Objective: To report a rare case of anti-GAD associated encephalitis with pediatric Stiff Person Syndrome (SPS) highlighting the need for broad investigations and extensive treatment.

Background: Anti-GAD-associated encephalitis is exceedingly rare in the pediatric population. Anti-GAD antibodies target glutamic acid decarboxylase, essential for GABAergic neurotransmission, and are also implicated in autoimmune diabetes mellitus.

Method: A 19-month-old boy presented with dystonia with elevated CK, hyperreflexia, and behavioral changes. Initial treatment with levodopa improved ambulatory function, suggesting involvement of dopamine metabolism. Diagnostic testing revealed markedly elevated anti-GAD antibody titers in both blood (> 100 times normal) and cerebrospinal fluid (> 10 times normal). Clinical, EEG and CSF findings (positive anti-GAD, 9 cells and 15 oligoclonal bands) supported diagnosis of autoimmune limbic encephalitis with SPS. MRI was normal twice. Complete metabolic screening, neurotransmitters in CSF and exome sequencing for neurotransmitter and metabolic diseases were negative. Treatment included intravenous immunoglobulin (IVIG), high-dose steroids, plasma exchange, and subsequent rituximab administration to address persistent behavioral and language difficulties.

Results: IVIG and steroids had little effect. Plasma exchange was associated with gradual improvement of motor function and recovery. One year later, rituximab was initiated. Two weeks after the first infusion, however, the child developed diabetic ketoacidosis, underscoring the link between anti-GAD antibodies and autoimmune diabetes. At age four, he was diagnosed with autism spectrum disorder, with behavior more affected than language.

Conclusion: Acute dystonia is a rare presentation at this age and warrants a broad and timely investigation. Aggressive and early intervention led to a clear improvement in motor function, however he developed persistent behavioral and metabolic (diabetic) complications associated with anti-GAD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/subacute-severe-dystonia-in-a-19-month-old-boy-a-case-of-pediatric-stiff-person-syndrome-sps/