The fixation suppression test can uncover vertical nystagmus of central origin in some patients with dizziness.

Abstract

Identifying dangerous causes of dizziness is a challenging task for neurologists, as it requires interpretation of subtle bedside exam findings, which become even more subtle with time. Nystagmus can be instrumental in differentiating peripheral from central vestibular disorders. Conventional teaching is that peripheral vestibular nystagmus is accentuated by removal of visual fixation. We sought to systematically test the hypothesis that, in some cases, vertical nystagmus due to central vestibular disorders may also be easier to identify when fixation is removed. To identify patients with vertical nystagmus, we retrospectively reviewed clinical, MRI, and VNG data of consecutive patients undergoing VNG in our vestibular clinic over a 9-month period. We analyzed clinical features, bedside neuro-otological examination, MRI results, and VNG findings in fixation as well as those with fixation removed. Two hundred and fourteen charts were reviewed. Twenty-six patients had vertical nystagmus with fixation removed on VNG. Only three (11.5%) of these patients had vertical nystagmus apparent with fixation (and only two had nystagmus observed clearly at the bedside with the unaided eye). Thirteen (50%) of the patients had posterior fossa lesions on MRI and eight of the rest (30.8%) were diagnosed with central vestibular disorders. Of the 13 patients with MRI-confirmed lesions, 3 patients (23.1%) had no neurological signs or conventional bedside oculomotor signs; in these cases, vertical nystagmus without fixation was the only sign of a central lesion. Our findings go against conventional teaching and show that removing fixation can uncover subtle vertical nystagmus due to central vestibular disease, particularly from focal or chronic lesions.