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Tests and diagnosis
By Mayo Clinic staffThe key sign that points to the possibility of myasthenia gravis is muscle weakness that improves with rest. Tests to help confirm the diagnosis may include:
Edrophonium test
Injection of the chemical edrophonium (Tensilon) may result in a sudden, although temporary, improvement in your muscle strength — an indication that you may have myasthenia gravis. Edrophonium acts to block an enzyme that breaks down acetylcholine, the chemical that transmits signals from your nerve endings to your muscle receptor sites.
Blood analysis
A blood test may reveal the presence of abnormal antibodies that disrupt the receptor sites where nerve impulses signal your muscles to move.
Repetitive nerve stimulation
This is a type of nerve conduction study, in which electrodes are attached to your skin over the muscles to be tested. Small pulses of electricity are sent through the electrodes to measure the nerve's ability to send a signal to your muscle. To diagnose myasthenia gravis, the nerve will be tested many times to see if its ability to send signals worsens with fatigue.
Single-fiber electromyography (EMG)
Electromyography (EMG) measures the electrical activity traveling between your brain and your muscle. It involves inserting a very fine wire electrode through your skin and into a muscle. In single-fiber EMGs, a single muscle fiber is tested. Most people find this test to be somewhat uncomfortable.
Imaging scans
Your doctor may order a CT scan or an MRI to see if there's a tumor or other abnormality in your thymus.
