Anti-SOX1 antibody-associated paraneoplastic cerebellar degeneration without detectable tumor

A 56-year-old male presented with complaints of insidious onset difficulty in speech and head tremors for 2 years and imbalance while walking for 1 year. He did not have autonomic dysfunction and had a negative family history. The cerebrospinal fluid examination was normal and the workup for autoimmune disorders was negative. Brain magnetic resonance imaging showed mild superior cerebellar atrophy. The serum paraneoplastic antibody panel showed anti-Sry-like high mobility group box (SOX1) antibodies positivity. He was treated with steroids and steroid-sparing immunosuppressants resulting in mild improvement. Extensive workup for underlying malignancy was negative. Our case illustrates a small subset of individuals with anti-SOX1 antibody positivity who also exhibit neurological symptoms indicative of paraneoplastic cerebellar degeneration without having any underlying tumor and responded to immunotherapy.