Visual evoked potential abnormalities in spinocerebellar ataxia type 27B: a case report

Spinocerebellar ataxia type 27B (SCA27B) is an autosomal dominant late-onset cerebellar ataxia caused by a pathogenic GAA repeat expansion in the FGF14 gene. Although oculomotor abnormalities, particularly downbeat nystagmus, are well recognized, afferent visual pathway involvement has not been systematically investigated.

We report a 67-year-old man with genetically confirmed SCA27B who presented with a two-year history of progressive cerebellar and somatosensory ataxia, with a Scale for the Assessment and Rating of Ataxia (SARA) score of 13. Neurological examination revealed typical oculomotor abnormalities, including downbeat nystagmus elicited during head-shaking testing. Brain MRI showed mild cerebellar atrophy. Electrophysiological studies demonstrated axonal sensory polyneuropathy and prolonged central conduction times on somatosensory evoked potentials. Comprehensive ophthalmological examination, including optical coherence tomography, showed no structural abnormalities. Pattern-reversal visual evoked potentials, recorded according to ISCEV standards, demonstrated mild bilateral prolongation of P100 latency, measuring 116 ms in the left eye and 115 ms in the right eye, with preserved amplitudes. Compared with laboratory normative data from individuals aged 60-70 years, these values exceeded the upper age-related reference limit.

This case suggests possible subclinical functional involvement of the post-retinal afferent visual pathways in SCA27B. However, the findings should be interpreted cautiously and confirmed in larger, age-matched cohorts.