| Description |
Patients with MG have a higher prevalence of other autoimmune disorders than otherwise normal persons, about 14% in a Danish cohort and 15% in a Dutch patient series. Disorders of the thyroid gland, including subclinical disease indicated by serum antibody studies, are more common in a myasthenic population than in a normal population. Such patients are as commonly hypothyroid as hyperthyroid. Millikan and Haines found that the incidence of hyperthyroidism before, during, and after detectable MG is about 5%. If all thyroid disorders are considered, the prevalence may be as high as 9% in males and 18% in females. Many of these patients have dysthyroid symptoms that precede myasthenic symptoms by months or years. In other cases, dysthyroid symptoms may be subsiding when symptoms and signs of MG first appear, and vice versa. The linkage between them appears to be the genetic and other factors predisposing to autoimmunity. All diseases of the thyroid gland are overrepresented, including nontoxic goiter, spontaneous myxedema, Hashimotos disease, Graves disease, and thyroid carcinoma. A particularly difficult clinical issue relates to the infrequent coexistence of thyroid ophthalmopathy and ocular manifestations of MG. The mechanism of this association remains uncertain, but there may be an immunologic cross-reaction to shared epitopes of the target tissues in the orbital muscles. Thyroid ophthalmopathy has been reported to be present in about 60% of patients with Graves disease, whether or not MG is also known to be present. Anti-ACh receptor antibodies have been found in 8% of patients with thyroid ophthalmopathy, although none of these patients showed clinical signs of MG over median follow-up of 4.5 years. The incidence of clinical MG in patients with thyroid ophthalmopathy has been estimated to be less than 1%. On the other hand, the incidence of exophthalmos in patients with MG has been reported to be as high as 23% in a Japanese series, with thyrotropin inhibitory immunoglobulin present in 19%. In the case of coexistence of these two ocular motor disorders, the presence of exotropia has been suggested to be a myasthenic feature much more likely than a thyroid ophthalmopathy feature. When a patient with thyroid ophthalmopathy develops ptosis, orbicularis oculi weakness, exotropia, or new ocular deviations after a period of stability, the possibility of coexisting MG should always be considered. Similarly, if a patient with MG develops proptosis, lid retraction, hypotropia with infraorbital edema, chemosis, or superficial ocular irritation, the development of thyroid ophthalmopathy should be suspected. In treating such overlap patients, efforts must be made to delineate the contribution of each underlying ocular motor disorder to the patients clinical status, and the appropriate therapy given specific to the effects of each disorder. The relative contribution of each disorder to the patients problems may evolve over time. It has been well documented that the clinical features of MG tend to become milder and easier to control with acetylcholinesterase inhibitors when coexisting hyperthyroidism is effectively treated, although there are reported exceptions. |
