Comparison of the histological and immunohistochemical features of the thymus in young- and elderly-onset myasthenia gravis without thymoma.

We performed histological and immunohistochemical analyses of the removed thymuses from 20 elderly (onset age > 60 years) and 23 young (onset age < 40 years) patients with myasthenia gravis (MG) who tested positive for serum anti-acetylcholine receptor (AChR) antibodies, but who did not have associated thymoma. In the elderly group, nine (45%) patients had accumulations of lymphocytes, indicating an atrophied thymus with loss of the basic structure. The elderly MG patients with atrophied thymic tissues had higher titres of anti-AChR antibody (59.6+/-81.0 nmol/L) than those with adipose infiltration of the thymus alone (20.1+/-20.9 nmol/L). In immunohistochemical studies using image analysis, both young patients and elderly patients with atrophied thymic tissues were found to have significantly higher levels of CD20 than age-matched controls (p < 0.005). Atrophied thymic tissues, often seen immunohistochemically in young MG patients, may also be found in elderly patients, particularly in those with high titres of the anti-AChR antibody, even though adipose infiltration is marked in these patients.