Olivopontocerebellar atrophy with retinal degeneration. An electroretinographic and histopathologic investigation.

BACKGROUND: Olivopontocerebellar atrophy is an uncommon disorder with variable clinical manifestations that affects the cerebellum, the spinocerebellar tracts, and other structures of the brainstem. A deficiency of glutamate dehydrogenase, which results in an excess of glutamate, has been suggested to play a role in the pathogenesis of olivopontocerebellar atrophy. In experimental animals, toxic levels of glutamate are known to cause a selective loss of the b-wave on electroretinographic (ERG) testing and a degeneration of the inner retinal layers. One of the subtypes of olivopontocerebellar atrophy, type II, according to Harding's classification, is associated with retinal degeneration.
METHODS: The authors describe the ophthalmologic and ERG findings in a family with olivopontocerebellar atrophy type II. Histopathologic study of an eye from a 6-year-old family member who died of severe neurologic deterioration secondary to olivopontocerebellar atrophy type II was performed.
RESULTS: Electroretinographic changes may be present in affected family members who are entirely asymptomatic and have a normal ophthalmologic evaluation. The changes on the ERG in one patient suggest that cone dysfunction is one of the subtle changes that may be seen in olivopontocerebellar atrophy type II. Our ERG results did not show a selective loss of the b-wave but instead showed a loss of both the a-wave and b-wave in affected family members. Results of light and electron microscopic examination showed diffuse and extensive degeneration of the photoreceptors involving both rods and cones, the most prominent changes being present in the macula. An amorphous debris, presumably degenerated photoreceptors, was noted between the outer nuclear layer and retinal pigment epithelium.
CONCLUSION: Patients with olivopontocerebellar atrophy type II have photoreceptor abnormalities as revealed in abnormal ERGs seen in many patients and histopathologic study of an autopsy eye from an affected 6-year-old boy. Our results do not support the hypothesis that glutamate toxicity may be responsible for the development of retinal degeneration in this condition.