Glutamic Acid Decarboxylase Antibody in a Patient with Myelitis: A Retrospective Study.

Objective: The aim of this study was to evaluate the positive rate of serum glutamic acid decarboxylase (GAD) autoantibody in patients with myelitis and to describe the clinical findings in patients with positive GAD antibody. Methods: Serum samples were collected from 390 patients with myelitis, including 210 patients positive for aquaporin 4 (AQP4) antibody and 180 patients negative for AQP4. GAD65 antibody was measured by an indirect immunofluorescence assay. Results: Only 1 serum and cerebral spinal fluid sample from 390 patients (0.26%) was positive for anti-GAD antibodies. The patient was a female with relapsing myelitis and a thymic mass. Thymic resection was undertaken, and pathological examination revealed a benign thymic cyst. Extensive infiltration of lymphocytes positive for CD3, CD4, CD8 and CD20 was found. Immunohistochemistry showed positive expression of GAD65 in the cyst. Conclusions: Although serum GAD65 antibodies were present in a patient, it is not recommended to routinely screen for GAD65 antibodies in patients with myelitis because of their rare occurrence. However, screening for GAD65 antibodies should be considered in patients who have been diagnosed with cancer or a thymic abnormality.