Downbeat Nystagmus and Ataxia Preceding New-Onset Type 1 Diabetes Mellitus - Role of Anti-GAD Antibodies?

Autoantibodies to glutamic acid decarboxylase (anti-GAD Abs) are prevalent in diabetes mellitus type 1 (T1DM) and often considered a biologic marker of this disease. High titers of anti-GAD Abs have also been described in multiple neurological syndromes, including stiff-person syndrome, cerebellar ataxia, and epilepsy-syndromes which are considered to result from reduced γ-aminobutyric acid (GABA) transmission. However, the pathophysiology underlying the role of anti-GAD Ab in these relatively rare neurological syndromes remains poorly understood and no criteria exist to establish when a neurological syndrome is pathogenically linked to anti-GAD Ab, sometimes resulting in misdiagnosis and unnecessary treatment. Here, we report a patient presenting with cerebellar ataxia who developed late-onset T1DM with high anti-GAD Ab titers.