Introduction
The value of a defect in optokinetic nystagmus in the lateralization of cerebral lesions has been well established.1,2 However, in brain stem lesions it has been of little use in determining the site of the pathological process. This has been reportedly due to the fact that in brain stem disease the optokinetic response is usually depressed in both directions.3 Observations on patients in the past several years at the National Institutes of Health and Washington University neuroophthalmology services have suggested that frequently in brain stem lesions the optokinetic response is depressed asymmetrically and may be of significant value in lateralization.The following cases are presented, having been selected because of an asymmetric optokinetic nystagmus associated with other lateralizing signs of disease in the brain stem. Optokinetic testing was done with the head and eyes in the primary position. The optokinetic tape was either an ordinary tape measure