Relapsing Seronegative Immune-Mediated Cerebellar Ataxia with Evolving Intrathecal Immune Activation and Rituximab Consolidation in a Toddler: A Case Report

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Abstract
Seronegative immune-mediated cerebellar ataxia in young children is poorly defined and may be mistaken for self-limited post-infectious ataxia when magnetic resonance imaging and antibody panels are negative. A previously healthy 2-year-old girl developed acute cerebellar ataxia with gait instability, frequent falls, limb tremor, and choking while drinking. Brain/cerebellar magnetic resonance imaging was normal. At relapse, an equivocal cervical cord T2 signal was not confirmed on dedicated cervical imaging or accompanied by myelopathic signs. Extensive serum and cerebrospinal fluid antibody testing was negative. Initial cerebrospinal fluid showed mild pleocytosis; oligoclonal bands were reported as negative, although a single cerebrospinal fluid band was noted. She improved after intravenous immunoglobulin and high-dose corticosteroids but relapsed after early steroid discontinuation. At relapse, cerebrospinal fluid pleocytosis increased to 251 × 10⁶/L, with type 2 cerebrospinal fluid-restricted oligoclonal bands and intrathecal immunoglobulin M synthesis. Epstein-Barr virus serology suggested recent infection, but blood and cerebrospinal fluid Epstein-Barr virus DNA were negative. Genetic and metabolic testing was unrevealing. Rituximab was introduced as consolidation therapy after repeat first-line immunotherapy, while oral prednisone was tapered from 1 mg/kg/day to discontinuation in January 2026. Four weekly infusions depleted peripheral B cells, and a fifth infusion was given after B-cell reconstitution. No infusion reactions or infectious complications occurred. The retrospectively estimated Scale for the Assessment and Rating of Ataxia gait/posture subscore improved from 12/18 at relapse to 0/18 during follow-up. This case highlights serial evolution of intrathecal immune activation in relapsing seronegative pediatric cerebellar ataxia and adds pharmacodynamic and clinical follow-up support for the feasibility of B-cell-monitoring-guided rituximab consolidation in selected relapsing cases. Causal efficacy cannot be inferred from a single case treated with intravenous immunoglobulin, corticosteroids, and a subsequent prednisone taper.

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