Anti-SOX1 Antibodies in a 3-Year-old Girl, Post-Varicella.

Anti-SRY-related HMG-box gene 1 (SOX1) antibodies were initially described in adults with paraneoplastic neurological disorders, where they are considered high-risk onconeural autoantibodies. Only two pediatric cases of anti-SOX1 antibodies have been reported: a 17-year-old adolescent presenting with paraneoplastic limbic encephalitis due to Hodgkin lymphoma and a 12-year-old girl presenting with non-paraneoplastic encephalitis. We present a unique case of anti-SOX1 antibodies in a 3-year-old girl, post-varicella infection. Initially, she presented with ataxia and dysmetria, with subsequent reports from parents of urinary incontinence and significant behavior changes. Additionally, reflexes in the lower limbs were absent. Anti-SOX1 antibodies tested positive in both serum and cerebrospinal fluid. Oncological screening at presentation and a seven-month follow-up showed no malignancies. The patient exhibited favorable clinical progress without requiring treatment. At the seven-month follow-up, serum antibodies tested negative. This case report broadens the known clinical spectrum, being the first description of post-varicella anti-SOX1 antibodies.