Upbeat Nystagmus in a Progressive Cerebellar Ataxia

Although downbeat nystagmus is much more typically seen in patients with cerebellar ataxia, it is possible to see spontaneous upbeat nystagmus (UBN) as well. UBN due to posterior fossa dysfunction may have to do with lack of medullary nuclei (Roller and intercalatus) inhibition of the cerebellar flocculus, which causes disinhibited Purkinje cells that inhibit the anterior canal (upward/anti-gravity) pathways too strongly. This results in relatively more posterior canal (downward) tone, creating downward quick phases followed by upward fast phases. This is a patient with progressive limb and gait ataxia over 10 years, with vertical oscillopsia due to UBN. Hereditary and metabolic causes of cerebellar ataxia are being investigated. The UBN is very high frequency to the point where at times it is challenging to recognize the distinct slow and fast phases, making the examiner wonder whether this could be a high frequency pendular nystagmus or whether these could represent saccadic oscillations. At the bedside, ophthalmoscopy can be very helpful to better understand the waveform - e.g., it is much clearer when viewing the optic disc that a slow phase precedes each fast phase. There is also a subtle torsional component to the nystagmus, which is not well appreciated with the ophthalmoscope, since you're essentially viewing through the axis of rotation for torsion. Therefore, viewing a bit more peripherally with the ophthalmoscope will allow you to see the torsion, or more simply, just to view a conjunctival blood vessel (naked eye, ophthalmoscope, slit lamp, etc). Another helpful bedside tip when movements are rapid and high frequency is to take a video and then view it in slow motion as is seen here.