Episodic Ataxia, Type 2

A number sign (#) is used with this entry because of evidence that episodic ataxia type 2 (EA2) is caused by heterozygous mutation in the calcium ion channel gene CACNA1A (601011) on chromosome 19p13.

Description

Episodic ataxia is a genetically heterogeneous neurologic condition characterized by spells of incoordination and imbalance, often associated with progressive ataxia. Episodic ataxia type 2 is the most common form of EA (Jen et al., 2007).

For a discussion of genetic heterogeneity of episodic ataxia, see EA1 (160120).

Clinical Features

Parker (1946) may have been the first to describe this disorder. Hill and Sherman (1968) described episodic cerebellar ataxia occurring particularly in children in a large kindred with an autosomal dominant pattern of inheritance. The symptoms ameliorated in later life with no permanent or progressive cerebellar abnormalities. The cases presented by White (1969) showed gradual abatement of symptoms.

Donat and Auger (1979) reported ataxia in a 16-year-old boy and his 41-year-old mother, both of whom had 'downbeating nystagmus' of the eyes when in the primary position of gaze. The attacks of dizziness, which began at the age of 9 in the boy, were relieved with acetazolamide. Koller and Bahamon-Dussan (1987) reported a family with affected individuals in 3 generations, including 1 instance of male-to-male transmission. Stress or emotion precipitated attacks. Examination between attacks showed nystagmus, but no other neurologic signs. After adolescence, there was no progression of symptoms. The authors found, as have others (e.g., Zasorin et al., 1983), that acetazolamide therapy successfully abolished the attacks. This disorder may have first been recognized by Parker (1946).

Vighetto et al. (1988) indicated that 15 kindreds had been reported. They were the first to report selective atrophy of the cerebellar vermis in all 3 members of 2 affected families that were studied by magnetic resonance imaging. In the family reported by Boel and Casaer (1988), all affected members had their first attacks before the age of 10 and the symptoms usually disappeared during the second decade of life. Ataxia was precipitated by stressful classroom situations or exciting football or tennis contests. Ataxia usually lasted 3 to 8 minutes with no loss of consciousness but was followed by a period of fatigue which often lasted for more than an hour. Bain et al. (1992) reported that in 6 affected members of 2 unrelated families with familial periodic cerebellar ataxia, symptoms were relieved with oral acetazolamide. When untreated, all subjects showed abnormal intracellular pH levels in the cerebellum by (31)P nuclear magnetic resonance (NMR) spectroscopy. These levels returned to normal with treatment. In 1 family studied, cerebral pH values were normal before and after treatment. In 3 additional patients with similar attacks, but without a family history, normal pH values were found in both cerebellum and cerebrum.

In the 2 families reported by von Brederlow et al. (1995), physical and emotional stress was the most consistent precipitating factor, although attacks were also triggered occasionally by carbohydrate-rich meals. Attacks lasted between one-half hour and 6 hours. Typical attacks were observed in children as young as age 2 to 5 years, although onset was more common in the second decade. Frequency of the episodes ranged from 3 to 4 times per week to 1 to 2 times per year. Symptoms were fully controlled with acetazolamide. Attacks recurred promptly within 48 to 72 hours upon cessation of medication.

Subramony et al. (2003) reported a family with autosomal dominant episodic ataxia type 2 confirmed by mutation in the CACNA1A gene. Nine of 11 affected patients reported that fever or heat triggered ataxic episodes, headaches, weakness, vertigo, or nausea and vomiting. Six patients showed interictal cerebellar deficits.

Spacey et al. (2005) reported 2 unrelated patients with EA2 confirmed by genetic analysis who developed dystonia late in the disease, at ages 59 and 47 years, respectively. The authors suggested that dystonia may be a manifestation of a degenerative cerebellar process.

Imbrici et al. (2005) reported an Indian man with late-onset EA2 beginning at age 61, followed by a progressive cerebellar syndrome at age 62. The patient's medical history included juvenile-onset complex partial seizures and depression but no headaches. Molecular analysis confirmed a mutation in the CACNA1A gene in the patient and in his asymptomatic son. Imbrici et al. (2005) emphasized the unusual clinical presentation.

Clinical Variability

Reinson et al. (2016) reported a family from Estonia with a highly unusual phenotype best characterized in some of the patients as a form of EA2. The proband was a 5-year-old boy who had severe epileptic encephalopathy with onset of seizures at 4 months of age. He had severe hypotonia with almost no spontaneous movements, no eye contact, and EEG abnormalities. Brain imaging showed several abnormalities, including a small corpus callosum, dilated ventricles, and diffuse hypomyelination; he also had optic atrophy. He was blind and bedridden with developmental delay, marked muscular atrophy, and rigidity. An older sister had died at age 5 with a similar phenotype. Both parents had mild intellectual disability, and the mother also had probable alcohol-induced cerebellar ataxia with cerebellar atrophy on brain imaging. She had intention tremor of the hands, poor balance, and wide-based gate with horizontal nystagmus. The father had rare headaches without hemiplegia. Two other sisters of the proband had mild intellectual disability: one also had migraine, exercise-induced vertigo attacks, and psychiatric problems, whereas the other also had generalized epileptic activity on EEG without overt seizures. Exome sequencing identified compound heterozygosity for a missense (W1439R) and a frameshift (Ala158ThrfsTer6) mutation in the CACNA1A gene in the severely affected sibs, whereas each mildly affected parent and the 2 mildly affected sisters were heterozygous for 1 of the mutations. Functional studies of the variants and studies of patient cells were not performed, but neither mutation was found in the ExAC database or in an in-house database of 339 individuals. Reinson et al. (2016) noted the unusual inheritance pattern and variable phenotype in this family.

Clinical Management

Most patients with EA2 show a favorable response to acetazolamide. Strupp et al. (2004) reported 3 patients with EA2, 2 of whom were confirmed by genetic analysis, who showed a favorable response to the potassium channel blocker 4-aminopyridine (4-AP). Attacks recurred after treatment was stopped, and subsequent treatment alleviated the symptoms. Strupp et al. (2004) postulated that 4-AP increased the release of the inhibitory transmitter gamma-aminobutyrate (GABA) in Purkinje cells by prolonging the duration of action potentials through the blockade of several potassium channels, including the delayed rectifier channel.

Strupp et al. (2011) evaluated the therapeutic efficacy of 4-AP in a randomized double-blind placebo-controlled crossover study of 10 individuals with episodic ataxia with nystagmus, 7 of whom had a proven pathogenic mutation in the CACNA1A gene. After 7 months of treatment, those who took 4-AP had a significant reduction in the frequency of episodes and a marginally significant decrease in the duration of attacks compared to those who took a placebo. Recipients of the drug also had decreased disease burden, as measured by a questionnaire. Minor adverse events were uncommon, and 7 of 10 continued to take 4-AP 2 years after the study with good response. All patients had a family history of the disorder.

Mapping

In a large family with this form of episodic ataxia, Litt et al. (1994) excluded linkage to 12p where the locus for the episodic ataxia/myokymia syndrome (EA1; 160120) had been mapped. In 2 large kindreds with paroxysmal ataxia, von Brederlow et al. (1995) found linkage to 19p. The microsatellite marker UT705 was found to be linked to the ataxia locus with a 2-point analysis yielding a maximum lod score of 8.20 at theta = 0.00 in a 5-generation pedigree. Linkage to this region was confirmed in the second kindred. They referred to the disorder as acetazolamide-responsive hereditary paroxysmal cerebellar ataxia (APCA). Vahedi et al. (1995) reported linkage in a large family with this type of episodic ataxia to a 30-cM region on 19p flanked by D19S216 and D19S215.

Kramer et al. (1994) suggested that there are 2 autosomal dominant forms of episodic ataxia. In EA1, attacks last minutes and interictal myokymia may be present. This form maps to chromosome 12 and by the candidate gene approach was shown to be due to mutations in a specific potassium voltage-gated channel gene (KCNA1; 176260). The second form, EA2, is often associated with nystagmus or truncal instability and shows beneficial response to acetazolamide. Kramer et al. (1994) and Kramer et al. (1995) demonstrated that the nystagmus-associated form was mapped to 19p. They studied 3 families in which detailed clinical descriptions had been given by Zasorin et al. (1983), Gancher and Nutt (1986), and Baloh and Winder (1991). The strongest evidence for linkage occurred at D19S221; total lod score = 5.07 at theta = 0.01 with no obligate crossovers in any of the 3 kindreds.

Molecular Genetics

In a family initially classified as autosomal dominant cerebellar ataxia of unknown type, Jodice et al. (1997) found that an intergenerational allele size change showed that a (CAG)20 allele (601011.0008) was associated with an EA2 phenotype and a (CAG)25 allele with progressive cerebellar ataxia. These results suggested that EA2 and SCA6 (183086) are the same disorder with a high phenotypic variability, at least partly related to the number of repeats, and suggested that the small expansions in the CACNA1A gene may not be as stable as previously reported.

Hemiplegic migraine type 1 (141500) has also been mapped to 19p13. Ophoff et al. (1996) found mutations in the calcium ion channel gene CACNL1A4 in both familial hemiplegic migraine (e.g., 601011.0001) and episodic ataxia type 2 (see 601011.0005 and 601011.0006). The CACNL1A4 gene had previously been mapped to 19p13.

Riant et al. (2008) identified a heterozygous 39.5-kb deletion in the CACNA1A gene (601011.0026) in 3 affected members of a family with autosomal dominant EA2 in whom initial screening did not identify a point mutation.

For a comprehensive review of episodic ataxia type 2 and its causative mutations, see Brandt and Strupp (1997).

Jen et al. (2007) provided a detailed review of the pathophysiology and molecular genetics of known episodic ataxia syndromes.

Riant et al. (2010) identified 4 different exonic deletions in the CACNA1A gene in 4 (14%) of 27 patients with episodic ataxia, in whom sequencing analysis was negative for CACNA1A point mutations. The EA2 phenotype in patients with deletions was similar to that of patients with point mutations. The findings indicated that screening for deletions in the CACNA1A gene should also be done for a complete genetic workup.

Genotype/Phenotype Correlations

Jen et al. (2004) identified a total of 13 mutations in the CACNA1A gene in 9 of 11 families with EA2 showing linkage to chromosome 19p, and in 4 of 9 sporadic cases of EA2. Mutations were identified in a total of 46 affected and 8 unaffected family members, demonstrating incomplete penetrance. Almost all of the mutation carriers reported episodic ataxia, except 2 related patients who reported only progressive ataxia. Onset was before age 20 years in all cases except 1 with reported ataxia at age 30. Migraine headaches occurred in 24 of 40 mutation carriers, and 15 of 21 treated patients reported good response to acetazolamide. Four patients had episodes of hemiplegia. The mutations were scattered throughout the gene, and there were no obvious genotype/phenotype correlations. Jen et al. (2004) noted the phenotypic overlap between disorders caused by mutations in the CACNA1A gene.