How is myasthenia gravis diagnosed?
Myasthenia gravis is often diagnosed accurately by a careful medical history and a neuromuscular exam, but several tests are used to confirm the diagnosis. Other conditions causing worsening of bulbar and skeletal muscles must be considered, including drug-induced myasthenia, thyroid disease, Lambert-Eaton myasthenic syndrome, botulism, and
inherited muscular dystrophies.
Myasthenia gravis causes characteristic changes in the electrical responses of muscles that may be observed with an electromyogram, which measures muscular response to electrical stimulation. Repetitive nerve stimulation leads to reduction in the height of the measured muscle response, reflecting the muscle's tendency to become fatigued.
Blood tests may confirm the presence of the antibody to the acetylcholine receptor, though up to a quarter of myasthenia gravis patients will not have detectable levels. A chest x ray or chest
Another test is called the edrophonium test. This approach requires the intravenous administration of edrophonium chloride or Tensilon®, a drug that blocks the breakdown of acetylcholine and temporarily increases the levels of acetylcholine at the neuromuscular junction. In people with myasthenia gravis involving the eye muscles, edrophonium chloride will briefly relieve weakness. Other methods to confirm the diagnosis include a version of nerve conduction study which tests for specific muscle fatigue by repetitive nerve stimulation. This test records weakening muscle responses when the nerves are repetitively stimulated, and helps to differentiate nerve disorders from muscle disorders. Repetitive stimulation of a nerve during a nerve conduction study may demonstrate decrements of the muscle action potential due to impaired nerve-to-muscle transmission.