The Epidemiology of Myasthenia Gravis
Myasthenia gravis (MG) has been the subject of more than 50 population-based epidemiological studies since the 1950s. The majority of the populations surveyed have been European or North American, but increasing numbers of studies are being done in other geographic areas. The occurrence of the disease has been rather uniform throughout, and a pattern of increasing incidence and prevalence has been reported.[2,3] Since these two publications, additional studies have continued to appear, indicating ongoing research interest in the disease.[4,5,6]
The published data over the past 50 years have indicated that MG is a rare disease. The highest reported prevalence rate for the disease is 20.4 per 100,000 population. To put this into context, one can envision a large assembly of people, such as a typical crowd at a college football stadium. If there are 100,000 in attendance, you would have to examine an average of 5000 individuals to discover one with MG. Of course cases are not evenly distributed in a population, but we have estimated that the typical neurologist might expect to encounter a new case of MG every 3 to 4 years. A typical neurologist's practice would likely contain no more than two to four patients with MG at a time.
The reported prevalence of the disease has increased in every decade since the 1950s.[2,3] The prevalence of the disease was at least four times higher in the 1990s than it was in the 1950s. The increase in the number of patients with MG can be attributed to several factors including improved recognition of the disease, the availability of diagnostic tests with higher sensitivity and specificity, longer life span in affected patients due to more effective treatment, and an increase in the population at risk due to aging. Physicians can certainly take some credit for changing the natural history of MG to the extent that some no longer consider the name of the disease to be especially accurate. The "gravis" part of the name, when it was coined in the early 1900s, was meant to denote the severe, life-threatening nature of the disease. The disease-specific mortality of the disease is now a fraction of what it was in the early part of the last century.
Even though there has been an enormous amount of MG-related research, treatment-specific studies have lagged behind other diseases. There have been relatively few clinical trials of drugs specific for MG. This is partly attributable to the fact that MG is a rare disease. Based on epidemiological data, we can estimate that the number of patients with MG in the United States is ~60,000. Industry-sponsored research in drug development has become increasingly costly, and pharmaceutical companies tend to consider the size of the potential market for new drugs they are trying to develop. The result for development and testing of drugs for MG has been that new initiatives specific for the disease have been sparse. Instead, most treatment innovations in MG have been "borrowed" from treatments designed for other diseases. For example, all of the immunosuppressive therapies now in use for MG have been used first for treatment of other diseases. After they have been shown to be efficacious for suppressing transplant rejection or treatment of other autoimmune diseases, the drugs have been prescribed in myasthenic patients for off-label use. This situation is unlikely to change in the foreseeable future, although some industry-sponsored research is being done.
Semin Neurol. 2004;24(1) © 2004 Thieme Medical Publishers
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