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Ataxia-oculomotor apraxia type 1
Ataxia with oculomotor apraxia type 1 (AOA1) is a rare autosomal recessive cerebellar ataxia (ARCA; see this term), characterized by progressive cerebellar ataxia associated with oculomotor apraxia, severe neuropathy, and hypoalbuminemia.
AOA1 represents 3.6% of all ARCA in Portugal; in Japan, AOA1 seems to be the most frequent cause of ARCA. In a cohort of 227 patients mostly of French origin with progressive cerebellar ataxia selected after exclusion of Friedreich ataxia (see this term), the relative frequency of AOA1 was of 5%.
Cerebellar ataxia is the first manifestation of AOA1 with a mean age of onset of 4.3 years (2-10 years) and is characterized by progressive gait imbalance followed by dysarthria, and limb dysmetria. Later, peripheral axonal motor neuropathy dominates the clinical picture. Oculomotor apraxia (OMA; inability to coordinate eyes ± head movements: when the head turns toward a lateral target; the head reaches the target before the eyes) is present in almost all individuals with AOA1. Chorea is present at onset in 80% of patients and upper limb dystonia (see this term) occurs in about 50% of individuals. Additional features include square wave jerks, saccadic pursuit and gaze-evoked nystagmus, areflexia followed by severe peripheral neuropathy. Variable intellectual disability is observed.
AOA1 results from mutations in APTX gene (9p13.3) encoding aprataxin which plays a role in DNA-single-strand break repair. Most mutations identified so far are localized in exons 5, 6 and 7. Some correlations between genotype and phenotype have been established: for example severe and persistent choreic phenotype is associated with mutations A198V; truncating mutations are associated with earlier onset and deletions with more severe phenotype and intellectual disability.
Diagnosis of AOA1 is based on the clinical features, the progressive evolution, the absence of extraneurologic findings and family history. Electromyography findings reveal severe axonal sensory-motor neuropathy. Oculographic recordings demonstrate normal latencies, hypometric saccades, decrease mean gain in amplitude and broken saccades into multiple successive saccades. Cerebral magnetic resonance imagery displays cerebellar atrophy. Hypoalbuminemia and hypercholesterolemia are usual (disease duration is positively correlated with cholesterol and negatively correlated with albumin levels). Diagnosis is confirmed by molecular analysis of APTX gene.
Differential diagnosis includes Friedreich ataxia, ataxia with vitamin E deficiency, AOA2, ataxia-telangiectasia, ataxia-telangiectasia-like disorder, autosomal recessive spastic ataxia of Charlevoix-Saguenay (see these terms).
Carrier testing for at-risk family members and prenatal testing are possible if both disease-causing alleles in a family are known.
Transmission of AOA1 is autosomal recessive. Genetic counseling is recommended as each sib of an affected individual has 25% chance of being affected, 50% chance of being an asymptomatic carrier, and 25% chance of being neither affected nor a carrier.
Management and treatment
No specific treatment exists for AOA1 and management is mainly supportive. It includes physical therapy for cerebellar ataxia and disabilities resulting from peripheral neuropathy; educational support for reading and writing difficulties, speech therapy for dysarthria and cognitive impairment. Low-cholesterol diet and hypolipemiant treatment are recommended. Routine follow-up with a neurologist or neurogenetician is suggested. Some therapeutic trials are on the way such as the evaluation of efficacy of Coenzyme Q10 in evolution of the disease.
AOA1 is a progressive neurodegenerative disorder and most patients usually become wheelchair bound from seven to ten years after onset of the disease.