Myasthenia Gravis

Make the Diagnosis: Myasthenia Gravis

Prior Probability

Myasthenia gravis is a rare disease. The prevalence in the United States is reported at approximately 14.1 in 100 000.^{1, 2, and 3} Men older than 50 years have the highest incidence, with the peak at approximately aged 70 years. Women have 2 incidence peaks: one at approximately aged 20 to 40 years and one at approximately aged 70 years.^{4, 5} The prior probability of myasthenia gravis in the general population among patients presenting with symptoms is unknown. Because of the high prevalence of the disease in the included studies (close to 50%), the results may not be generalizable to the general population.

Population for Whom Myasthenia Gravis Could Be Considered

• Patients with asymmetric fluctuating eyelid ptosis

• Patients with extraocular dysmotility not referable to a single nerve

• Patients with weakness of other specific muscles

• Young women of childbearing age and men and women aged approximately 70 years

Detecting the Likelihood of Myasthenia Gravis

The clinical findings, when applied to the correct patient population, are important (Table 34-3).

Reference Standard Tests

The reference standard for definite myasthenia gravis is typical clinical presentation plus one of the following: elevated acetylcholine receptor antibody level or abnormal electrodiagnostic study results (repetitive nerve stimulation or single-fiber electromyography). These criteria should also be fulfilled in clinical practice for definite diagnosis.

Original Article: Does This Patient Have Myasthenia Gravis?

Clinical Scenarios

Case 1

A 45-year-old man has a 2-month history of fluctuating double vision, a droopy right eye that improves with ...