Clinical and Genetic Characteristics of SCA27B: A Global Systematic Review and Meta-Analysis

Spinocerebellar ataxia 27B (SCA27B) has been recognised as a major cause of sporadic late-onset cerebellar ataxias, accounting for 9-61% of previously unexplained cases. We aimed to describe the clinical, radiological and genetic spectrum of SCA27B through a systematic review of the published literature. A systematic literature search of PubMed, Embase (Ovid), Ovid Medline and Scopus was conducted. Studies including genetically confirmed SCA27B patients were selected. Individual patient and aggregate data were extracted, pooled and summarised. A total of 45 studies including 1267 patients were analysed. The pooled mean age at onset was 53.14±15.23 years. Gait ataxia (pooled prevalence 0.97; 95% CI: 0.93-0.98), oculomotor abnormalities [excluding all nystagmus] (0.79; 95% CI: 0.68-0.87), all nystagmus except downbeat nystagmus(0.57; 95% CI: 0.43-0.70), downbeat nystagmus (0.45; 95% CI: 0.35-0.55), dysarthria (0.51; 95% CI: 0.44-0.59), episodic symptoms (0.44; 95% CI: 0.31-0.58) and vertigo/dizziness (0.37;95% CI: 0.31-0.43) were the most frequent clinical features. Substantial heterogeneity occurred across outcomes. Genotype-phenotype correlation analysis showed that the high-repeat group (≥250 repeats) was associated with significantly higher odds of gait ataxia (3.7 (95% confidence interval 1.9-6.9; p<0.001) and dysarthria (3.5 (1.6-7.7; p<0.001) compared to the low-repeat group (180- 249 repeats). SCA27B is a common and genetically defined cause of late-onset cerebellar ataxia with a heterogeneous clinical phenotype. Recognition of key clinical clues, including gait ataxia, downbeat nystagmus, vestibular features, oculomotor abnormalities and episodic symptoms, is essential to facilitate timely diagnosis and appropriate genetic testing. Repeat size of 250 and above seems to be associated with higher odds of gait ataxia and dysarthria compared to repeat size below 250.