Spinocerebellar ataxia type 6 (SCA6) is a calcium channelopathy due to a pathological CAG repeat expansion in
CACNL1A4. Patients frequently describe paroxysmal vertigo early in the disease course, but it is not clear whether this is central
or labyrinthine in origin. To address this issue we studied 21 SCA6 patients. Symptoms of vertigo were defined using a structured
questionnaire. Signs were recorded during a standardised bed-side vestibular examination that included systematic positional
testing with Frenzel goggles.
Brief, recurrent attacks of vertigo occurred in 13 patients, usually preceding the onset of ataxia. Nystagmus was observed
behind Frenzel goggles in 14 patients, and was induced either during positional testing, or head shaking in 20 patients. Only
one patient had findings typical of benign paroxysmal positional vertigo (BPPV). Combined downbeat and horizontal gaze-evoked
nystagmus (“side-pocket”) was the most common form, occurring most commonly in supine and head-hanging positions, and following
horizontal head-shaking. Nystagmus beating away from the ground (apogeotropic) occurred in 9 patients as they lay on their
side.
In conclusion, vertigo and abnormalities on bedside vestibular examination are common in SCA6, with forms of nystagmus typical
of cerebellar, rather than labyrinthine, disease. These findings demonstrate phenotypic overlap between SCA6 and episodic
ataxia type 2, which are both due to mutations in CACNL1A4.
Key words spinocerebellar ataxia - stereopsis - nystagmus - ocular motility - vertigo - vestibular