Seronegative generalized myasthenia gravis: low frequency of thymic pathology.

We have reviewed thymus histology, preoperative serum acetylcholine receptor antibody status, and clinical features of all 50 patients who underwent thymectomy for generalized myasthenia gravis in the University of British Columbia-affiliated hospitals over the last 8 years. Seven patients had thymoma, 25 had hyperplasia, and 18 had a normal thymus. The seven thymoma patients all had severe limb involvement and all had circulating antibodies against the acetylcholine receptor. Patients without circulating antibodies had only minimal limb involvement and half of them had a normal thymus. Only two of the 32 (6%) patients with abnormal thymus were antibody-negative; this contrasts with six of the 18 patients (33%) with normal thymus. We conclude that seronegative patients with generalized myasthenia gravis constitute a distinct subgroup which may not be immunologically homogeneous and could be subgrouped with normal or abnormal thymic histology.