Unidirectional Nystagmus in Lateral Medullary Syndrome

This is a 70-yo-man who presented with acute vertigo. Examination demonstrated very mild spontaneous torsional nystagmus (towards the right ear) in primary (not seen well in this video), with robust downbeat-torsional (towards right ear) nystagmus in right gaze and (less robust) almost pure torsional nystagmus (still beating towards right ear) in left gaze. This video makes the a small unilateral posterior fossa lesion can cause unidirectional nystagmus even in accordance with Alexander's law that might make the clinician suspect a peripheral lesion. Arguing strongly in his case for a central localization were: 1) the vertical-torsional nystagmus (acute peripheral nystagmus is generally horizontal-torsional), and 2) his saccadic dysmetria (hyper to right and hypo to left). The saccadic dysmetria, in this case, does have localizing value as hypermetric saccades are always ipsilesional in Wallenberg syndrome. The nystagmus seen in Wallenberg is usually contralesional, but can also be ipsilesional as in this case.