Factors affecting response to thymectomy for myasthenia gravis

Abstract

Timing of operation, thymic disease, and immunologic factors affect results of thymectomy for myasthenia gravis. Review of hospital records of 97 patients with myasthenia gravis revealed 22 (17 female and five male patients) who had undergone thymectomy from 1961 to 1980. There were no surgical deaths. Pathological examination recorded thymic hyperplasia in 13, no thymic disease in five, involuted glands in three, and thymoma in one. Thymectomy resulted in 91% improvement: complete remission in 10, minimal residual symptoms on decreased medication in 10, and moderate improvement in two patients operated upon within the last year. This study demonstrates that factors predictive of symptom amelioration were short duration of disease, thymic hyperplasia with abundant germinal centers or normal thymus gland, and acetylcholine receptor (AChR) antibodies present before and absent following complete transsternal thymectomy. Factors predictive of delayed or no response to thymectomy were long duration of disease before operation, muscle wasting, thymic atrophy, incomplete excision, and persistent AChR antibodies. Response was independent of sex and age. Our findings support the current theory that the thymus in myasthenia gravis participates in the production of AChR antibodies, which interfere with motor end-plates and neuromuscular transmission causing myasthenic weakness. The autoimmunizing event is unknown. Excision of hyperplastic thymus early in the course of myasthenia gravis is associated with early remission. Prolonged post-thymectomy impairment may be expected in some patients who have had myasthenia for years prior to operation. Measurement of AChR antibodies routinely before and after thymectomy may yield valuable prognostic data and distinguish patients with active autoimmune disease possibly due to residual or regenerated thymus from those with severe muscle atrophy.