Myasthenia gravis “could be underdiagnosed in the elderly,” according to Angela Vincent, MD, Professor in the Department of Clinical Neurology at the University of Oxford, United Kingdom. She based that assertion on her new two-part study showing steep rises in the rate of myasthenia gravis in 45- to 79-year-olds that reached 4.8 per 100,000 among women and 9.9 per 100,000 among men in that age-group (the overall rate was 1.8 per 100,000).

That those rates fell off sharply after age 80 strongly suggests that physicians are missing the diagnosis in very old patients, Professor Vincent told NEUROLOGY REVIEWS. So does the fact that the 75-and-older age-group was more likely than younger persons to test positive for the acetylcholine receptor (AChR) antibodies present in myasthenia gravis.

IT TAKES TWO

In the first part of the study, Professor Vincent and colleagues estimated the age- and sex-specific incidence of myasthenia gravis among 3,183 people who tested positive for AChR antibodies between 1997 and 1999. Twelve of the AChR-positive individuals were excluded from the study because they were infants with neonatal myasthenia gravis.

When formulating their myasthenia gravis incidence estimates, the investigators assumed that each AChR-positive person in the study represented a myasthenia gravis case and that, together, these cases accurately reflected the total myasthenia gravis incidence among UK adults. Such assumptions are reasonable, they argued, because the AChR antibody assay is highly specific and rarely requested without clinical suspicion of myasthenia gravis. Also, it is routinely used to confirm clinical diagnoses of myasthenia gravis.

In the second part of the study, the investigators determined the prevalence of positive AChR antibodies among 2,000 Oxford-area residents 60 and older. Most of these subjects were identified through the Oxford Healthy Aging Project of England’s Medical Research Council. However, 351 were outpatients treated for nonneurological complaints from late 1999 to early 2000.

MYASTHENIA GRAVIS OR STROKE?

The results of both parts of the study were published jointly in the August Journal of Neurology, Neurosurgery, and Psychiatry. As expected, in the 15-to-45 age-group, part I of the study showed that the proportion of myasthenia gravis cases was higher among females than males, the researchers reported. After age 45, males were just as likely to develop myasthenia gravis as females, and over time their incidence of the disease became far greater, even after age 80 when the incidence dropped sharply for both sexes, they noted.

Of the Oxford-area residents who participated in part II of the study, only one (0.12%) of the 853 persons in the 60-to-74 age-group tested positive for AChR antibodies versus eight (0.7%) of the 1,147 in the 75-and- older age-group. “All but one of the [AChR] values were at the lower end of the range, but all were compatible with a clinical diagnosis of myasthenia gravis,” noted the investigators.

Despite the 75-and-older group’s greater chance of testing positive for AChR antibodies, only one person in that group was diagnosed with myasthenia gravis. “Notably, however, four of them had a history of stroke or transient ischemic attacks,” the investigators reported.

That is understandable, since myasthenia gravis has a number of symptoms in common with stroke—including fatigue, muscle weakness, slurred speech, and difficulty swallowing. “Doctors should think of myasthenia gravis when a patient complains of these symptoms, irrespective of the age of the patient,” Professor Vincent advised.

NR

—Timothy Begany

Suggested Reading
Aragones JM, Bolibar I, Bonfill X, et al. Myasthenia gravis: a higher than expected incidence in the elderly. Neurology. 2003;60:1024-1026.
Vincent A, Clover L, Buckley C, et al. Evidence of underdiagnosis of myasthenia gravis in older people. J Neurol Neurosurg Psychiatry. 2003;74:1105-1108.

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