Episodic Ataxias

Episodic ataxia type 1 (EA1) is a rare disorder caused by mutations in the voltage-gated potassium channel Kv1.1. Affected individuals have brief episodes of cerebellar ataxia lasting seconds or minutes (120). Interictal myokymia, detected clinically or by demonstration of continuous motor unit activity on EMG, is the principal diagnostic feature. As well as this paroxysmal ataxia being confused with a partial epileptic seizure, there exists a real over-representation of epilepsy in families with EA1 (103,121). The potassium channel is expressed throughout the central and peripheral nervous system. Whether the phenotype comprises ataxia, myokymia (or neuromyotonia), or epilepsy or a combination of these seems to relate to the functional consequences of the mutation and its tissue-specific developmental expression (121).

Episodic ataxia type 2 (EA2) is less frequently mistaken for epilepsy because the attacks are longer (minutes to hours), and there may be interictal cerebellar signs including eye movement control impairments. This disorder is associated with mutations in the voltage-gated calcium channel gene CACNA1A located on chromosome 19 (122). It is allelic with familial hemiplegic migraine and spinocerebellar ataxia type 6. In their pure forms, these are distinct disorders but overlap syndromes do occur. Partial seizures have been documented in familial hemi-plegic migraine families, and there is a case report of a child with a de novo truncating mutation in CACNA1A who has EA2 and absence epilepsy (123,124).

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