Reaching a Diagnosis of Myasthenia Gravis

Tests and Techniques Neurologists Use to Diagnose Myasthenia Gravis

Hospital Patient
Hospital Patient. Adam Berry / Stringer / Getty Images

Myasthenia gravis causes muscle weakness that in very rare cases can become life-threatening. The disease occurs when the body's own immune system attacks receptors for the neurotransmitter acetylcholine, where the nerve tells muscles to contract (the neuromuscular junction). Fortunately, good treatments are available to control the symptoms. First, though, the diagnosis has to be confirmed.

Diagnosing myasthenia gravis usually begins with a careful history and physical performed by a neurologist.

During the physical exam, a neurologis is looking for a key sign that raises suspicion for myasthenia gravis: muscle weakness that improves with rest. 

Tests to Reach a Diagnosis of Myasthenia Gravis

Edrophonium test: The chemical edrophonium chloride (Tensilon) is injected, and patients with myasthenia gravis may experience a sudden, but temporary, improvement in muscle strength. This medication reduces the natural metabolism of acetylcholine, increasing the level of the neurotransmitter at the neuromuscular junction, and briefly overcoming the weakness caused by the myasthenia gravis. 

Ice pack test: An icepack is held on closed eyelids for two minutes. Improvement of droopy eyelids suggests a diagnosis of myasthenia gravis.

Serum antibodies: A very large proportion of people with myasthenia gravis have antibodies targeting receptors on the muscle. Most of these are against the acetylcholine receptor (AChR), with a smaller proportion against muscle specific tyrosine kinase (MuSK).

Testing for these antibodies can help confirm the diagnosis. If these are negative, other antibodies such as titin, the ryanodine receptor, and anti-striated muscle antibodies may also be checked, but the significance of these antibodies is still uncertain. It must also be remembered that a small portion of people with myasthenia gravis have no antibodies that can be found.

Electrodiagnostic tests: Electromyography (EMG) and nerve conduction studies (NCSs) can help both to confirm the diagnosis of myasthenia gravis and help rule out other possibilities like motor neuron disease or Lambert-Eaton syndrome.

Techniques Used to Diagnose Myasthenia Gravis

In addition to the regular testing, there are specific techniques that can hone in on a diagnosis of myasthenia gravis:

Repetitive nerve stimulation test: A nerve receives a short series of electrical shocks, and the strength of the muscle response is measured each time. In myasthenia gravis, the strength of the muscle decreases over time as the muscle fatigues. 

Single-fiber electromyographyA more technically demanding method of electrodiagnostic evaluation, this technique is less widely available but it is the most sensitive technique used to diagnose myasthenia gravis. In this test, a thin, specialized needle measures electrical activity in just two muscle fibers. This test finds positive results in more than 95 percent of those with generalized myasthenia. However, it can also be abnormal in other diseases, including motor neuron disease, polio, peripheral neuropathy, and Lambert-Eaton syndrome.

Testing for Other Causes of Weakness and Comorbid Diseases

In addition to these tests which look more specifically for myasthenia gravis, additional tests may be done to rule out mimics of myasthenia gravis, as well as to investigate for other autoimmune diseases that frequently accompany myasthenia.

For example, the thymus should be evaluated using a chest CT or MRI scan after the diagnosis of myasthenia gravis has been established, as thymic tumors occur more frequently in people with myasthenia gravis.

Once the test of myasthenia gravis has been established, there are various treatments available. Because these treatments are not without some risk of potential side effects, though, it's important to make sure the correct disease is being targeted for therapy. It's equally important to ensure that no additional disease is present that could additionally endanger someone weakened by this autoimmune disorder.


Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.

Braunwald E, Fauci ES, et al. Harrison's Principles of Internal Medicine. 16th ed. 2005.

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