Slowing of all eye movements thought to be caused
by a brainstem lesion of the paramedian pontine reticular formation. Clinical features include an
abnormal accompanying movements of head and neck; spinocerebellar
degeneration with abnormal gait (ataxia); progressive intellectual impairment,
extrapyramidal dysfunction and peripheral neuropathy. Skeletal abnormalities
can occur. Muscle biopsy shows nonspecific mitochondrial abnormalities.
Magnetic resonance imaging studies of the brain show a significant degree of
cerebellar and brainstem atrophy. Death occurs within 10 years of onset.