A rare case of crescent-shaped peridentate hyperintensity preceding cerebellar atrophy in GAD65 antibody-associated ataxia.

Glutamic acid decarboxylase 65 (GAD65) antibody-associated cerebellar ataxia (GAD65-CA) is a rare autoimmune neurological disorder wherein antibodies against the GAD65 enzyme impair GABAergic neurotransmission, ultimately causing Purkinje cell dysfunction. In this report, a 33-year-old woman exhibited right-dominant limb ataxia and dysarthria with high-titer anti-GAD65 antibodies (> 2,000 U/mL) in serum and cerebrospinal fluid. Three months after symptom onset, magnetic resonance imaging (MRI) revealed a crescent-shaped hyperintensity in the white matter surrounding the right dentate nucleus, along with hilum hyperintensity on T2-weighted imaging and fluid-attenuated inversion recovery sequences, without cerebellar atrophy. However, during follow-up, progressive right cerebellar atrophy developed, and iron accumulated within the dentate nucleus. These early MRI signal changes around the dentate nucleus suggested secondary axonal degeneration resulting from Purkinje cell injury. Therefore, this case highlights a distinctive imaging pattern, providing new insights into autoantibody-induced cerebellar synaptic impairment.