Anti‐Ma2 Antibody‐Associated Paraneoplastic Cerebellar Degeneration Mimicking the Cerebellar Ataxic Subtype of Hashimoto’s Encephalopathy: A Case Report

Autoimmune cerebellar ataxia (ACA) is an immune‐mediated cerebellar disorder arising from various underlying conditions. Because ACA is potentially treatable, identifying associated neuronal antibodies is important for diagnosis. We report the case of a 55‐year‐old woman with subacute progressive ataxia who presented with dysarthria and impaired handwriting and gait. Her scale for the assessment and rating of ataxia (SARA) score was 20.5, with particularly high subscores for gait and stance. She had a history of Hashimoto’s thyroiditis. Laboratory testing showed markedly elevated antithyroid antibody levels with normal thyroid function, initially suggesting the cerebellar ataxic subtype of Hashimoto’s encephalopathy (HE). Cerebral blood flow single‐photon emission computed tomography revealed cerebellar hypoperfusion, which was most prominent in the superior cerebellar vermis. Intravenous methylprednisolone (1000 mg/day for 3 days) was administered; however, her response was poor. Further evaluation revealed positivity for anti‐Ma2 antibodies, leading to a diagnosis of anti‐Ma2‐associated ACA. Subsequent intravenous immunoglobulin therapy improved the clinical symptoms and imaging findings, and the follow‐up SARA score decreased to 10. Comprehensive imaging revealed no underlying malignancy. This case is unique not only because both rare diseases were key considerations in the differential diagnosis, but also because it is instructive in two respects: the construct of HE remains insufficiently specific and should not preclude consideration of alternative immune‐mediated etiologies, and patients positive for anti‐Ma2 antibody require sustained oncological surveillance, even in the absence of malignancy at presentation.