Eye movements localize objects of regard on the retina and maintain them in steady, clear focus. Involuntary nystagmoid movements, congenital or acquired, moving retinal imagery more than a few degrees may produce blurring of vision, spatial disorientation, and/or oscillipsia.A 40 YOWF complained of progressive vertical oscillopsia and incoordination. Examination revealed a small refractive error and downbeat nystagmus, worse in lateral and downgaze. Downbeating nystagmus is usually pathognomonic of a lesion at the craniocervical junction. MRI demonstrated an ectopic cerebellar vermis was present in the foramen magnum--a congenital hind brain anomaly.Posterior fossa decompensation, the treatment of choice, produced some improvement in symptoms and prevented further progression. Nystagmus and oscillopsia, however, may persist in spite of surgical therapy. Drug therapy (clonzepam, isoniazid) and/or prisms may then be of benefit. In this case, we demonstrate the successful trial of horizontal prisms to achieve a subjective improvement of oscillopsia by exploiting the effect of vergence on downbeat nystagmus. Long-term improvement was maintained with a full manifest spectacle prescription incorporating base out prism.