Episodic ataxia type 1 (EA1) is a potassium channelopathy characterized by constant myokymia and dramatic episodes of spastic contractions of the skeletal muscles of the head, arms, and legs with loss of both motor coordination and balance. During attacks individuals may experience a number of variable symptoms including vertigo, blurred vision, diplopia, nausea, headache, diaphoresis, clumsiness, stiffening of the body, dysarthric speech, and difficulty in breathing, among others. EA1 may be associated with epilepsy. Other possible associations include delayed motor development, cognitive disability, choreoathetosis, and carpal spasm. Usually, onset is in childhood or early adolescence.

Brody disease (BROD) is an autosomal recessive skeletal muscle disorder characterized by exercise-induced muscle stiffness and cramps primarily affecting the arms, legs, and eyelids, although more generalized muscle involvement may also occur. Symptom onset is most often in the first decade, but many patients present and are diagnosed later in life. Skeletal muscle biopsy typically shows variation in fiber size, increased internal nuclei, and atrophy of type II muscle fibers. Rare patients have been reported to develop malignant hyperthermia after administration of anesthesia, suggesting that patients with the disorder should be tested. The disorder results from defective relaxation of fast-twitch (type II) skeletal muscle fibers due to defects in calcium homeostasis and reuptake in the muscle fiber (summary by Odermatt et al., 2000 and Molenaar et al., 2020).

A very rare form of hereditary episodic ataxia with characteristics of vestibular ataxia, vertigo, tinnitus and interictal myokymia.

KCNQ2-related disorders represent a continuum of overlapping neonatal epileptic phenotypes ranging from self-limited familial neonatal epilepsy (SLFNE) at the mild end to neonatal-onset developmental and epileptic encephalopathy (NEO-DEE) at the severe end. Additional, less common phenotypes consisting of neonatal encephalopathy with non-epileptic myoclonus, infantile or childhood-onset developmental and epileptic encephalopathy (DEE), and isolated intellectual disability (ID) without epilepsy have also been described. KCNQ2-SLFNE is characterized by seizures that start in otherwise healthy infants between two and eight days after term birth and spontaneously disappear between the first and the sixth to 12th month of life. There is always a seizure-free interval between birth and the onset of seizures. Seizures are characterized by sudden onset with prominent motor involvement, often accompanied by apnea and cyanosis; video EEG identifies seizures as focal onset with tonic stiffening of limb(s) and some migration during each seizure's evolution. About 30% of individuals with KCNQ2-SLFNE develop epileptic seizures later in life. KCNQ2-NEO-DEE is characterized by multiple daily seizures beginning in the first week of life that are mostly tonic, with associated focal motor and autonomic features. Seizures generally cease between ages nine months and four years. At onset, EEG shows a burst-suppression pattern or multifocal epileptiform activity; early brain MRI can show basal ganglia hyperdensities and later MRIs may show white matter or general volume loss. Moderate-to-profound developmental impairment is present.

Spastic paraplegia-79B (SPG79B) is an autosomal recessive progressive neurologic disorder characterized by onset of spastic paraplegia and optic atrophy in the first decade of life. Additional features are variable, but may include peripheral neuropathy, cerebellar ataxia, and cognitive impairment (summary by Rydning et al., 2017). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

A rare hereditary ataxia characterized by recurrent episodes of ataxia with variable frequency and duration, associated with slurred speech, generalized muscle weakness and balance disturbance. Other symptoms may occur between episodes, including intention tremor, gait ataxia, mild dysarthria, myokymia, migraine and nystagmus.

Familial paroxysmal nonkinesigenic dyskinesia (PNKD) is characterized by unilateral or bilateral involuntary movements. Attacks are typically precipitated by coffee, tea, or alcohol; they can also be triggered by excitement, stress, or fatigue, or can be spontaneous. Attacks involve dystonic posturing with choreic and ballistic movements, may be accompanied by a preceding aura, occur while the individual is awake, and are not associated with seizures. Attacks last minutes to hours and rarely occur more than once per day. Attack frequency, duration, severity, and combinations of symptoms vary within and among families. Age of onset is typically in childhood or early teens but can be as late as age 50 years.

Progressive spastic tetraplegia and axial hypotonia (STAHP) is an autosomal recessive neurologic disorder characterized by onset of severe and progressive motor dysfunction in the first year of life. Affected individuals have severe axial hypotonia combined with spastic tetraplegia, hyperekplexia, hypertonia, and myokymia, reflecting upper motor neuron involvement. Cognitive development may be affected, but only 2 unrelated patients have been reported (Andersen et al., 2019; Park et al., 2019).

Charcot-Marie-Tooth disease type 2Z (CMT2Z) is an autosomal dominant axonal peripheral neuropathy characterized by onset, usually in the first decade, of distal lower limb muscle weakness and sensory impairment. The disorder is progressive, and affected individuals tend to develop upper limb and proximal muscle involvement in an asymmetric pattern, resulting in severe disability late in adulthood. Rare occurrence of global developmental delay with impaired intellectual development or learning difficulties has been observed. In some instances, the same mutation may result in different phenotypic manifestations (CMT2Z or DIGFAN), which highlights the clinical spectrum associated with MORC2 mutations and may render the classification of patients into one or the other disorder challenging (summary by Sevilla et al., 2016, Ando et al., 2017, Guillen Sacoto et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

NMAN is an autosomal recessive neurologic disorder characterized by onset in the first or second decade of a peripheral axonal neuropathy predominantly affecting motor more than sensory nerves. The axonal neuropathy is reminiscent of Charcot-Marie-Tooth disease type 2 (see, e.g., CMT2A1, 118210) and distal hereditary motor neuropathy (see, e.g., HMND1, 182960). Individuals with NMAN also have delayed muscle relaxation and action myotonia associated with neuromyotonic discharges on needle EMG resulting from hyperexcitability of the peripheral nerves (summary by Zimon et al., 2012).

Intellectual developmental disorder with muscle tone abnormalities and distal skeletal defects (IDDMDS) is an autosomal recessive disorder characterized by global developmental delay apparent in infancy manifest as speech delay and late walking by a few years. Affected individuals have hypertonia or, more rarely, hypotonia; a notable common feature is facial myokymia with corresponding EMG findings. Additional features include distal skeletal defects such as joint contractures, hypo- or areflexia, and hernia (Marafi et al., 2022).