There are several forms of autosomal dominant

Ataxia in which ataxia occurs in intermittent


1  In episodic ataxia with myokymia (EAM/EA type 1), the attacks start in early childhood and last seconds to minutes with myokymia (rippling of muscles, especially facial) evident between attacks. Persistent ataxia does not develop. Mutation analysis of the voltage gated potassium channel gene, KCNA1, on chromosome 12p has identified different mis-sense point mutations. The attacks may respond to acetazolamide or carbamazepine.

2  In episodic ataxia with nystagmus (EAN/EA type 2), the attacks develop in later childhood or adolescence, last hours or days and may be accompanied by headache, nausea and vertigo; they are relieved by acetazolamide. There is no myokymia. In between the episodes, there may be nystagmus and mild gait ataxia; some patients do not experience any acute attacks. Cerebellar atrophy is seen on brain MRI.

Similar neurological signs are seen in some patients with autosomal dominant familial hemiplegic migraine. Familial hemiplegic migraine and periodic ataxia without myokymia are allelic disorders on chromosome 19p13 caused by point mutations of the alpha 1A calcium channel gene (CACNL1A4). Trinucleotide CAG expansions within the same gene cause a mild autosomal dominant adult-onset cerebellar ataxia (SCA6, see below under Autosomal dominant cerebellar ataxia type III). Other point mutations of CACNL1A4 cause

Familial hemiplegic migraine which has similar symptoms to EA2.

Episodic ataxia type 3 is rare; there is episodic acetazolamide-responsive ataxia, vertigo and tinnitus; myokymia can occur. There is linkage to a gene on chromosome 1q42.

Episodic ataxia type 4 (periodic vestibulocerebellar ataxia) can also be associated with a mild persistent ataxia; it is not linked to episodic ataxia type 1, 2 or 3. There are attacks of ataxia, oscillopsia, diplopia and vertigo. A persisting ataxia may develop. Episodic ataxia type 5 has been seen in one family. It was caused by a mutation of the alpha 4 subunit of the calcium channel gene CACNB4 on chromosome 2q22-23. The same mutation rarely causes epilepsy.

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