Labyrinthine functional tests and a case of midline cerebellar teratoma.

Electronystagmographic (ENG) examination may indicate clinically abnormal but nonspecific or even diagnostically deceptive results. Insights into the processes producing such observations sometimes can be suggested from correlations of findings to pathologic conditions in cases of demonstrated vestibular lesions. Such an opportunity occurred when unusual ENG results were obtained from a patient who had a history of equilibrium disturbances. The ENG revealed a direction-changing nystagmus in lateral head and body positions but not with the head similarly turned. Ocular counterroll measurements indicated hypofunctioning otolithic reactions despite normal vestibuloocular responses to calorizations. Radiologic follow-up showed a midline cerebellar tumor, later surgically confirmed, and found to be a rare teratoma. After partial excision and decompression, repeat tests revealed changes in both the positional nystagmus and ocular counterroll. These findings have implications for diagnosis, for mechanisms underlying ataxia, for inclusion of ocular counterroll testing in diagnostic evaluations, and for understanding the mechanisms underlying direction-changing positional nystagmus.