Downbeat nystagmus without ataxia as an early manifestation of spinocerebellar ataxia, autosomal recessive type 10 (SCAR10): a case report.

The differential diagnosis of dizziness is broad and can include both vestibular and autonomic pathology. Vestibular dizziness is typically described as a sensation of movement (e.g. the world is spinning) while dizziness related to autonomic dysfunction is typically described as symptoms of orthostatic intolerance (e.g. postural lightheadedness). It is important to differentiate the type of dizziness to guide proper diagnostic and therapeutic workup. We describe the case of a young patient evaluated in the autonomic clinic for dizziness, ultimately found to have a rare cerebellar neurodegenerative disorder. A 23-year-old female with a past medical history of migraine without aura presented in autonomic clinic with a four-month history of slow, progressive onset of vestibular dizziness. Neurological exam was notable for downbeat nystagmus, but no appendicular or truncal ataxia was appreciated. Subsequent vestibular evaluation was consistent with central vestibular dysfunction. Magnetic resonance imaging (MRI) of the brain with and without contrast was notable for severe cerebellar atrophy. The patient subsequently underwent genetic testing which demonstrated a pathogenic and likely pathogenic variant in the Anoctamin 10 (ANO10) gene, which is seen in Autosomal Recessive Spinocerebellar Ataxia, Autosomal Recessive Type 10 (SCAR10). To our knowledge, this case represents the first reported instance of SCAR10 presenting with the sole neurologic exam finding of downbeat nystagmus without cerebellar ataxia. Additionally, the finding of severe cerebellar atrophy seen on MRI, without gait or limb ataxia on exam is an interesting clinicoradiological dissociation. This case suggests that nystagmus may represent an early disease marker, even in the absence of clinical ataxia in patients with SCAR10.