Describe a case of cerebellar ataxia which inspired an exam maneuver to detect nodulus symptoms
Case Description: A 27 year-old woman reported turning-induced dizziness after Chiari decompression. Actively turning 180° rightward triggered vertigo and left-beating nystagmus lasting >30 seconds. Turning left produced comparable vertigo and right-beating nystagmus. Passive 180° rotations in a swivel chair at ~50°/s elicited similar responses. Head-impulse-testing was normal and nonprovocative. Examination demonstrated saccadic pursuits and gaze-evoked & rebound nystagmus. Review of presurgical MRI revealed compression of both cerebellar tonsils and uvula/nodulus.
Neuroscientific Correlate: Nodular Purkinje neurons provide GABA-mediated inhibitiory signals to vestibular nuclei which augment the velocity storage mechanisms (VSMs). The VSMs improve responses to prolonged low-frequency movements by perseverating peripheral vestibular signals after semicircular afferent signals decay. VSM disinhibition can cause central patterns of head-shaking nystagmus and periodic alternating nystagmus (PAN). We propose turn-induced-nystagmus may reflect perseverated vestibular signals from disinhibited VSMs, a possible manifestation of nodulus impairment on a spectrum with PAN.
This is an ongoing prospective study. Thus far ten patients with dizziness and cerebellar ataxia and two with dizziness but no cerebellar ataxia were tested. Subjects were seated on a swivel chair with room light, rotated en bloc 180°/s for 1 second, stopped, and the presence and duration of nystagmus was recorded.
9 patients with cerebellar ataxia exhibited prolonged (5-10 seconds) nystagmus after the 180° turn. Patients without cerebellar ataxia exhibited no nystagmus after the turn. Notably, the one cerebellar ataxia patient without nystagmus had severe vestibular loss.
Inspired by observations from a patient with cerebellar ataxia and pre-operative nodulus compression, the swivel-chair test may help identify patients with clinically significant nodulus impairment. Patients with cerebellar ataxia should be asked about turn-induced dizziness, and the presence/absence of this sign may help localization and/or guide therapeutics (e.g., trial of GABA-ergic medication).