MedGen

If untreated, young children with profound biotinidase deficiency usually exhibit neurologic abnormalities including seizures, hypotonia, ataxia, developmental delay, vision problems, hearing loss, and cutaneous abnormalities (e.g., alopecia, skin rash, candidiasis). Older children and adolescents with profound biotinidase deficiency often exhibit motor limb weakness, spastic paresis, and decreased visual acuity. Once vision problems, hearing loss, and developmental delay occur, they are usually irreversible, even with biotin therapy. Individuals with partial biotinidase deficiency may have hypotonia, skin rash, and hair loss, particularly during times of stress. [from GeneReviews]

The dystrophinopathies cover a spectrum of X-linked muscle disease ranging from mild to severe that includes Duchenne muscular dystrophy, Becker muscular dystrophy, and DMD-associated dilated cardiomyopathy (DCM). The mild end of the spectrum includes the phenotypes of asymptomatic increase in serum concentration of creatine phosphokinase (CK) and muscle cramps with myoglobinuria. The severe end of the spectrum includes progressive muscle diseases that are classified as Duchenne/Becker muscular dystrophy when skeletal muscle is primarily affected and as DMD-associated DCM when the heart is primarily affected. Duchenne muscular dystrophy (DMD) usually presents in early childhood with delayed motor milestones including delays in walking independently and standing up from a supine position. Proximal weakness causes a waddling gait and difficulty climbing stairs, running, jumping, and standing up from a squatting position. DMD is rapidly progressive, with affected children being wheelchair dependent by age 12 years. Cardiomyopathy occurs in almost all individuals with DMD after age 18 years. Few survive beyond the third decade, with respiratory complications and progressive cardiomyopathy being common causes of death. Becker muscular dystrophy (BMD) is characterized by later-onset skeletal muscle weakness. With improved diagnostic techniques, it has been recognized that the mild end of the spectrum includes men with onset of symptoms after age 30 years who remain ambulatory even into their 60s. Despite the milder skeletal muscle involvement, heart failure from DCM is a common cause of morbidity and the most common cause of death in BMD. Mean age of death is in the mid-40s. DMD-associated DCM is characterized by left ventricular dilation and congestive heart failure. Females heterozygous for a DMD pathogenic variant are at increased risk for DCM. [from GeneReviews]

Krabbe disease comprises a spectrum ranging from infantile-onset disease (i.e., onset of extreme irritability, spasticity, and developmental delay before age 12 months) to later-onset disease (i.e., onset of manifestations after age 12 months and as late as the seventh decade). Although historically 85%-90% of symptomatic individuals with Krabbe disease diagnosed by enzyme activity alone have infantile-onset Krabbe disease and 10%-15% have later-onset Krabbe disease, the experience with newborn screening (NBS) suggests that the proportion of individuals with possible later-onset Krabbe disease is higher than previously thought. Infantile-onset Krabbe disease is characterized by normal development in the first few months followed by rapid severe neurologic deterioration; the average age of death is 24 months (range 8 months to 9 years). Later-onset Krabbe disease is much more variable in its presentation and disease course. [from GeneReviews]

Multiple acyl-CoA dehydrogenase deficiency (MADD) represents a clinical spectrum in which presentations can be divided into type I (neonatal onset with congenital anomalies), type II (neonatal onset without congenital anomalies), and type III (late onset). Individuals with type I or II MADD typically become symptomatic in the neonatal period with severe metabolic acidosis, which may be accompanied by profound hypoglycemia and hyperammonemia. Many affected individuals die in the newborn period despite metabolic treatment. In those who survive the neonatal period, recurrent metabolic decompensation resembling Reye syndrome and the development of hypertrophic cardiomyopathy can occur. Congenital anomalies may include dysmorphic facial features, large cystic kidneys, hypospadias and chordee in males, and neuronal migration defects (heterotopias) on brain MRI. Individuals with type III MADD, the most common presentation, can present from infancy to adulthood. The most common symptoms are muscle weakness, exercise intolerance, and/or muscle pain, although metabolic decompensation with episodes of rhabdomyolysis can also be seen. Rarely, individuals with late-onset MADD (type III) may develop severe sensory neuropathy in addition to proximal myopathy. [from GeneReviews]

Epimerase deficiency galactosemia (GALE deficiency galactosemia) is generally considered a continuum comprising several forms: Generalized. Enzyme activity is profoundly decreased in all tissues tested. Peripheral. Enzyme activity is deficient in red blood cells (RBC) and circulating white blood cells, but normal or near normal in all other tissues. Intermediate. Enzyme activity is deficient in red blood cells and circulating white blood cells and less than 50% of normal levels in other cells tested. Infants with generalized epimerase deficiency galactosemia develop clinical findings on a regular milk diet (which contains lactose, a disaccharide of galactose and glucose); manifestations include hypotonia, poor feeding, vomiting, weight loss, jaundice, hepatomegaly, liver dysfunction, aminoaciduria, and cataracts. Prompt removal of galactose/lactose from their diet resolves or prevents these acute symptoms. Longer-term features that may be seen in those with generalized epimerase deficiency include short stature, developmental delay, sensorineural hearing loss, and skeletal anomalies. In contrast, neonates with the peripheral or intermediate form generally remain clinically well even on a regular milk diet and are usually only identified by biochemical testing, often in newborn screening programs. [from GeneReviews]

Pompe disease is classified by age of onset, organ involvement, severity, and rate of progression. Infantile-onset Pompe disease (IOPD; individuals with onset before age 12 months with cardiomyopathy) may be apparent in utero but more typically onset is at the median age of four months with hypotonia, generalized muscle weakness, feeding difficulties, failure to thrive, respiratory distress, and hypertrophic cardiomyopathy. Without treatment by enzyme replacement therapy (ERT), IOPD commonly results in death by age two years from progressive left ventricular outflow obstruction and respiratory insufficiency. Late-onset Pompe disease (LOPD; including: (a) individuals with onset before age 12 months without cardiomyopathy; and (b) all individuals with onset after age 12 months) is characterized by proximal muscle weakness and respiratory insufficiency; clinically significant cardiac involvement is uncommon. [from GeneReviews]

HEXA disorders are best considered as a disease continuum based on the amount of residual beta-hexosaminidase A (HEX A) enzyme activity. This, in turn, depends on the molecular characteristics and biological impact of the HEXA pathogenic variants. HEX A is necessary for degradation of GM2 ganglioside; without well-functioning enzymes, GM2 ganglioside builds up in the lysosomes of brain and nerve cells. The classic clinical phenotype is known as Tay-Sachs disease (TSD), characterized by progressive weakness, loss of motor skills beginning between ages three and six months, decreased visual attentiveness, and increased or exaggerated startle response with a cherry-red spot observable on the retina followed by developmental plateau and loss of skills after eight to ten months. Seizures are common by 12 months with further deterioration in the second year of life and death occurring between ages two and three years with some survival to five to seven years. Subacute juvenile TSD is associated with normal developmental milestones until age two years, when the emergence of abnormal gait or dysarthria is noted followed by loss of previously acquired skills and cognitive decline. Spasticity, dysphagia, and seizures are present by the end of the first decade of life, with death within the second decade of life, usually by aspiration. Late-onset TSD presents in older teens or young adults with a slowly progressive spectrum of neurologic symptoms including lower-extremity weakness with muscle atrophy, dysarthria, incoordination, tremor, mild spasticity and/or dystonia, and psychiatric manifestations including acute psychosis. Clinical variability even among affected members of the same family is observed in both the subacute juvenile and the late-onset TSD phenotypes. [from GeneReviews]

Familial dysautonomia, which affects the development and survival of sensory, sympathetic, and parasympathetic neurons, is a debilitating disorder present from birth. Neuronal degeneration progresses throughout life. Affected individuals have gastrointestinal dysfunction, autonomic crises (i.e., hypertensive vomiting attacks), recurrent pneumonia, altered pain sensitivity, altered temperature perception, and blood pressure instability. Hypotonia contributes to delay in acquisition of motor milestones. Optic neuropathy results in progressive vision loss. Older individuals often have a broad-based and ataxic gait that deteriorates over time. Developmental delay / intellectual disability occur in about 21% of individuals. Life expectancy is decreased. [from GeneReviews]

The phenotype of acid sphingomyelinase deficiency (ASMD) occurs along a continuum. Individuals with the severe early-onset form, infantile neurovisceral ASMD, were historically diagnosed with Niemann-Pick disease type A (NPD-A). The later-onset, chronic visceral form of ASMD is also referred to as Niemann-Pick disease type B (NPD-B). A phenotype with intermediate severity is also known as chronic neurovisceral ASMD (NPD-A/B). The most common presenting symptom in NPD-A is hepatosplenomegaly, usually detectable by age three months; over time the liver and spleen become massive in size. Psychomotor development progresses no further than the 12-month level, after which neurologic deterioration is relentless. Failure to thrive typically becomes evident by the second year of life. A classic cherry-red spot of the macula of the retina, which may not be present in the first few months, is eventually present in all affected children. Interstitial lung disease caused by storage of sphingomyelin in pulmonary macrophages results in frequent respiratory infections and often respiratory failure. Most children succumb before the third year of life. NPD-B generally presents later than NPD-A, and the manifestations are less severe. NPD-B is characterized by progressive hepatosplenomegaly, gradual deterioration in liver and pulmonary function, osteopenia, and atherogenic lipid profile. No central nervous system (CNS) manifestations occur. Individuals with NPD-A/B have symptoms that are intermediate between NPD-A and NPD-B. The presentation in individuals with NPD-A/B varies greatly, although all are characterized by the presence of some CNS manifestations. Survival to adulthood can occur in individuals with NPD-B and NPD-A/B. [from GeneReviews]

PMM2-CDG, the most common of a group of disorders of abnormal glycosylation of N-linked oligosaccharides, is divided into three clinical stages: infantile multisystem, late-infantile and childhood ataxia–intellectual disability, and adult stable disability. The clinical manifestations and course are highly variable, ranging from infants who die in the first year of life to mildly affected adults. Clinical findings tend to be similar in sibs. In the infantile multisystem presentation, infants show axial hypotonia, hyporeflexia, esotropia, and developmental delay. Feeding problems, vomiting, faltering growth, and developmental delay are frequently seen. Subcutaneous fat may be excessive over the buttocks and suprapubic region. Two distinct clinical courses are observed: (1) a nonfatal neurologic course with faltering growth, strabismus, developmental delay, cerebellar hypoplasia, and hepatopathy in infancy followed by neuropathy and retinitis pigmentosa in the first or second decade; and (2) a more severe neurologic-multivisceral course with approximately 20% mortality in the first year of life. The late-infantile and childhood ataxia–intellectual disability stage, which begins between ages three and ten years, is characterized by hypotonia, ataxia, severely delayed language and motor development, inability to walk, and IQ of 40 to 70; other findings include seizures, stroke-like episodes or transient unilateral loss of function, coagulopathy, retinitis pigmentosa, joint contractures, and skeletal deformities. In the adult stable disability stage, intellectual ability is stable; peripheral neuropathy is variable, progressive retinitis pigmentosa and myopia are seen, thoracic and spinal deformities with osteoporosis worsen, and premature aging is observed; females may lack secondary sexual development and males may exhibit decreased testicular volume. Hypogonadotropic hypogonadism and coagulopathy may occur. The risk for deep venous thrombosis is increased. [from GeneReviews]

Systemic primary carnitine deficiency (CDSP) is a disorder of the carnitine cycle that results in defective fatty acid oxidation. It encompasses a broad clinical spectrum including the following: Metabolic decompensation in infancy typically presenting between age three months and two years with episodes of hypoketotic hypoglycemia, poor feeding, irritability, lethargy, hepatomegaly, elevated liver transaminases, and hyperammonemia triggered by fasting or common illnesses such as upper respiratory tract infection or gastroenteritis. Childhood myopathy involving heart and skeletal muscle with onset between age two and four years. Pregnancy-related decreased stamina or exacerbation of cardiac arrhythmia. Fatigability in adulthood. Absence of symptoms. The latter two categories often include mothers diagnosed with CDSP after newborn screening has identified low carnitine levels in their infants. [from GeneReviews]

Deficiency of very long-chain acyl-coenzyme A dehydrogenase (VLCAD), which catalyzes the initial step of mitochondrial beta-oxidation of long-chain fatty acids with a chain length of 14 to 20 carbons, is associated with three phenotypes. The severe early-onset cardiac and multiorgan failure form typically presents in the first months of life with hypertrophic or dilated cardiomyopathy, pericardial effusion, and arrhythmias, as well as hypotonia, hepatomegaly, and intermittent hypoglycemia. The hepatic or hypoketotic hypoglycemic form typically presents during early childhood with hypoketotic hypoglycemia and hepatomegaly, but without cardiomyopathy. The later-onset episodic myopathic form presents with intermittent rhabdomyolysis provoked by exercise, muscle cramps and/or pain, and/or exercise intolerance. Hypoglycemia typically is not present at the time of symptoms. [from GeneReviews]

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. [from GeneReviews]

Disorders of intracellular cobalamin metabolism have a variable phenotype and age of onset that are influenced by the severity and location within the pathway of the defect. The prototype and best understood phenotype is cblC; it is also the most common of these disorders. The age of initial presentation of cblC spans a wide range: In utero with fetal presentation of nonimmune hydrops, cardiomyopathy, and intrauterine growth restriction. Newborns, who can have microcephaly, poor feeding, and encephalopathy. Infants, who can have poor feeding and slow growth, neurologic abnormality, and, rarely, hemolytic uremic syndrome (HUS). Toddlers, who can have poor growth, progressive microcephaly, cytopenias (including megaloblastic anemia), global developmental delay, encephalopathy, and neurologic signs such as hypotonia and seizures. Adolescents and adults, who can have neuropsychiatric symptoms, progressive cognitive decline, thromboembolic complications, and/or subacute combined degeneration of the spinal cord. [from GeneReviews]

Alpha-mannosidosis encompasses a continuum of clinical findings from mild to severe. Three major clinical subtypes have been suggested: A mild form recognized after age ten years with absence of skeletal abnormalities, myopathy, and slow progression (type 1). A moderate form recognized before age ten years with presence of skeletal abnormalities, myopathy, and slow progression (type 2). A severe form manifested as prenatal loss or early death from progressive central nervous system involvement or infection (type 3). Individuals with a milder phenotype have mild-to-moderate intellectual disability, impaired hearing, characteristic coarse features, clinical or radiographic skeletal abnormalities, immunodeficiency, and primary central nervous system disease – mainly cerebellar involvement causing ataxia. Periods of psychiatric symptoms are common. Associated medical problems can include corneal opacities, hepatosplenomegaly, aseptic destructive arthritis, and metabolic myopathy. Alpha-mannosidosis is insidiously progressive; some individuals may live into the sixth decade. [from GeneReviews]

Leigh syndrome is a clinical diagnosis based primarily on characteristic brain imaging findings associated with progressive and severe neurodegenerative features with onset within the first months or years of life, sometimes resulting in early death. Affected individuals usually show global developmental delay or developmental regression, hypotonia, ataxia, dystonia, and ophthalmologic abnormalities, such as nystagmus or optic atrophy. The neurologic features are associated with the classic findings of T2-weighted hyperintensities in the basal ganglia and/or brainstem on brain imaging. Leigh syndrome can also have detrimental multisystemic affects on the cardiac, hepatic, gastrointestinal, and renal organs. Biochemical studies in patients with Leigh syndrome tend to show increased lactate and abnormalities of mitochondrial oxidative phosphorylation (summary by Lake et al., 2015). Genetic Heterogeneity of Nuclear Leigh Syndrome Leigh syndrome is a presentation of numerous genetic disorders resulting from defects in the mitochondrial OXPHOS complex. Accordingly, the genes implicated in Leigh syndrome most commonly encode structural subunits of the OXPHOS complex or proteins required for their assembly, stability, and activity. Mutations in both nuclear and mitochondrial genes have been identified. For a discussion of genetic heterogeneity of mitochondrial Leigh syndrome, see MILS (500017). Nuclear Leigh syndrome can be caused by mutations in nuclear-encoded genes involved in any of the mitochondrial respiratory chain complexes: complex I deficiency (see 252010), complex II deficiency (see 252011), complex III deficiency (see 124000), complex IV deficiency (cytochrome c oxidase; see 220110), and complex V deficiency (see 604273) (summary by Lake et al., 2015). Some forms of combined oxidative phosphorylation deficiency (COXPD) can present as Leigh syndrome (see, e.g., 617664). Leigh syndrome may also be caused by mutations in components of the pyruvate dehydrogenase complex (e.g., DLD, 238331 and PDHA1, 300502). Deficiency of coenzyme Q10 (607426) can present as Leigh syndrome. [from OMIM]

Down syndrome, the most frequent form of mental retardation caused by a microscopically demonstrable chromosomal aberration, is characterized by well-defined and distinctive phenotypic features and natural history. It is caused by triplicate state (trisomy) of all or a critical portion of chromosome 21. [from OMIM]

Medium-chain acyl-coenzyme A dehydrogenase (MCAD) is one of the enzymes involved in mitochondrial fatty acid ß-oxidation. Fatty acid ß-oxidation fuels hepatic ketogenesis, which provides a major source of energy once hepatic glycogen stores become depleted during prolonged fasting and periods of higher energy demands. MCAD deficiency is the most common disorder of fatty acid ß-oxidation and one of the most common inborn errors of metabolism. Most children are now diagnosed through newborn screening. Clinical symptoms in a previously apparently healthy child with MCAD deficiency include hypoketotic hypoglycemia and vomiting that may progress to lethargy, seizures, and coma triggered by a common illness. Hepatomegaly and liver disease are often present during an acute episode. Children appear normal at birth and – if not identified through newborn screening – typically present between age three and 24 months, although presentation even as late as adulthood is possible. The prognosis is excellent once the diagnosis is established and frequent feedings are instituted to avoid any prolonged periods of fasting. [from GeneReviews]

Hypophosphatasia is characterized by defective mineralization of growing or remodeling bone, with or without root-intact tooth loss, in the presence of low activity of serum and bone alkaline phosphatase. Clinical features range from stillbirth without mineralized bone at the severe end to pathologic fractures of the lower extremities in later adulthood at the mild end. While the disease spectrum is a continuum, seven clinical forms of hypophosphatasia are usually recognized based on age at diagnosis and severity of features: Perinatal (severe): characterized by pulmonary insufficiency and hypercalcemia. Perinatal (benign): prenatal skeletal manifestations that slowly resolve into one of the milder forms. Infantile: onset between birth and age six months of clinical features of rickets without elevated serum alkaline phosphatase activity. Severe childhood (juvenile): variable presenting features progressing to rickets. Mild childhood: low bone mineral density for age, increased risk of fracture, and premature loss of primary teeth with intact roots. Adult: characterized by stress fractures and pseudofractures of the lower extremities in middle age, sometimes associated with early loss of adult dentition. Odontohypophosphatasia: characterized by premature exfoliation of primary teeth and/or severe dental caries without skeletal manifestations. [from GeneReviews]

Arylsulfatase A deficiency (also known as metachromatic leukodystrophy or MLD) is characterized by three clinical subtypes: late-infantile MLD, juvenile MLD, and adult MLD. Age of onset within a family is usually similar. The disease course may be from several years in the late-infantile-onset form to decades in the juvenile- and adult-onset forms. Late-infantile MLD. Onset is before age 30 months. Typical presenting findings include weakness, hypotonia, clumsiness, frequent falls, toe walking, and dysarthria. As the disease progresses, language, cognitive, and gross and fine motor skills regress. Later signs include spasticity, pain, seizures, and compromised vision and hearing. In the final stages, children have tonic spasms, decerebrate posturing, and general unawareness of their surroundings. Juvenile MLD. Onset is between age 30 months and 16 years. Initial manifestations include decline in school performance and emergence of behavioral problems, followed by gait disturbances. Progression is similar to but slower than in the late-infantile form. Adult MLD. Onset occurs after age 16 years, sometimes not until the fourth or fifth decade. Initial signs can include problems in school or job performance, personality changes, emotional lability, or psychosis; in others, neurologic symptoms (weakness and loss of coordination progressing to spasticity and incontinence) or seizures initially predominate. Peripheral neuropathy is common. Disease course is variable – with periods of stability interspersed with periods of decline – and may extend over two to three decades. The final stage is similar to earlier-onset forms. [from GeneReviews]