Myasthenia gravis

Abstract

The annual incidence rate of myasthenia gravis (MG) was estimated as 4.6 per million population, and the rate was found to be constant in calendar-time. Age- & sex-specific incidence rates disclosed a bimodal appearance for both sexes. According to the epidemiological data, the age limit between early onset and late onset of MG should be set at 50 years. The point prevalence rate has increased in time from 41 per million population in 1950 to 77 in 1988, and model calculation predicts an increased rate of 83 in year 2000. This reflects an improvement in the prognosis of MG. Standardized mortality rate for MG was estimated as 1.87. The total patient population was weighted 6 to 4 with predominance of females. A decrement of the muscle action potential (AP) and of the muscle twitch (TW) was found in 72% and 49% of the cases respectively, the diagnostic yield being proportional to the severity of MG. Post-tetanic facilitation of AP and TW was found in 25% and 20% of the cases respectively. The analogue estimates for post-tetanic exhaustion were 44% and 25% respectively. An abnormal staircase phenomenon was present in 37% of MG patients. Neurophysiological evidence of myopathy, which was not correlated to the degree of neuromuscular transmission failure, was found in 19% of the patients, more often in cases with late onset than in those with early onset (31% compared with 11%). The occurrence of myopathy was associated with pathological muscular biopsy (78%), and with anti-striated muscle antibodies (53%) suggesting that a myopathy of possibly autoimmune origin may coexist with MG. The overall diagnostic sensitivity of anti-AChR antibodies RIA was found to be 88%, the positivity being proportionate to the clinical severity of MG (ocular MG: early onset 71%, late onset 88%; severe generalized MG: early onset 89%, late onset 98%). The finding of anti-AChR antibodies is highly specific for MG (>99.9%). The distribution of antibodies titres was approximately lognormal. The concentration of the antibodies correlated with the clinical severity of the disease, female or male gender, and the pathology of the thymus gland. The improved outlook for MG during the last 4 decades may be ascribed to the combined results of all the new modalities of treatment. Experimental results towards selective plasma exchange are expounded. As opposed to MG patients with hyperplasia or involution of the thymus gland, thymoma cases exhibited exacerbation of the clinical course and also of the anti-AChR antibody titres after thymectomy. Possible steps to circumvent the therapeutical impasse in thymoma cases are discussed, such as combined immunosuppressive therapy along with surgery or conservative treatment for as long as possible.