Atypical Ocular Motor Features (Gaze-evoked Nystagmus) in PSP

This is a 70-yo-woman who met clinical and radiologic diagnostic criteria for progressive supranuclear palsy (PSP). Typical ocular motor features of PSP include square wave jerks, hypometric saccades, choppy pursuit/VORS, impaired down>upgaze (supranuclear in origin) and impaired down>upward saccades may be one of the first ocular motor exam findings in this population (this can be nicely demonstrated using an optokinetic stimulus where the downward fast phase can be lost). This particular patient had all of these ocular motor features, and they were relatively mild aside from the clearly impaired downward saccades. Atypical for PSP was her prominent gaze-evoked nystagmus in lateral and up gaze which implies dysfunction of the neural integrators/gaze-holding machinery. With ophthalmoscopy, there was very subtle spontaneous upbeat nystagmus (UBN), and clear UBN in upgaze. In such a patient, a neurodegenerative condition that affects the posterior fossa such as multiple system atrophy (MSA) should also be considered, although there were no prominent autonomic symptoms/signs, no limb ataxia, and no hot cross bun sign on MRI. Taken together, she fit best with the diagnosis of PSP, and in fact, PSP with cerebellar ataxia is a rare phenotype of PSP although the clinical and radiological features of this disorder remain poorly characterized. Although PSP and MSA typically have characteristic ocular motor findings, it is important to know that overlap between the two conditions can exist and may lead to diagnostic confusion. Positional downbeat nystagmus should also be evaluated for in parkinsonian patients, as this finding has been well-described in MSA (also not seen in this patient).