Episodic ataxia
| Episodic ataxia | |
|---|---|
| Classification and external resources | |
| OMIM | 160120 108500 606554 606552 |
| DiseasesDB | 32380 |
| eMedicine | neuro/556 |
Episodic ataxia (EA) is an autosomal dominant disorder characterized by sporadic bouts of ataxia (severe discoordination) with or without myokymia (continuous muscle movement). There are seven types recognised but the majority are due to two recognized entities. Ataxia can be provoked by stress, startle, or heavy exertion such as exercise. Symptoms can first appear in infancy. There are at least 6 loci for EA, of which 4 are known genes. Some patients with EA also have migraine or progressive cerebellar degenerative disorders, symptomatic of either familial hemiplegic migraine or spinocerebellar ataxia. Some patients respond to acetazolamide though others do not.
Typically, episodic ataxia presents as bouts of ataxia induced by startle, stress, or exertion. Some patients also have continuous tremors of various motor groups, known as myokymia. Other patients have nystagmus, vertigo, tinnitus, diplopia or seizures.
The various symptoms of EA are caused by dysfunction of differing areas. Ataxia, the most common symptom, is due to misfiring of Purkinje cells in the cerebellum. This is either due to direct malfunction of these cells, such as in EA2, or improper regulation of these cells, such as in EA1. Seizures are likely due to altered firing of hippocampal neurons (KCNA1 null mice have seizures for this reason).
Type 1 episodic ataxia (EA1) is characterized by attacks of generalized ataxia induced by emotion or stress, with myokymia both during and between attacks. This disorder is also known as episodic ataxia with myokymia (EAM), hereditary paroxysmal ataxia with neuromyotonia and Isaacs-Mertens syndrome. Onset of EA1 occurs during early childhood to adolescence and persists throughout the patient's life. Attacks last from seconds to minutes. Mutations of the gene KCNA1, which encodes the voltage-gated potassium channel KV1.1, are responsible for this subtype of episodic ataxia. KV1.1 is expressed heavily in basket cells and interneurons that form GABAergic synapses on Purkinje cells. The channels aid in the repolarization phase of action potentials, thus affecting inhibitory input into Purkinje cells and, thereby, all motor output from the cerebellum. EA1 is an example of a synaptopathy. There are currently 17 KV1.1 mutations associated with EA1, Table 1 and Figure 1. 15 of these mutations have been at least partly characterized in cell culture based electrophysiological assays wherein 14 of these 15 mutations have demonstrated drastic alterations in channel function. As described in Table 1, most of the known EA1 associated mutations result in a drastic decrease in the amount of current through KV1.1 channels. Furthermore, these channels tend to activate at more positive potentials and slower rates, demonstrated by positive shifts in their V½ values and slower τ activation time constants, respectively. Some of these mutations, moreover, produce channels that deactivate at faster rates (deactivation τ), which would also result in decreased current through these channels. While these biophysical changes in channel properties likely underlie some of the decrease in current observed in experiments, many mutations also seem to result in misfolded or otherwise mistrafficked channels, which is likely to be the major cause of dysfunction and disease pathogenesis. It is assumed, though not yet proven, that decrease in KV1.1 mediated current leads to prolonged action potentials in interneurons and basket cells. As these cells are important in the regulation of Purkinje cell activity, it is likely that this results increased and aberrant inhibitory input into Purkinje cells and, thus, disrupted Purkinje cell firing and cerebellum output.
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