MILD CLINICAL EXPRESSION OF MYASTHENIA-GRAVIS ASSOCIATED WITH AUTOIMMUNE THYROID-DISEASES

Myasthenia gravis (MG) may occur in association with autoimmune thyroi d diseases (AITD). The aim of this study was to evaluate the features of MG associated with AITD compared to those of MG without AITD. A tot al of 129 MG patients (34 men and 95 women; age range, 11-81 yr) were subdivided into: group A, 56 MG patients with AITD [25 with autoimmune thyroiditis and 31 with Graves' disease (GD)I; group B, 21 MG patient s with nonautoimmune thyroid diseases; and group C, 52 MG patients wit hout thyroid disease. The severity of MG was ranked according to the O sserman score. Laboratory evaluation included assays for antithyroid a nd antiacetylcholine receptor (AchRAb) antibodies. Ocular MG (Osserman 's class 1) was more frequent in group A (41.0%) than in group B (14.2 %; P < 0.03) or C (21.4%; P < 0.03). Severe generalized MG (classes gr eater than or equal to 2B) was more frequent in groups B (57.1%; P < 0 .03) and C (51.9%; P < 0.02) than in group A (28.5%). GD patients with clinical evidence of ophthalmopathy had a higher frequency (P = 0.05) of ocular MG (57.8%) than GD patients without clinical ophthalmopathy (16.6%). Thymic disease was less frequent in group A (26.7%) than in group B (71.4%; P = 0.001) or C (59.7%; P = 0.001). The prevalence of thymic hyperplasia was 17.8%, 38.0%, and 40.3% in groups A, B, and C, respectively; the prevalence of thymoma was 8.9%, 33.4%, and 19.4%. Wh en only patients with generalized MG were considered, thymic disease w as less frequent (P < 0.02) in group A (40.6%) than in the remaining g roups (69.4%). AchRAb was more frequent in groups B (57.1%) and C (57. 6%; P < 0.03) than in group A (35.7%). In conclusion, MG associated wi th AITD has a mild clinical expression, with preferential ocular invol vement and lower frequency of thymic disease and AchRAb. This supports the hypothesis that ocular and generalized MG are separate diseases w ith different spectra of associated diseases. Nonautoimmune thyroid di seases have no influence on the features of MG. The association of ocu lar MG and AITD might be due to a common autoimmune mechanism and/or a peculiar genetic background.