The Epidemiology of Myasthenia Gravis

Lawrence H. Phillips II, M.D.

Disclosures

Semin Neurol. 2004;24(1) 

In This Article

New or Evolving Trends in the Epidemiology of MG

Although classic epidemiological research continues to be valuable, newer approaches and techniques are providing additional insight into the clinical spectrum of manifestations of MG. New trends are emerging, and more analytic techniques are necessary to appreciate them.

For example, the population as a whole is becoming older. The incidence and prevalence of MG have been shown to be substantially higher in patients over the age of 55.[5,8] There is some feeling among experienced clinicians that the natural history of the disease may differ in older individuals. There may be more cases of disease limited to the ocular muscles (class I MG in the Myasthenia Gravis Foundation of America classification scheme)[9] in patients who develop the disease later in life. No studies have looked at this directly as yet, but a recent publication by Vincent and colleagues indicated that there is underdiagnosis of MG in older patients.[10] Their data were obtained in an innovative way through looking at the rate of positive acetylcholine receptor antibody assays in the population of the United Kingdom. A steady increase in the incidence of positive assays in patients up to the age of 75 was observed, but a steep drop-off was observed after the age of 80. This pattern could be explained by two characteristics of the disease. The first is that the rate of antibody positivity is much lower in class I patients, and the second is that MG is often harder to detect in patients who have ocular-only disease. There is no reason to presume that the rising trend in incidence should stop at age 75; thus, one would expect the trend to continue into the oldest age groups. The drop-off in older age groups observed in this study may well be related to a higher proportion of ocular-only disease in the older segment of the population.

Another emerging concept is that there are subtypes of MG that have clinical and immunological differences from the standard form of the disease. For example, patients who present with weakness in distal extremity muscles have been described.[11,12] In addition to a different pattern of muscle weakness, these patients often have different electrophysiological and immunological findings. Some patients with this form of MG have been given the mistaken diagnosis of peripheral neuropathy due to the atypical distribution of muscle weakness.[12] At least one study, from Austria, has looked at how common this form of MG is in a population.[13] There were 6 of 84 MG patients, or 7%, who had either distal onset of disease or predominance of weakness in distal muscles. This number, although small, is large enough to suggest that we should be looking more diligently for such cases among our patients who present with symptoms of distal weakness.

The patients who have generalized MG but lack acetylcholine receptor antibodies in their serum (or seronegative MG) have presented a special problem in diagnosis and treatment. New observations that some patients with seronegative MG have antibodies directed against muscle specific tyrosine kinase have broadened our understanding of another subset of patients.[14,15] The manifestations of the disease in these patients differs in some important respects from more typical MG. The distribution of weakness, age distribution of affected patients, and response to treatment interventions may all differ in significant ways from the acetylcholine receptor antibody-related disease. The incidence and prevalence of this myasthenic disease have yet to be defined, and a more complete picture of the full clinical spectrum of the disease is not known.

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