Cerebellar Atrophy in Pediatric-Onset Opsoclonus-Myoclonus-Ataxia Syndrome: A Large International Cohort Study.

Background And Objectives Pediatric-onset opsoclonus-myoclonus-ataxia syndrome (POOMAS) is a rare, neuroinflammatory syndrome that targets the cerebellum and can cause irreversible cerebellar structural changes, namely cerebellar atrophy. We investigated the frequency of cerebellar atrophy, as well as associated risk factors and functional outcome measures, using the largest active POOMAS registry.Methods This was a retrospective observational study of participants with POOMAS with disease onset from 1995 to 2025 using data from the multinational, multicenter POOMAS registry. Variables were compared between the cerebellar atrophy subgroups using the Fisher exact test or Wilcoxon rank-sum test.Results Of participants with follow-up imaging, cerebellar atrophy developed in 9.5% (6/63) of those with an MRI scan obtained at least 6 months after disease onset and 12.5% (3/24) with an MRI scan obtained at least 48 months after disease onset. Half of the participants who developed cerebellar atrophy (3/6) were identified more than 48 months after disease onset. Cerebellar atrophy was associated with older age at diagnosis (p = 0.023), but not with other demographic or clinical characteristics. There were no statistically significant differences in functional outcome measures or educational setting.Discussion The overall frequency of cerebellar atrophy was likely underestimated in this study because of the lack of standardized MRI timing and the large number of patients without MRI scans beyond 48 months.