Olivopontocerebellar atrophy: A group of genetic diseases in which there is progressive degeneration of the olivopontocerebellar pathway in the brain connecting the inferior olive, pons, and cerebellum. The cerebellum is the part of the brain that plays a role in maintaining balance and posture as well as coordinating voluntary movement. The pons is part of the brainstem and contains important neuronal pathways between the cerebrum, spinal cord, and cerebellum. The pons serves as a relay point for messages between these structures. The inferior olives are two round structures that contain nuclei that are involved with balance, coordination and motor activity.
Olivopontocerebellar atrophy (OPCA) usually has its clinical onset in youth or the middle years. It may be confused with Parkinson's disease. The features characteristic of OPCA are progressive disequilibrium (increasing problems with balance problems), progressive cerebellar ataxia (increasing impairment of the capacity to coordinate voluntary movements), and progressive dysarthria (increasing difficulty in speaking clearly).
There are at least five forms of OPCA. All are inherited in an autosomal manner. Both autosomal dominant and autosomal recessive types are known. There is considerable complexity and some confusion about the names for the different forms of OPCA, which is also known as spinocerebellar ataxia or SPA:
- OPCA I (or SPA 1) -- Autosomal dominant. Onset of symptoms usually in the third or fourth decade of life, most often around age 30. Due to expansion of a CAG trinucleotide sequence in the ataxin-1 gene (ATX1) on chromosome 6p23.
- OPCA II -- Autosomal recessive. Called the Fickler-Winkler type of OPCA. Differs from OPCA I in a lack of involuntary movements and of sensory changes. Gene not known.
- OPCA II (or SCA 2) -- Autosomal dominant. Called the Cuban type of OPCA. Due to expansion of a CAG trinucleotide sequence in the ataxin-2 gene (ATX2) on chromosome 12q24.
- OPCA III (or SCA 7) -- Autosomal dominant. OPCA with macular degeneration and ophthalmoplegia. Due to expansion of a trinucleotide sequence in the ataxin-7 gene (SCA7) on chromosome 3p.
- OPCA IV -- Autosomal dominant. Similar clinically to OPCAI but with spastic paraplegia. May be due to a different mutation in the ataxin-1 gene (ATX1) and so be alleleic to OPCA I.
- OPCA V -- Autosomal dominant. OPCA with progressive dementia and extrapyramidal neurologic signs. The gene responsible for this disease has not been identified.