Potential role of anti-GAD antibodies in abnormal eye movements

Glutamic acid decarboxylase (GAD) catalyzes the conversion of glutamic acid to gamma-aminobutyric acid (GABA). Autoantibodies directed against GAD (antiGAD-Ab) have been described in patients with insulin-dependent diabetes mellitus, stiff-man syndrome, and in a few patients with progressive cerebellar ataxia. The presence of these autoantibodies suggests an autoimmune pathophysiological mechanism for the neurological manifestations in these disorders. However, the exact role of antiGAD-Ab and GABAergic neurotransmission in the pathogenesis of the neurological manifestations, particularly in progressive cerebellar ataxia, is not fully understood. The cases of two patients with subacute cerebellar ataxia associated with antiGAD-Ab presenting with abnormal eye movements are reported. One patient presented a periodic alternating nystagmus (PAN), whereas the other presented a downbeat nystagmus (DBN) and slow vertical saccades. The potential role of antiGAD-Ab and the resultant GABAergic neurotransmission deficit in oculomotor manifestations is discussed.