Pediatric opsoclonus myoclonus syndrome with improvement of symptoms and decrease in cerebrospinal fluid B cell percentage after rituxmab administration: Case report

Opsoclonus myoclonus syndrome (OMS) is a rare neurological disorder that presents with opsoclonus, ataxia or myoclonus, behavioral changes, or sleep disturbances. Cognitive and behavioral deficits are the most problematic sequelae in pediatric patients with OMS. Patients with OMS often have increased cerebrospinal fluid (CSF) B cell counts, which are considered a biomarker of disease activity and may be an important indicator in selecting optimal treatment. A 1-year-and-5-month-old boy diagnosed with paraneoplastic OMS was started on immunotherapy with intravenous immunoglobulin and dexamethasone (DEX) pulse therapy 3 months after disease onset. After one course of DEX pulse therapy, rituximab (RTX) was added due to a worsening of symptoms, resulting in an OMS Rating Scale score of 13. Two weeks after starting RTX therapy, the patient's symptoms started to improve, and he was able to walk 5 months later. The percentage of B cells in the CSF was 10.7 % before the introduction of RTX therapy but decreased to 0.16 % 3 months after starting RTX therapy. The patient achieved remission 12 months after the disease onset and had no recurrence. The high percentage of B-cells in the patient's CSF indicated severe disease activity. Remission could have been achieved sooner if RTX had been administered earlier. CSF lymphocyte subset analysis should be performed aggressively in OMS as it is a potential indicator for RTX introduction.