Autoantibodies to GABAergic neurons and response to plasmapheresis in stiff-man syndrome.

Stiff-man syndrome is a rare disorder of unknown etiology in which progressive rigidity, spasms, and continuous motor unit activity may be due to dysfunction of GABAergic inhibition of alpha motor neurons. Some patients with this disorder have evidence of autoantibodies reactive with glutamic acid decarboxylase (GAD). We describe a patient with progressive stiff-man syndrome and high titers of GAD-like immunoreactivity in serum but not spinal fluid. Plasmapheresis resulted in lowered antibody titers, decreased exteroceptive reflex responses, reduced motor unit activity, and marked clinical improvement. Immunohistochemistry using patient serum and plasma produced specific labeling of human and experimental animal tissue consistent with GABAergic neurons and terminal fields. This antibody response was not present in samples from more than 200 other patients. These results provide additional evidence of an autoimmune mechanism for stiff-man syndrome and indicate plasmapheresis may be beneficial in some patients.