may worsen during puberty. In females, symptoms are more severe just before and during menstruation, after taking oral contraceptives, and around the time of menopause.

McLeod neuroacanthocytosis syndrome (designated as MLS throughout this review) is a multisystem disorder with central nervous system (CNS), neuromuscular, cardiovascular, and hematologic manifestations in males: CNS manifestations are a neurodegenerative basal ganglia disease including movement disorders, cognitive alterations, and psychiatric symptoms. Neuromuscular manifestations include a (mostly subclinical) sensorimotor axonopathy and muscle weakness or atrophy of different degrees. Cardiac manifestations include dilated cardiomyopathy, atrial fibrillation, and tachyarrhythmia. Hematologically, MLS is defined as a specific blood group phenotype (named after the first proband, Hugh McLeod) that results from absent expression of the Kx erythrocyte antigen and weakened expression of Kell blood group antigens. The hematologic manifestations are red blood cell acanthocytosis and compensated hemolysis. Alloantibodies in the Kell and Kx blood group system can cause strong reactions to transfusions of incompatible blood and severe anemia in affected male newborns of Kell-negative mothers. Females heterozygous for XK pathogenic variants have mosaicism for the Kell and Kx blood group antigens. Although they usually lack CNS and neuromuscular manifestations, some heterozygous females may develop clinical manifestations including chorea or late-onset cognitive decline.

Homocystinuria caused by cystathionine ß-synthase (CBS) deficiency is characterized by involvement of the eye (ectopia lentis and/or severe myopia), skeletal system (excessive height, long limbs, scolioisis, and pectus excavatum), vascular system (thromboembolism), and CNS (developmental delay/intellectual disability). All four ? or only one ? of the systems can be involved; expressivity is variable for all of the clinical signs. It is not unusual for a previously asymptomatic individual to present in adult years with only a thromboembolic event that is often cerebrovascular. Two phenotypic variants are recognized, B6-responsive homocystinuria and B6-non-responsive homocystinuria. B6-responsive homocystinuria is usually milder than the non-responsive variant. Thromboembolism is the major cause of early death and morbidity. IQ in individuals with untreated homocystinuria ranges widely, from 10 to 138. In B6-responsive individuals the mean IQ is 79 versus 57 for those who are B6-non-responsive. Other features that may occur include: seizures, psychiatric problems, extrapyramidal signs (e.g., dystonia), hypopigmentation of the skin and hair, malar flush, livedo reticularis, and pancreatitis.

Genetic prion disease generally manifests with cognitive difficulties, ataxia, and myoclonus (abrupt jerking movements of muscle groups and/or entire limbs). The order of appearance and/or predominance of these features and other associated neurologic and psychiatric findings vary. The three major phenotypes of genetic prion disease are genetic Creutzfeldt-Jakob disease (gCJD), fatal familial insomnia (FFI), and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Although these phenotypes display overlapping clinical and pathologic features, recognition of these phenotypes can be useful when providing affected individuals and their families with information about the expected clinical course. The age at onset typically ranges from 50 to 60 years. The disease course ranges from a few months in gCJD and FFI to a few (up to 4, and in rare cases up to 10) years in GSS syndrome.

Gomez-Lopez-Hernandez syndrome (GLHS), also known as cerebellotrigeminal dermal dysplasia, is a rare neurocutaneous syndrome classically characterized by the triad of rhombencephalosynapsis, trigeminal anesthesia, often giving rise to corneal opacities, and bilateral parietal or parietooccipital alopecia. However, trigeminal anesthesia is an inconsistent finding (summary by Sukhudyan et al., 2010).

X-linked intellectual developmental disorder-12 (XLID12) is characterized by borderline to severe intellectual disability with variable neurologic features, short stature, and elevated body mass index (BMI) (Kumar et al., 2015).

CLCN4-related neurodevelopmental disorder (CLCN4-NDD), an X-linked disorder, is characterized in the 36 males reported to date by developmental delay or intellectual disability, behavioral/mental health issues (e.g., autism spectrum disorder, anxiety, hyperactivity, and bipolar disorder), epilepsy, and gastrointestinal dysfunction. The five heterozygous females with a de novo CLCN4 variant reported to date had findings very similar to those of affected males. Twenty-two of 25 heterozygous females identified in family studies following identification of an affected male were unaffected or had only mild specific learning difficulties and/or mental health concerns, whereas three were more severely affected.

Depression (also known as major depression or major depressive disorder) is a psychiatric disorder that affects mood, behavior, and overall health. It causes prolonged feelings of sadness, emptiness, or hopelessness, and a loss of interest in activities that were once enjoyed. People with depression may also have changes in appetite (leading to overeating or not eating enough), changes in sleeping patterns (sleeping too much or not being able to sleep), loss of energy, and difficulty concentrating. Although depression is considered primarily a mental health disorder, it can also have physical features including headaches, other unexplained aches and pains, unusually slow or fast movements, and digestive problems. To be diagnosed with depression, an individual must have signs and symptoms nearly every day for at least 2 weeks. However, the features of this condition vary widely.\n\nDepression most commonly begins in late adolescence or early adulthood, although it can appear at any age. If untreated, episodes of depression can last for weeks, months, or years, and can go away and come back (recur). Affected individuals may have difficulty functioning in their daily lives, including at school or work. People with depression have a higher risk of substance abuse problems and dying by suicide than the general population.\n\nSeveral health conditions are closely related to depression or have depression as a characteristic feature. These include dysthymia (which has long-lasting signs and symptoms that are similar to, but not as severe as, those of depression), perinatal or postpartum depression (which occurs around or following the birth of a child), seasonal affective disorder (which is triggered by the changing of the seasons), bipolar disorder (which can include both "highs," or manic episodes, and depressive episodes), and generalized anxiety disorder. In people with schizoaffective disorder, depression or another mood disorder occurs together with features of schizophrenia (a brain disorder that affects a person's thinking, sense of self, and perceptions).

Andersen-Tawil syndrome (ATS) is characterized by a triad of: episodic flaccid muscle weakness (i.e., periodic paralysis); ventricular arrhythmias and prolonged QT interval; and anomalies including low-set ears, widely spaced eyes, small mandible, fifth-digit clinodactyly, syndactyly, short stature, and scoliosis. Affected individuals present in the first or second decade with either cardiac symptoms (palpitations and/or syncope) or weakness that occurs spontaneously following prolonged rest or following rest after exertion. Mild permanent weakness is common. Mild learning difficulties and a distinct neurocognitive phenotype (i.e., deficits in executive function and abstract reasoning) have been described.

Any familial hypocalciuric hypercalcemia in which the cause of the disease is a mutation in the AP2S1 gene.

Neuronal ceroid lipofuscinosis-4 (CLN4) is an autosomal dominant neurodegenerative disorder characterized by onset of symptoms in adulthood. It belongs to a group of progressive neurodegenerative diseases characterized by accumulation of intracellular autofluorescent lipopigment storage material in the brain and other tissues. Several different forms have been described according to age of onset (see, e.g., CLN3, 204200). Individuals with the adult form, sometimes referred to as Kufs disease, develop psychiatric manifestations, seizures, cerebellar ataxia, and cognitive decline. Retinal degeneration is usually not present (summary by Benitez et al., 2011 and Velinov et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

SGCE myoclonus-dystonia (SGCE-M-D) is a movement disorder characterized by a combination of rapid, brief muscle contractions (myoclonus) and/or sustained twisting and repetitive movements that result in abnormal postures (dystonia). The myoclonic jerks typical of SGCE-M-D most often affect the neck, trunk, and upper limbs with less common involvement of the legs. Approximately 50% of affected individuals have additional focal or segmental dystonia, presenting as cervical dystonia and/or writer's cramp. Non-motor features may include alcohol abuse, obsessive-compulsive disorder (OCD), and anxiety disorders. Symptom onset is usually in the first decade of life and almost always by age 20 years, but ranges from age six months to 80 years. Most affected adults report a dramatic reduction in myoclonus in response to alcohol ingestion. SGCE-M-D is compatible with an active life of normal span.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Disease with characteristics of early-onset tremor, dyskinesia and slowly progressive cerebellar ataxia. Fewer than 30 cases have been reported to date. This disease is caused by a mutation in the fibroblast growth factor 14 FGF14 gene (13q34). Prognosis is relatively good. Life-threatening status epilepticus and intractable seizure or severe dysphagia is rare.

Progressive external ophthalmoplegia is characterized by multiple mitochondrial DNA deletions in skeletal muscle. The most common clinical features include adult onset of weakness of the external eye muscles and exercise intolerance. Patients with C10ORF2-linked adPEO may have other clinical features including proximal muscle weakness, ataxia, peripheral neuropathy, cardiomyopathy, cataracts, depression, and endocrine abnormalities (summary by Fratter et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640). PEO caused by mutations in the POLG gene (174763) are associated with more complicated phenotypes than those forms caused by mutations in the SLC25A4 (103220) or C10ORF2 genes (Lamantea et al., 2002).

FMR1 disorders include fragile X syndrome (FXS), fragile X-associated tremor/ataxia syndrome (FXTAS), and fragile X-associated primary ovarian insufficiency (FXPOI). Fragile X syndrome occurs in individuals with an FMR1 full mutation or other loss-of-function variant and is nearly always characterized in affected males by developmental delay and intellectual disability along with a variety of behavioral issues. Autism spectrum disorder is present in 50%-70% of individuals with FXS. Affected males may have characteristic craniofacial features (which become more obvious with age) and medical problems including hypotonia, gastroesophageal reflux, strabismus, seizures, sleep disorders, joint laxity, pes planus, scoliosis, and recurrent otitis media. Adults may have mitral valve prolapse or aortic root dilatation. The physical and behavioral features seen in males with FXS have been reported in females heterozygous for the FMR1 full mutation, but with lower frequency and milder involvement. FXTAS occurs in individuals who have an FMR1 premutation and is characterized by late-onset, progressive cerebellar ataxia and intention tremor followed by cognitive impairment. Psychiatric disorders are common. Age of onset is typically between 60 and 65 years and is more common among males who are hemizygous for the premutation (40%) than among females who are heterozygous for the premutation (16%-20%). FXPOI, defined as hypergonadotropic hypogonadism before age 40 years, has been observed in 20% of women who carry a premutation allele compared to 1% in the general population.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

MECP2 duplication syndrome is a severe neurodevelopmental disorder characterized by early-onset hypotonia, feeding difficulty, gastrointestinal manifestations including gastroesophageal reflux and constipation, delayed psychomotor development leading to severe intellectual disability, poor speech development, progressive spasticity, recurrent respiratory infections (in ~75% of affected individuals), and seizures (in ~50%). MECP2 duplication syndrome is 100% penetrant in males. Occasionally females have been described with a MECP2 duplication and a range of findings from mild intellectual disability to a phenotype similar to that seen in males. In addition to the core features, autistic behaviors, nonspecific neuroradiologic findings on brain MRI, mottled skin, and urogenital anomalies have been observed in several affected boys.

Spinocerebellar ataxia type 17 (SCA17) is characterized by ataxia, dementia, and involuntary movements, including chorea and dystonia. Psychiatric symptoms, pyramidal signs, and rigidity are common. The age of onset ranges from three to 55 years. Individuals with full-penetrance alleles develop neurologic and/or psychiatric symptoms by age 50 years. Ataxia and psychiatric abnormalities are frequently the initial findings, followed by involuntary movement, parkinsonism, dementia, and pyramidal signs. Brain MRI shows variable atrophy of the cerebrum, brain stem, and cerebellum. The clinical features correlate with the length of the polyglutamine expansion but are not absolutely predictive of the clinical course.

Huntington disease-like 2 (HDL2) typically presents in midlife with a relentless progressive triad of movement, emotional, and cognitive abnormalities which lead to death within ten to 20 years. HDL2 cannot be differentiated from Huntington disease clinically. Neurologic abnormalities include chorea, hypokinesia (rigidity, bradykinesia), dysarthria, and hyperreflexia in the later stages of the disease. There is a strong correlation between the duration of the disease and the progression of the motor and cognitive disorder.

The neuronal ceroid lipofuscinoses (NCL; CLN) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by the intracellular accumulation of autofluorescent lipopigment storage material in different patterns ultrastructurally. The lipopigment pattern seen most often in CLN1 is referred to as granular osmiophilic deposits (GROD). The patterns most often observed in CLN2 and CLN3 are 'curvilinear' and 'fingerprint' profiles, respectively. CLN4, CLN5, CLN6, CLN7, and CLN8 show mixed combinations of granular, curvilinear, fingerprint, and rectilinear profiles. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). Zeman and Dyken (1969) referred to these conditions as the 'neuronal ceroid lipofuscinoses.' Goebel (1995) provided a comprehensive review of the NCLs and noted that they are possibly the most common group of neurodegenerative diseases in children. Mole et al. (2005) provided a detailed clinical and genetic review of the neuronal ceroid lipofuscinoses. Genetic Heterogeneity of Neuronal Ceroid Lipofuscinosis See also CLN2 (204500), caused by mutation in the TPP1 gene (607998) on chromosome 11p15; CLN3 (204200), caused by mutation in the CLN3 gene (607042) on 16p12; CLN4 (162350), caused by mutation in the DNAJC5 gene (611203) on 20q13; CLN5 (256731), caused by mutation in the CLN5 gene (608102) on 13q22; CLN6A (601780) and CLN6B (204300), both caused by mutation in the CLN6 gene (606725) on 15q21; CLN7 (610951), caused by mutation in the MFSD8 gene (611124) on 4q28; CLN8 (600143) and the Northern epilepsy variant of CLN8 (610003), both caused by mutation in the CLN8 gene (607837) on 8p23; CLN10 (610127), caused by mutation in the CTSD gene (116840) on 11p15; CLN11 (614706), caused by mutation in the GRN gene (138945) on 17q21; CLN13 (615362), caused by mutation in the CTSF gene (603539) on 11q13; and CLN14 (611726), caused by mutation in the KCTD7 gene (611725) on 7q11. CLN9 (609055) has not been molecularly characterized. A disorder that was formerly designated neuronal ceroid lipofuscinosis-12 (CLN12) is now considered to be a variable form of Kufor-Rakeb syndrome (KRS; 606693).

DYT1 early-onset isolated dystonia typically presents in childhood or adolescence and only on occasion in adulthood. Dystonic muscle contractions causing posturing or irregular tremor of a leg or arm are the most common presenting findings. Dystonia is usually first apparent with specific actions such as writing or walking. Over time, the contractions frequently (but not invariably) become evident with less specific actions and spread to other body regions. No other neurologic abnormalities are present. Disease severity varies considerably even within the same family. Isolated writer's cramp may be the only sign.

Bipolar disorder is a mental health condition that causes extreme shifts in mood, energy, and behavior. This disorder most often appears in late adolescence or early adulthood, although symptoms can begin at any time of life.\n\nPeople with bipolar disorder experience both dramatic "highs," called manic episodes, and "lows," called depressive episodes. These episodes can last from hours to weeks, and many people have no symptoms between episodes.\n\nManic episodes are characterized by increased energy and activity, irritability, restlessness, an inability to sleep, and reckless behavior. Some people with bipolar disorder experience hypomanic episodes, which are similar to but less extreme than manic episodes.\n\nDepressive episodes are marked by low energy and activity, a feeling of hopelessness, and an inability to perform everyday tasks. People with bipolar disorder often have repeated thoughts of death and suicide, and they have a much greater risk of dying by suicide than the general population.\n\nManic and depressive episodes can include psychotic symptoms, such as false perceptions (hallucinations) or strongly held false beliefs (delusions). Mixed episodes, which have features of manic and depressive episodes at the same time, also occur in some affected individuals.\n\nBipolar disorder is classified into several types based on the mood changes that occur. Bipolar I involves manic episodes, which can be accompanied by psychotic symptoms, and hypomanic or depressive episodes. Bipolar II involves hypomanic episodes and depressive episodes. Cyclothymic disorder involves hypomanic episodes and depressive episodes that are typically less severe than those in bipolar I or bipolar II.\n\nBipolar disorder often occurs with other mental health conditions, including anxiety disorders (such as panic attacks), behavioral disorders (such as attention-deficit/hyperactivity disorder), and substance abuse.

PINK1 type of young-onset Parkinson disease is characterized by early onset (mean age 33 years) of tremor, bradykinesia, and rigidity that are often indistinguishable from other causes of Parkinson disease. Lower-limb dystonia may be a presenting sign. Postural instability, hyperreflexia, abnormal behavior, and psychiatric manifestations have been described. The disease is usually slowly progressive. Individuals have a marked and sustained response to oral administration of levodopa (L-dopa), frequently associated with L-dopa-induced fluctuations and dyskinesias.

Spinocerebellar ataxia type 14 (SCA14) is characterized by slowly progressive cerebellar ataxia, dysarthria, and nystagmus. Axial myoclonus, cognitive impairment, tremor, and sensory loss may also be observed. Parkinsonian features including rigidity and tremor have been described in some families. Findings seen in other ataxia disorders (e.g., dysphagia, dysphonia) may also occur in SCA14. The average age of onset is in the 30s, with a range from childhood to the seventh decade. Life span is not shortened.

ACTH-independent macronodular adrenal hyperplasia (AIMAH) is an endogenous form of adrenal Cushing syndrome characterized by multiple bilateral adrenocortical nodules that cause a striking enlargement of the adrenal glands. Although some familial cases have been reported, the vast majority of AIMAH cases are sporadic. Patients typically present in the fifth or sixth decade of life, approximately 10 years later than most patients with other causes of Cushing syndrome (Swain et al., 1998; Christopoulos et al., 2005). Approximately 10 to 15% of adrenal Cushing syndrome is due to primary bilateral ACTH-independent adrenocortical pathology. The 2 main subtypes are AIMAH and primary pigmented nodular adrenocortical disease (PPNAD; see 610489), which is often a component of the Carney complex (160980) and associated with mutations in the PRKAR1A gene (188830). AIMAH is rare, representing less than 1% of endogenous causes of Cushing syndrome (Swain et al., 1998; Christopoulos et al., 2005). See also ACTH-independent Cushing syndrome (615830) due to somatic mutation in the PRKACA gene (601639). Cushing 'disease' (219090) is an ACTH-dependent disorder caused in most cases by pituitary adenomas that secrete excessive ACTH. Genetic Heterogeneity of ACTH-Independent Macronodular Adrenal Hyperplasia AIMAH2 (615954) is caused by germline mutation on 1 allele of the ARMC5 gene (615549) coupled with a somatic mutation in the other allele.

Wolfram syndrome-2 (WFS2) is an autosomal recessive neurodegenerative disorder characterized by diabetes mellitus, high frequency sensorineural hearing loss, optic atrophy or neuropathy, and defective platelet aggregation resulting in peptic ulcer bleeding (summary by Mozzillo et al., 2014). For a discussion of genetic heterogeneity of Wolfram syndrome, see WFS1 (222300).

Rare disease with manifestations of action tremor associated with relatively mild cerebellar ataxia. Associated pyramidal and extrapyramidal signs and dementia have been reported. Prevalence is unknown. Approximately 40 families have been reported. The pathogenesis seems to be related to a toxic effect at the RNA level as it is caused by a CAG expansion at the 5'' end of the PPP2R2B gene on chromosome 5q31-5q32.

Celiac disease is a systemic autoimmune disease that can be associated with gastrointestinal findings (diarrhea, malabsorption, abdominal pain and distension, bloating, vomiting, and weight loss) and/or highly variable non-gastrointestinal findings (dermatitis herpetiformis, chronic fatigue, joint pain/inflammation, iron deficiency anemia, migraines, depression, attention-deficit disorder, epilepsy, osteoporosis/osteopenia, infertility and/or recurrent fetal loss, vitamin deficiencies, short stature, failure to thrive, delayed puberty, dental enamel defects, and autoimmune disorders). Classic celiac disease, characterized by mild to severe gastrointestinal symptoms, is less common than non-classic celiac disease, characterized by absence of gastrointestinal symptoms.

Genetic prion disease generally manifests with cognitive difficulties, ataxia, and myoclonus (abrupt jerking movements of muscle groups and/or entire limbs). The order of appearance and/or predominance of these features and other associated neurologic and psychiatric findings vary. The three major phenotypes of genetic prion disease are genetic Creutzfeldt-Jakob disease (gCJD), fatal familial insomnia (FFI), and Gerstmann-Sträussler-Scheinker (GSS) syndrome. Although these phenotypes display overlapping clinical and pathologic features, recognition of these phenotypes can be useful when providing affected individuals and their families with information about the expected clinical course. The age at onset typically ranges from 50 to 60 years. The disease course ranges from a few months in gCJD and FFI to a few (up to 4, and in rare cases up to 10) years in GSS syndrome.

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is characterized by clusters of nocturnal motor seizures, which are often stereotyped and brief (5 seconds to 5 minutes). They vary from simple arousals from sleep to dramatic, often bizarre hyperkinetic events with tonic or dystonic features. Affected individuals may experience aura. Retained awareness during seizures is common. A minority of individuals experience daytime seizures. Onset ranges from infancy to adulthood. About 80% of individuals develop ADNFLE in the first two decades of life; mean age of onset is ten years. Clinical neurologic examination is normal and intellect is usually preserved, but reduced intellect, psychiatric comorbidity, or cognitive deficits may occur. Within a family, the manifestations of the disorder may vary considerably. ADNFLE is lifelong but not progressive. As an individual reaches middle age, attacks may become milder and less frequent.

Primary pigmented nodular adrenocortical disease (PPNAD) is a form of ACTH-independent adrenal hyperplasia resulting in Cushing syndrome. It is usually seen as a manifestation of the Carney complex (CNC1; 160980), a multiple neoplasia syndrome. However, PPNAD can also occur in isolation (Groussin et al., 2002). Genetic Heterogeneity of Primary Pigmented Nodular Adrenocortical Disease See also PPNAD2 (610475), caused by mutation in the PDE11A gene (604961) on chromosome 2q31; PPNAD3 (614190), caused by mutation in the PDE8B gene (603390) on chromosome 5q13; and PPNAD4 (615830), caused by a duplication on chromosome 19p13 that includes the PRKACA gene (601639).

Any primary pigmented nodular adrenocortical disease in which the cause of the disease is a mutation in the PDE11A gene.

Spastic paraplegia 4 (SPG4; also known as SPAST-HSP) is characterized by insidiously progressive bilateral lower-limb gait spasticity. More than 50% of affected individuals have some weakness in the legs and impaired vibration sense at the ankles. Sphincter disturbances are very common. Onset is insidious, mostly in young adulthood, although symptoms may start as early as age one year and as late as age 76 years. Intrafamilial variation is considerable.

LMNB1-related autosomal dominant leukodystrophy (ADLD) is a slowly progressive disorder of central nervous system white matter characterized by onset of autonomic dysfunction in the fourth to fifth decade, followed by pyramidal and cerebellar abnormalities resulting in spasticity, ataxia, and tremor. Autonomic dysfunction can include bladder dysfunction, constipation, postural hypotension, erectile dysfunction, and (less often) impaired sweating. Pyramidal signs are often more prominent in the lower extremities (e.g., spastic weakness, hypertonia, clonus, brisk deep tendon reflexes, and bilateral Babinski signs). Cerebellar signs typically appear at the same time as the pyramidal signs and include gait ataxia, dysdiadochokinesia, intention tremor, dysmetria, and nystagmus. Many individuals have sensory deficits starting in the lower limbs. Pseudobulbar palsy with dysarthria, dysphagia, and forced crying and laughing may appear in the seventh or eighth decade. Although cognitive function is usually preserved or only mildly impaired early in the disease course, dementia and psychiatric manifestations can occur as late manifestations. Affected individuals may survive for decades after onset.

The spectrum of DCTN1-related neurodegeneration includes Perry syndrome, distal hereditary motor neuronopathy type 7B (dHMN7B), frontotemporal dementia (FTD), motor neuron disease / amyotrophic lateral sclerosis (ALS), and progressive supranuclear palsy. Some individuals present with overlapping phenotypes (e.g., FTD-ALS, Perry syndrome-dHMN7B). Perry syndrome (the most common of the phenotypes associated with DCTN1) is characterized by parkinsonism, neuropsychiatric symptoms, hypoventilation, and weight loss. The mean age of onset in those with Perry syndrome is 49 years (range: 35-70 years), and the mean disease duration is five years (range: 2-14 years). In most affected persons, the reported cause/circumstance of death relates to sudden death/hypoventilation or suicide.

Parkinson disease is the second most common neurogenic disorder after Alzheimer disease (AD; 104300), affecting approximately 1% of the population over age 50. Clinical manifestations include resting tremor, muscular rigidity, bradykinesia, and postural instability. Additional features are characteristic postural abnormalities, dysautonomia, dystonic cramps, and dementia (Polymeropoulos et al., 1996). For a general phenotypic description and a discussion of genetic heterogeneity of Parkinson disease, see 168600.

ATP1A3-related neurologic disorders represent a clinical continuum in which at least three distinct phenotypes have been delineated: rapid-onset dystonia-parkinsonism (RDP); alternating hemiplegia of childhood (ACH); and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS). However, some affected individuals have intermediate phenotypes or only a few features that do not fit well into one of these major phenotypes. RDP has been characterized by: abrupt onset of dystonia over days to weeks with parkinsonism (primarily bradykinesia and postural instability); common bulbar involvement; and absence or minimal response to an adequate trial of L-dopa therapy, with few exceptions. Often fever, physiologic stress, or alcoholic binges trigger the onset of symptoms. After their initial appearance, symptoms often stabilize with little improvement; occasionally second episodes occur with abrupt worsening of symptoms. Rarely, affected individuals have reported a more gradual onset of symptoms over weeks to months. Anxiety, depression, and seizures have been reported. Age of onset ranges from four to 55 years, although a childhood variation of RDP with onset between ages nine and 14 months has been reported. AHC is a complex neurodevelopmental syndrome most frequently manifesting in infancy or early childhood with paroxysmal episodic neurologic dysfunction including alternating hemiparesis or dystonia, quadriparesis, seizure-like episodes, and oculomotor abnormalities. Episodes can last for minutes, hours, days, or even weeks. Remission of symptoms occurs with sleep and immediately after awakening. Over time, persistent neurologic deficits including oculomotor apraxia, ataxia, choreoathetosis, dystonia, parkinsonism, and cognitive and behavioral dysfunction develop in the majority of those affected; more than 50% develop epilepsy in addition to their episodic movement disorder phenotype. CAPOS (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) syndrome is characterized by episodes of ataxic encephalopathy and/or weakness during and after a febrile illness. Onset is between ages six months and four years. Some acute symptoms resolve; progression of sensory losses and severity vary.

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disorder of the central nervous system (CNS) with various degrees of axonal damage. MS affects mainly young adults with predominance for females. The disorder often leads to substantial disability (summary by Bomprezzi et al., 2003). Genetic Heterogeneity of Susceptibility to Multiple Sclerosis Additional MS susceptibility loci include MS2 (612594) on chromosome 10p15, MS3 (612595) on chromosome 5p13, MS4 (612596) on chromosome 1p36, and MS5 (614810), conferred by variation in the TNFRSF1A gene (191190) on chromosome 12p13.

Spinocerebellar ataxia type 10 (SCA10) is characterized by slowly progressive cerebellar ataxia that usually starts as poor balance and unsteady gait, followed by upper-limb ataxia, scanning dysarthria, and dysphagia. Abnormal tracking eye movements are common. Recurrent seizures after the onset of gait ataxia have been reported with variable frequencies among different families. Some individuals have cognitive dysfunction, behavioral disturbances, mood disorders, mild pyramidal signs, and peripheral neuropathy. Age of onset ranges from 12 to 48 years.

Four phenotypes comprise the RRM2B mitochondrial DNA maintenance defects (RRM2B-MDMDs): RRM2B encephalomyopathic MDMD, the most severe phenotype, usually manifesting shortly after birth as hypotonia, poor feeding, and faltering growth requiring hospitalization. Subsequent assessments are likely to reveal multisystem involvement including sensorineural hearing loss, renal tubulopathy, and respiratory failure. Autosomal dominant progressive external ophthalmoplegia (adPEO), typically adult onset; other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness. RRM2B autosomal recessive progressive external ophthalmoplegia (arPEO), a typically childhood-onset predominantly myopathic phenotype of PEO, ptosis, proximal muscle weakness, and bulbar dysfunction. RRM2B mitochondrial neurogastrointestinal encephalopathy (MNGIE)-like, characterized by progressive ptosis, ophthalmoplegia, gastrointestinal dysmotility, cachexia, and peripheral neuropathy. To date, 78 individuals from 52 families with a molecularly confirmed RRM2B-MDMD have been reported.

Attention-deficit/hyperactivity disorder (ADHD) is a behavioral disorder that typically begins in childhood and is characterized by a short attention span (inattention), an inability to be calm and stay still (hyperactivity), and poor impulse control (impulsivity). Some people with ADHD have problems with only inattention or with hyperactivity and impulsivity, but most have problems related to all three features.\n\nIn people with ADHD, the characteristic behaviors are frequent and severe enough to interfere with the activities of daily living such as school, work, and relationships with others. Because of an inability to stay focused on tasks, people with inattention may be easily distracted, forgetful, avoid tasks that require sustained attention, have difficulty organizing tasks, or frequently lose items.\n\nHyperactivity is usually shown by frequent movement. Individuals with this feature often fidget or tap their foot when seated, leave their seat when it is inappropriate to do so (such as in the classroom), or talk a lot and interrupt others.\n\nImpulsivity can result in hasty actions without thought for the consequences. Individuals with poor impulse control may have difficulty waiting for their turn, deferring to others, or considering their actions before acting.\n\nMore than two-thirds of all individuals with ADHD have additional conditions, including insomnia, mood or anxiety disorders, learning disorders, or substance use disorders. Affected individuals may also have autism spectrum disorder, which is characterized by impaired communication and social interaction, or Tourette syndrome, which is a disorder characterized by repetitive and involuntary movements or noises called tics.\n\nIn most affected individuals, ADHD continues throughout life, but in about one-third of individuals, signs and symptoms of ADHD go away by adulthood.

Generally, Parkinson's disease that begins after age 50 is called late-onset disease. The condition is described as early-onset disease if signs and symptoms begin before age 50. Early-onset cases that begin before age 20 are sometimes referred to as juvenile-onset Parkinson's disease.\n\nParkinson's disease can also affect emotions and thinking ability (cognition). Some affected individuals develop psychiatric conditions such as depression and visual hallucinations. People with Parkinson's disease also have an increased risk of developing dementia, which is a decline in intellectual functions including judgment and memory.\n\nOften the first symptom of Parkinson's disease is trembling or shaking (tremor) of a limb, especially when the body is at rest. Typically, the tremor begins on one side of the body, usually in one hand. Tremors can also affect the arms, legs, feet, and face. Other characteristic symptoms of Parkinson's disease include rigidity or stiffness of the limbs and torso, slow movement (bradykinesia) or an inability to move (akinesia), and impaired balance and coordination (postural instability). These symptoms worsen slowly over time.\n\nParkinson's disease is a progressive disorder of the nervous system. The disorder affects several regions of the brain, especially an area called the substantia nigra that controls balance and movement.

Congenital hypertryptophanemia, which is accompanied by hyperserotonemia, does not appear to have significant clinical consequences (Ferreira et al., 2017).

Generally, Parkinson's disease that begins after age 50 is called late-onset disease. The condition is described as early-onset disease if signs and symptoms begin before age 50. Early-onset cases that begin before age 20 are sometimes referred to as juvenile-onset Parkinson's disease.\n\nParkinson's disease can also affect emotions and thinking ability (cognition). Some affected individuals develop psychiatric conditions such as depression and visual hallucinations. People with Parkinson's disease also have an increased risk of developing dementia, which is a decline in intellectual functions including judgment and memory.\n\nOften the first symptom of Parkinson's disease is trembling or shaking (tremor) of a limb, especially when the body is at rest. Typically, the tremor begins on one side of the body, usually in one hand. Tremors can also affect the arms, legs, feet, and face. Other characteristic symptoms of Parkinson's disease include rigidity or stiffness of the limbs and torso, slow movement (bradykinesia) or an inability to move (akinesia), and impaired balance and coordination (postural instability). These symptoms worsen slowly over time.\n\nParkinson's disease is a progressive disorder of the nervous system. The disorder affects several regions of the brain, especially an area called the substantia nigra that controls balance and movement.

Autosomal dominant Wolfram-like syndrome (WFSL) is characterized by the clinical triad of congenital progressive hearing impairment, diabetes mellitus, and optic atrophy. The hearing impairment, which is usually diagnosed in the first decade of life, is relatively constant and alters mainly low- and middle-frequency ranges (summary by Valero et al., 2008). Wolfram syndrome (WFS1; 222300) is an autosomal recessive allelic disorder characterized by optic atrophy, diabetes mellitus, hearing loss, and diabetes insipidus, and is caused by homozygous or compound heterozygous mutation in the WFS1 gene. An autosomal dominant syndrome involving optic atrophy with or without deafness, ophthalmoplegia, myopathy, ataxia, and neuropathy (125250), is caused by heterozygous mutation in the OPA1 gene (605290).

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is characterized initially by gait changes followed by progressive spastic paresis, progressive dystonia (which may be limited to the hands and feet or more generalized), neuropsychiatric abnormalities (emotional lability, depression, anxiety, impulsivity, compulsions, hallucinations, perseveration, inattention, and hyperactivity), and cognitive decline. Additional early findings can include dysphagia, dysarthria, optic atrophy, axonal neuropathy, parkinsonism, and bowel/bladder incontinence. Survival is usually well into adulthood. End-stage disease is characterized by severe dementia, spasticity, dystonia, and parkinsonism.

AMACR deficiency is a rare autosomal recessive peroxisomal disorder characterized by adult onset of variable neurodegenerative symptoms affecting the central and peripheral nervous systems. Features may include seizures, visual failure, sensorimotor neuropathy, spasticity, migraine, and white matter hyperintensities on brain imaging. Serum pristanic acid and C27 bile acid intermediates are increased (summary by Smith et al., 2010).

Autosomal recessive mitochondrial complex III deficiency is a severe multisystem disorder with onset at birth of lactic acidosis, hypotonia, hypoglycemia, failure to thrive, encephalopathy, and delayed psychomotor development. Visceral involvement, including hepatopathy and renal tubulopathy, may also occur. Many patients die in early childhood, but some may show longer survival (de Lonlay et al., 2001; De Meirleir et al., 2003). Genetic Heterogeneity of Mitochondrial Complex III Deficiency Mitochondrial complex III deficiency can be caused by mutation in several different nuclear-encoded genes. See MC3DN2 (615157), caused by mutation in the TTC19 gene (613814) on chromosome 17p12; MC3DN3 (615158), caused by mutation in the UQCRB gene (191330) on chromosome 8q; MC3DN4 (615159), caused by mutation in the UQCRQ gene (612080) on chromosome 5q31; MC3DN5 (615160), caused by mutation in the UQCRC2 gene (191329) on chromosome 16p12; MC3DN6 (615453), caused by mutation in the CYC1 gene (123980) on chromosome 8q24; MC3DN7 (615824), caused by mutation in the UQCC2 gene (614461) on chromosome 6p21; MC3DN8 (615838), caused by mutation in the LYRM7 gene (615831) on chromosome 5q23; MC3DN9 (616111), caused by mutation in the UQCC3 gene (616097) on chromosome 11q12; and MC3DN10 (618775), caused by mutation in the UQCRFS1 gene (191327) on chromosome 19q12. See also MTYCB (516020) for a discussion of a milder phenotype associated with isolated mitochondrial complex III deficiency and mutations in a mitochondrial-encoded gene.

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is characterized by clusters of nocturnal motor seizures, which are often stereotyped and brief (5 seconds to 5 minutes). They vary from simple arousals from sleep to dramatic, often bizarre hyperkinetic events with tonic or dystonic features. Affected individuals may experience aura. Retained awareness during seizures is common. A minority of individuals experience daytime seizures. Onset ranges from infancy to adulthood. About 80% of individuals develop ADNFLE in the first two decades of life; mean age of onset is ten years. Clinical neurologic examination is normal and intellect is usually preserved, but reduced intellect, psychiatric comorbidity, or cognitive deficits may occur. Within a family, the manifestations of the disorder may vary considerably. ADNFLE is lifelong but not progressive. As an individual reaches middle age, attacks may become milder and less frequent.

Primary familial brain calcification (PFBC) is a neurodegenerative disorder with characteristic calcium deposits in the basal ganglia and other brain areas visualized on neuroimaging. Most affected individuals are in good health during childhood and young adulthood and typically present in the fourth to fifth decade with a gradually progressive movement disorder and neuropsychiatric symptoms. The movement disorder first manifests as clumsiness, fatigability, unsteady gait, slow or slurred speech, dysphagia, involuntary movements, or muscle cramping. Neuropsychiatric symptoms, often the first or most prominent manifestations, range from mild difficulty with concentration and memory to changes in personality and/or behavior, to psychosis and dementia. Seizures of various types occur frequently, some individuals experience chronic headache and vertigo; urinary urgency or incontinence may be present.

PEOA6 is characterized by muscle weakness, mainly affecting the lower limbs, external ophthalmoplegia, exercise intolerance, and mitochondrial DNA (mtDNA) deletions on muscle biopsy. Symptoms may appear in childhood or adulthood and show slow progression (summary by Ronchi et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

Mitochondrial complex III deficiency nuclear type 2 is an autosomal recessive severe neurodegenerative disorder that usually presents in childhood, but may show later onset, even in adulthood. Affected individuals have motor disability, with ataxia, apraxia, dystonia, and dysarthria, associated with necrotic lesions throughout the brain. Most patients also have cognitive impairment and axonal neuropathy and become severely disabled later in life (summary by Ghezzi et al., 2011). The disorder may present clinically as spinocerebellar ataxia or Leigh syndrome, or with psychiatric disturbances (Morino et al., 2014; Atwal, 2014; Nogueira et al., 2013). For a discussion of genetic heterogeneity of mitochondrial complex III deficiency, see MC3DN1 (124000).

Neuronal ceroid lipofuscinosis-13 (CLN13) is an autosomal recessive neurodegenerative disorder characterized by adult onset of progressive cognitive decline and motor dysfunction leading to dementia and often early death. Some patients develop seizures. Neurons show abnormal accumulation of autofluorescent material (summary by Smith et al., 2013). Adult-onset neuronal ceroid lipofuscinosis is sometimes referred to as Kufs disease (see 204300). In a review of the classification of CLN disease, Gardner and Mole (2021) noted that the CLN13 phenotype corresponds to 'Kufs type B', which is characterized by dementia and a variety of motor signs (Smith et al., 2013). For a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis (CLN), see CLN1 (256730).

Macrocephaly refers to an abnormally enlarged head inclusive of the scalp, cranial bones, and intracranial contents. Macrocephaly may be due to megalencephaly (true enlargement of the brain parenchyma), and the 2 terms are often used interchangeably in the genetic literature (reviews by Olney, 2007 and Williams et al., 2008). Autosomal recessive macrocephaly/megalencephaly syndrome is characterized by an enlarged cranium apparent at birth or in early childhood. Affected individuals have intellectual disability and may have dysmorphic facial features resulting from the macrocephaly (summary by Alfaiz et al., 2014).

ADCADN is an autosomal dominant neurologic disorder characterized by adult onset of progressive cerebellar ataxia, narcolepsy/cataplexy, sensorineural deafness, and dementia. More variable features include optic atrophy, sensory neuropathy, psychosis, and depression (summary by Winkelmann et al., 2012).

Advanced sleep phase syndrome is characterized by very early sleep onset and offset (summary by Jones et al., 1999). Genetic Heterogeneity of Advanced Sleep Phase Syndrome See also FASPS2 (615224), caused by mutation in the CSNK1D gene (600864) on chromosome 17q25, and FASPS3 (616882), caused by mutation in the PER3 gene (603427) on chromosome 1p36.

Primary familial brain calcification (PFBC) is a neurodegenerative disorder with characteristic calcium deposits in the basal ganglia and other brain areas visualized on neuroimaging. Most affected individuals are in good health during childhood and young adulthood and typically present in the fourth to fifth decade with a gradually progressive movement disorder and neuropsychiatric symptoms. The movement disorder first manifests as clumsiness, fatigability, unsteady gait, slow or slurred speech, dysphagia, involuntary movements, or muscle cramping. Neuropsychiatric symptoms, often the first or most prominent manifestations, range from mild difficulty with concentration and memory to changes in personality and/or behavior, to psychosis and dementia. Seizures of various types occur frequently, some individuals experience chronic headache and vertigo; urinary urgency or incontinence may be present.

C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) is characterized most often by frontotemporal dementia (FTD) and upper and lower motor neuron disease (MND); however, atypical presentations also occur. Age at onset is usually between 50 and 64 years (range: 20-91 years) irrespective of the presenting manifestations, which may be pure FTD, pure amyotrophic lateral sclerosis (ALS), or a combination of the two phenotypes. The clinical presentation is highly heterogeneous and may differ between and within families, causing an unpredictable pattern and age of onset of clinical manifestations. The presence of MND correlates with an earlier age of onset and a worse overall prognosis.

Cushing syndrome is a clinical designation for the systemic signs and symptoms arising from excess cortisol production. Affected individuals typically show hypertension, impaired glucose tolerance, central obesity, osteoporosis, and sometimes depression. Corticotropin-independent Cushing syndrome results from autonomous cortisol production by the adrenal glands, often associated with adrenocortical tumors. Adrenocortical tumors are most common in adult females (summary by Cao et al., 2014; Sato et al., 2014).

Progressive leukoencephalopathy with ovarian failure is an autosomal recessive neurodegenerative disorder characterized by loss of motor and cognitive skills, usually with onset in young adulthood. Some patients may have a history of delayed motor development or learning difficulties in early childhood. Neurologic decline is severe, usually resulting in gait difficulties, ataxia, spasticity, and cognitive decline and dementia. Most patients lose speech and become wheelchair-bound or bedridden. Brain MRI shows progressive white matter signal abnormalities in the deep white matter. Affected females develop premature ovarian failure (summary by Dallabona et al., 2014).

ACTH-independent macronodular adrenal hyperplasia-2 is an autosomal dominant tumor susceptibility with syndromic incomplete penetrance, as a second hit to the ARMC5 gene is required to develop macronodular hyperplasia (Assie et al., 2013).

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Advanced sleep phase syndrome is characterized by early sleep time (sleep onset) and early morning awakening (sleep offset) (summary by Zhang et al., 2016). For a discussion of genetic heterogeneity of advanced sleep phase syndrome, see FASPS1 (604348).

Spinocerebellar ataxia-42 (SCA42) is an autosomal dominant neurologic disorder characterized predominantly by gait instability and additional cerebellar signs such as dysarthria, nystagmus, and saccadic pursuits. The age at onset and severity of the disorder is highly variable. The disorder is slowly progressive (Coutelier et al., 2015). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Any Parkinson disease in which the cause of the disease is a mutation in the CHCHD2 gene.

Kosaki overgrowth syndrome (KOGS) is characterized by a facial gestalt involving prominent forehead, proptosis, downslanting palpebral fissures, broad nasal bridge, thin upper lip, and pointed chin. Affected individuals are tall, with an elongated lower segment, and have large hands and feet. Skin is hyperelastic and fragile. Patients exhibit progressive dilatory and vascular changes in basilar/vertebral and coronary arteries starting in the teenage years (Takenouchi et al., 2015; Takenouchi et al., 2021).

Primary familial brain calcification (PFBC) is a neurodegenerative disorder with characteristic calcium deposits in the basal ganglia and other brain areas visualized on neuroimaging. Most affected individuals are in good health during childhood and young adulthood and typically present in the fourth to fifth decade with a gradually progressive movement disorder and neuropsychiatric symptoms. The movement disorder first manifests as clumsiness, fatigability, unsteady gait, slow or slurred speech, dysphagia, involuntary movements, or muscle cramping. Neuropsychiatric symptoms, often the first or most prominent manifestations, range from mild difficulty with concentration and memory to changes in personality and/or behavior, to psychosis and dementia. Seizures of various types occur frequently, some individuals experience chronic headache and vertigo; urinary urgency or incontinence may be present.

Myoclonic dystonia-26 (DYT26) is an autosomal dominant neurologic disorder characterized by onset of myoclonic jerks affecting the upper limbs in the first or second decade of life. The disorder is progressive, and patients later develop dystonia with predominant involvement of the craniocervical regions and sometimes the trunk and/or lower limbs. Dystonia dominates the clinical picture (summary by Mencacci et al., 2015).

Neurodegeneration with brain iron accumulation refers to a group of neurodegenerative disorders characterized by progressive motor and cognitive dysfunction beginning in childhood or young adulthood. Patients show extrapyramidal motor signs, such as spasticity, dystonia, and parkinsonism. Brain imaging shows iron accumulation in the basal ganglia (summary by Dusi et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of NBIA, see NBIA1 (234200).

Mitochondrial myopathy and ataxia (MMYAT) is an autosomal recessive mtDNA depletion disorder characterized by cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic, and pigmentary retinopathy (summary by Donkervoort et al., 2019).

Primary familial brain calcification (PFBC) is a neurodegenerative disorder with characteristic calcium deposits in the basal ganglia and other brain areas visualized on neuroimaging. Most affected individuals are in good health during childhood and young adulthood and typically present in the fourth to fifth decade with a gradually progressive movement disorder and neuropsychiatric symptoms. The movement disorder first manifests as clumsiness, fatigability, unsteady gait, slow or slurred speech, dysphagia, involuntary movements, or muscle cramping. Neuropsychiatric symptoms, often the first or most prominent manifestations, range from mild difficulty with concentration and memory to changes in personality and/or behavior, to psychosis and dementia. Seizures of various types occur frequently, some individuals experience chronic headache and vertigo; urinary urgency or incontinence may be present.

SCA48 is an autosomal dominant neurodegenerative disorder characterized by onset of gait ataxia and/or cognitive-affective symptoms in midadulthood. Patients may present with involvement of either system, but most eventually develop impairment in both. Features include gait ataxia, dysarthria, and dysphagia, as well as cognitive decline, deficits in executive function, and psychiatric or affective manifestations, such as depression, anxiety, and apathy. Additional more variable features may include movement abnormalities, such as parkinsonism, tremor, chorea, dystonia, and dysmetria; spasticity is not observed. Brain imaging shows selective atrophy of the posterior areas of the cerebellar vermis, often with bilateral T2-weighted hyperintensities in the dentate nuclei (the 'crab sign'), and diffusion tensor imaging (DTI) may show paucity of cerebellar connections to the brainstem and cerebrum. The presentation is consistent with a clinical diagnosis of cerebellar cognitive-affective syndrome (CCAS). The phenotype shows both inter- and intrafamilial variability as well as some clinical overlap with SCAR16, suggesting that mutations in the STUB1 gene result in a spectrum of neurodegenerative manifestations (summary by Genis et al., 2018; Cocozza et al., 2020; Palvadeau et al., 2020; Ravel et al., 2021). Magri et al. (2022) found evidence that heterozygous STUB1 variants alone do not cause disease but require a concurrent expanded repeat allele of the TBP gene (600075) for disease manifestation; see MOLECULAR GENETICS.

Autosomal recessive spinocerebellar ataxia-27 (SCAR27) is an adult-onset neurologic disorder characterized by gait difficulties and other cerebellar signs, such as eye movement abnormalities, dysarthria, and difficulty writing. The disorder is progressive, and some patients may lose independent ambulation. Additional features include spasticity of the lower limbs and cognitive impairment. Brain imaging shows cerebellar atrophy (summary by Eidhof et al., 2018).

Multiple types of congenital heart defects-7 (CHTD7) is an autosomal dominant disorder with incomplete penetrance characterized mainly by tetralogy of Fallot but also including right-sided aortic arch, absent pulmonary valve, and other cardiac abnormalities (Jin et al., 2017, Reuter et al., 2019).

Beck-Fahrner syndrome (BEFAHRS) is a developmental disorder characterized by global developmental delay with variably impaired intellectual development. Affected individuals often have behavioral abnormalities, such as autistic features or attention deficit-hyperactivity disorder (ADHD), as well as learning disabilities. Most patients have hypotonia and dysmorphic facies. Some may have growth abnormalities, including overgrowth or poor growth, poor feeding, and rarely, seizures. Although both monoallelic and biallelic mutations have been reported, some heterozygous carriers in autosomal recessive families may have milder symptoms; thus, both groups are included in this entry (summary by Beck et al., 2020).

Lissencephaly-10 (LIS10) is a neurologic disorder characterized by variably delayed development with mildly to moderately impaired intellectual development and language delay, as well as seizures, which are often intractable. There is a spectrum of severity, with some patients having normal early development and only borderline to mild cognitive impairment. Brain imaging shows features consistent with neuronal migration defects, including posterior-predominant lissencephaly, pachygyria, agyria, and subcortical band heterotopia (summary by Tsai et al., 2020). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

Microcephaly, developmental delay, and brittle hair syndrome (MDBH) is a multisystem disorder with clinical variability. Affected individuals show cognitive and motor disabilities, as well as some degree of fine, brittle hair with microscopic shaft abnormalities. Other shared features include failure to thrive in early childhood and short stature, with some patients exhibiting feeding difficulties and hepatic steatosis (Kuo et al., 2019).

Frontotemporal dementia and/or amyotrophic lateral sclerosis-6 (FTDALS6) is an autosomal dominant neurodegenerative disorder with highly variable manifestations. Some patients present in adulthood with progressive FTD, often classified as the 'behavioral variant,' which is characterized by reduced empathy, impulsive behavior, personality changes, and reduced verbal output. Other patients present with features of amyotrophic lateral sclerosis (ALS), which is a fatal neurodegenerative disease characterized by upper and lower motor neuron dysfunction resulting in rapidly progressive paralysis and death from respiratory failure. The pathologic hallmarks of this disease include pallor of the corticospinal tract due to loss of motor neurons (in ALS). In both ALS and FTD, there are ubiquitin-positive inclusions within surviving neurons as well as deposition of pathologic TDP43 (TARDBP; 605078) or p62 (SQSTM1; 601530) aggregates. Patients with a D395G mutation (601023.0014) have been shown to develop pathologic tau (MAPT; 157140) aggregates. Some patients with the disorder may have features of both diseases, and there is significant interfamilial and intrafamilial phenotypic variability (summary by Johnson et al., 2010; Wong et al., 2018; Al-Obeidi et al., 2018; Darwich et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of FTDALS, see FTDALS1 (105550).

Progressive myoclonic epilepsy-12 (EPM12) is an autosomal recessive neurologic disorder characterized by onset of tonic-clonic seizures and/or myoclonus in the second decade of life. Affected individuals develop cerebellar ataxia associated with progressive cerebral and cerebellar atrophy on brain imaging. Most patients lose ambulation and become wheelchair-bound. Additional more variable features include mild cognitive dysfunction or psychiatric manifestations, such as depression or anxiety (summary by Mazzola et al., 2021). For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

Neurodegeneration with ataxia and late-onset optic atrophy (NDAXOA) is an autosomal dominant disorder with somewhat variable manifestations. Most affected individuals present in mid-adulthood with slowly progressive cerebellar and gait ataxia, optic atrophy, and myopathy or myalgia. Some patients may have a childhood history of neurologic features, including limited extraocular movements. Additional features can include cardiomyopathy, psychiatric disturbances, and peripheral sensory impairment (summary by Taylor et al., 1996 and Courage et al., 2017).

Parkinsonism with polyneuropathy (PKNPY) is an autosomal dominant disorder characterized by asymmetrical tremor-dependent parkinsonism. The age of onset ranges from the late forties to mid-sixties, and patients have a good response to levodopa (summary by Lin et al., 2020).

Mitochondrial DNA depletion syndrome-16B (MTDPS16B) is an autosomal recessive childhood-onset and progressive neuroophthalmic mtDNA depletion disorder characterized by optic atrophy, mixed polyneuropathy, spinal and cerebellar ataxia, and generalized chorea (Dosekova et al., 2020).

Neuronal ceroid lipofuscinosis-6B (CLN6B) is an autosomal recessive form of 'Kufs disease,' which refers in general to adult-onset neuronal ceroid lipofuscinosis without retinal involvement. CLN6B is a neurodegenerative disorder with a mean onset of symptoms at around age 28 years, although onset in the teens and later adulthood may also occur. Patients typically present with progressive myoclonus epilepsy, ataxia, loss of motor function, dysarthria, progressive dementia, and progressive cerebral and cerebellar atrophy on brain imaging. Ultrastructural examination typically shows fingerprint profiles and granular osmiophilic deposits in some tissues, including brain samples (summary by Arsov et al., 2011 and Berkovic et al., 2019). However, pathologic findings in peripheral tissues in adults is not as accurate for diagnosis as it is in children with the disease (Cherian et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

CSF1R-related adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is characterized by executive dysfunction, memory decline, personality changes, motor impairments, and seizures. A frontal lobe syndrome (e.g., loss of judgment, lack of social inhibitors, lack of insight, and motor persistence) usually appears early in the disease course. The mean age of onset is usually in the fourth decade. Affected individuals eventually become bedridden with spasticity and rigidity. The disease course ranges from two to 30 or more years (mean: 8 years).

Autosomal dominant Usmani-Riazzudin syndrome (USRISD) is a neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development and speech delay, hypotonia, and behavioral abnormalities, most commonly aggressive behavior. More variable additional features may include seizures and distal limb anomalies (summary by Usmani et al., 2021).

Developmental delay, impaired speech, and behavioral abnormalities (DDISBA) is characterized by global developmental delay apparent from early childhood. Intellectual disability can range from mild to severe. Additional variable features may include dysmorphic facial features, seizures, hypotonia, motor abnormalities such as Tourette syndrome or dystonia, and hearing loss (summary by Cousin et al., 2021).

Parkinson disease-24 (PARK24) is an autosomal dominant disorder characterized by classic Parkinson disease features, including adult onset, asymmetric limb involvement initially, and slowly progressive motor dysfunction. PARK24 shows incomplete penetrance, consistent with the presence of the PSAP mutation being a susceptibility factor for development of the disease (Oji et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of Parkinson disease, see 168600.

Dystonia-31 (DYT31) is an autosomal recessive progressive neurologic disorder characterized by involuntary muscle twisting movements and postural abnormalities affecting the upper and lower limbs, neck, face, and trunk. Some patients may have orofacial dyskinesia resulting in articulation and swallowing difficulties. The age at onset ranges from childhood to young adulthood. There are usually no additional neurologic symptoms, although late-onset parkinsonism was reported in 1 family (summary by Zech et al., 2022).

Childhood-onset parkinsonism-dystonia-3 (PKDYS3) is an autosomal recessive neurodegenerative disorder with onset in infancy or early childhood. Affected individuals present with progressive movement abnormalities, including parkinsonism with tremor, dystonia, myoclonus ataxia, and hyperkinetic movements such as ballismus. The parkinsonism features may be responsive to treatment with levodopa, although many patients develop levodopa-induced dyskinesia. Some patients may have mild cognitive impairment or psychiatric disturbances (summary by Burke et al., 2018 and Skorvanek et al., 2022). For a discussion of genetic heterogeneity of PKDYS, see 613135.

Neurodevelopmental disorder with speech impairment and with or without seizures (NEDSIS) is a phenotypically heterogeneous neurologic disorder whose severity appears to depend on the functional effect of the CACNA1I mutation. Severely affected individuals present in infancy with profound global developmental delay, hypotonia, delayed or absent walking, absent speech, feeding difficulties, cortical visual impairment, and onset of hyperexcitability and seizures in the first months or years of life. They achieve little or no developmental progress and may be tube-fed. Mutations in these individuals occurred de novo. In contrast, a milder phenotype associated with an inherited mutation has been found in a family with mild to moderate cognitive impairment and mild speech delay, usually without seizures (El Ghaleb et al., 2021).

Neurodevelopmental disorder with poor growth and behavioral abnormalities (NEDGBA) is an autosomal recessive disorder characterized by global developmental delay, moderately to severely impaired intellectual development, often with absent speech, and behavioral abnormalities, including hyperactivity, short attention span, and ADHD. Affected individuals show failure to thrive with poor overall growth; some have microcephaly. Additional features may include nonspecific facial dysmorphism, hypotonia, and feeding difficulties (Vogt et al., 2022; Meng et al., 2023).

Megalencephalic leukoencephalopathy with subcortical cysts-3 (MLC3) is a neurodegenerative disorder characterized by increased head circumference in infancy followed by progressive motor and cognitive decline in early childhood. Affected individuals either do not achieve walking or lose independent ambulation in the first or second decades. Cognitive impairment is variable and accompanied by poor speech and dysarthria. Most patients have early-onset seizures, which may be mild or refractory. Brain imaging shows unremitting megalencephalic leukoencephalopathy with subcortical cysts and swelling of the cerebral white matter (Passchier et al., 2023). For a discussion of genetic heterogeneity of megalencephalic leukoencephalopathy with subcortical cysts, see MLC1 (604004).