Making the Diagnosis
There's a quick and simple test for myasthenia gravis. Anticholinesterase
medications are used to control symptoms in this disease. One such medication,
edrophonium,* has a very short duration of action (its effects don't last very
long), and is used for diagnosis. A tiny bit is injected to start with. If there
are no problems, the rest is injected. In people with myasthenia gravis, this
will produce a sudden major improvement in muscle strength and control, lasting
about five minutes.
The test is also valuable in people with diagnosed myasthenia because it distinguishes
symptoms due to the disease from similar symptoms that are sometimes caused
by the medications. The drugs given for myasthenia gravis can trigger a cholinergic
crisis, which is characterized by increasing muscle weakness. Edrophonium
will relieve symptoms of myasthenic crisis, but may temporarily worsen a cholinergic
crisis.
Another important diagnostic test is called electromyography (EMG)
and single-fiber EMG. These tests are used to determine whether the nerve-to-muscle
contact is working properly.
Blood tests can be done to detect the anti-acetylcholine antibodies.
A chest X-ray or CAT scan of the chest may be done to look for a thymoma.