Myasthenia Gravis

Diagnosis

In addition to the standard tests for myasthenia gravis listed below, Mayo Clinic offers diagnosis capabilities not found at any other medical center, including:

• The largest and most experienced electromyography (EMG) lab in the nation, which performs more than 15,000 studies each year. This experience helps Mayo diagnose myasthenia gravis with as few recordings as possible, diminishing patient discomfort. Additionally, Mayo Clinic routinely performs single-fiber electromyography, a technique that samples one or two muscle fibers rather than the standard 10 to 15. It is difficult to perform, not widely available, and is 95 to 100 percent sensitive in diagnosing myasthenia gravis.
• An internationally known immunologic laboratory, led by Vanda Lennon, M.D., Ph.D., provides serologic tests to identify immune system abnormalities in myasthenia gravis patients. Physicians from all over the world send samples to this laboratory for testing.
• A muscle research laboratory, led by Andrew Engel, M.D., uses specialized tissue, chemical and genetic analysis to diagnose the less-common congenital myasthenic syndromes.

These resources help ensure thorough testing and accurate diagnoses of myasthenia gravis.

Tests

Several tests may be conducted to diagnose myasthenia gravis. Tests can all be completed on the Mayo campus, often over the course of one or two days through Mayo's coordinated and efficient scheduling system.

Neurologic Exam

The key sign is muscle weakness that improves with rest. A physical exam and accurate medical history are important in making a diagnosis. The neurologist may test reflexes, muscle strength and tone, senses of touch and sight, gait, posture, coordination, balance and mental skills.

Edrophonium Test

A chemical (edrophonium) is injected into the bloodstream. Sudden but temporary improvement of muscle strength can indicate myasthenia gravis.

Nerve Conduction Studies and Single-Fiber Electromyography(EMG)

These tests, conducted by the EMG lab, measure the strength of muscle contraction by stimulating nerves with a small electrical impulse applied to the skin. Repeat impulses are applied to test for specific muscle weakness. Then, a thin needle inserted into a muscle measures patterns of electrical activity in a muscle at rest and with slight contraction.

Blood Analysis

A blood test checks for the abnormal antibodies in myasthenia gravis that are markers for autoimmunity to the neuromuscular junction.

Additional testing may be done to look for evidence of myasthenia gravis in specific muscle groups. These tests may include:

• Chest X-ray or CT scan of the chest to check the thymus gland for enlargement or the presence of a tumor in the gland (thymoma)
• Pulmonary function tests to determine whether respiratory muscles are affected by myasthenia gravis, and, if so, to what degree
• Eye exam to rule out other causes of double vision and drooping eyelids