Introduction: OMS is a rare and severe autoimmune neurological disorder poorly recognized and undertreated, causing lifelong neurological disabilities. OMS classically causes opsoclonus, myoclonus, and ataxia, as well as sleep disturbance, cognitive dysfunction, and behavioral disruption. Early diagnosis and treatment may yield better prognosis. Neuroblastoma is associated in about one half of the cases. The current work focus on diagnostic difficulties and atypical presentations.

Methods: To define clinical aspects in a series of OMS, were transversally reviewed 30 children over an 1-year period (April 2017/April2018). The diagnosis was made by a pediatric neurologist and applied the diagnostic criteria proposed by the international workshop on OMS.

Results: The mean age of onset was 1 year and 6 months. The earliest neurological symptoms by rank order were: myoclonus/tremor, falling, and opsoclonus. Other signs and symptoms infrequently described on OMS were: vomiting, dystonia, torticollis, and strabismus. The average delay to diagnosis was 12 weeks. Neuroblastoma was found in 38% of the cases, and in only 6 patients the neuroblastoma was detected by traditional methods of diagnosis. Infection prodromal symptoms were infrequent (23%). All patients used conventional treatments (corticosteroids, adrenocorticotropic hormone, and intravenous immunoglobulin) and six patients unconventional treatments (rituximab and cyclophosphamide). Persistence of symptoms was observed in 50% of patients.

Conclusions: OMS diagnosis requires a high level of suspicion and a methodical diagnostic approach for neuroblastoma. The delay in diagnosis (03 months) is unacceptably long. An early and aggressive therapy can improve the neurological outcome.