Isolated Vestibular Suppression Impairment With Vestibular Migraine: A Phenotypic CANVAS Variant.

HYPOTHESIS: Cerebellar Ataxia with Neuropathy and Vestibular Areflexia (CANVAS) is likely to have a genetic basis. We describe the unique eye movement features of a possible phenotypic CANVAS variant.

BACKGROUND: The patient comes from a large CANVAS kindred (four out of nine siblings) and has sensory neuropathy, cerebellar eye signs, and vestibular migraine (VM), but otherwise normal vestibular function.

METHODS: We recorded eye and head movements using the gold standard scleral search coil technique: in the patient, a close relative with mild sensory neuropathy, and a normal control.

RESULTS: At ≥ 0.8 Hz vestibulo-ocular reflex suppression (VORS) was significantly smaller in the patient. At 1 Hz, the patient's VORS was almost two times worse than the control, and five times worse at 1.6 Hz. The patient's VORS deficiency was observed with the naked eye as an inability to keep the eye stationary during imposed sinusoidal head rotation at ∼ 1 Hz. At ≤ 0.8 Hz the patient had 10 to 20% lower smooth pursuit function compared with both the patient-relative and control subjects. This difference was difficult to detect by the naked eye. Saccadic oculomotor and vestibular function was normal.

CONCLUSION: We propose that impaired VORS and VM are because of similar, but distinct, consequences of selective partial cerebellar dysfunction. The patient's VORS data are consistent with a CANVAS neuropathological study showing selective degeneration of the dorsal vermis of the cerebellum, a region thought to be important for VORS. Taken together our findings suggest the patient is a CANVAS variant. We hypothesise VORS impairment is part of CANVAS, but not revealed because of vestibular loss.