Myasthenia gravis diagnosis

Last updated May 16, 2023, by Lindsey Shapiro, PhD
Fact-checked by Joana Carvalho, PhD

Getting a correct diagnosis of the rare autoimmune disease myasthenia gravis (MG) can be challenging, because its symptoms — typically muscle weakness and fatigue — are nonspecific and can be caused by many other disorders. Diagnosing MG may require a series of blood, nerve, muscle, and imaging tests.

In MG, self-reactive antibodies attack the neuromuscular junction, which is the site where nerves and muscles communicate to coordinate voluntary movements. If physical and neurological examinations indicate signs of MG, a blood test to look for these disease-causing antibodies is the main way of definitively diagnosing the neuromuscular disease.

If that test isn’t clear, a doctor may need to order additional tests to rule out other possible diagnoses and establish the presence of MG. Such assessments may include the ice pack or tensilon test, blood tests and imaging scans, and/or nerve and respiratory function tests.

Physical and neurological examination

In the course of determining a myasthenia gravis diagnosis, a doctor will first review the person’s symptoms and medical history. In addition, a thorough physical and neurological exam will be conducted. These tests may include evaluations of reflexes, muscle strength and tone, sensations, coordination, and balance — all areas that often are affected by MG.

The clinician may be looking for a few common myasthenia gravis symptoms:

• Ocular, or eye-related, issues are often among the first manifestations of MG for patients. Such eye symptoms, in some cases, may be the only signs of the disease. Given that, doctors may look for problems with the eye or eyelid movements or vision issues.
• Fluctuating muscle weakness that worsens with fatigue is common among people with MG. For many patients, muscle weakness tends to get worse after periods of activity and improves with rest.
• Shortness of breath is a frequent problem reported by MG patients. When respiratory muscles are affected, individuals with the disorder may have difficulty breathing.
• Problems with muscles of the face and throat commonly occur among people with myasthenia gravis. Such issues can lead to speech impairments, difficulty chewing or swallowing, or changes in facial expressions.

MG patients do not typically show signs of neurological impairment. Thus, if a person appears to have significant neurological dysfunction or impairments, it may be a sign of a different condition. These other conditions can sometimes be ruled out with MRI imaging.

Ice pack test

If patients show signs of eyelid droopiness, called ptosis and a common early symptom of MG, a doctor may perform an ice pack test to determine if the neuromuscular disorder is the cause.

During this test, a doctor places a bag filled with ice on a patient’s droopy eyelid. After a few minutes, the ice is removed and droopiness is re-evaluated. A temporary improvement indicates the possible presence of MG, which will not be observed with other causes of ptosis.

This improvement is thought to occur because cold temperatures reduce the activity of the acetylcholinesterase enzyme that breaks down acetylcholine — the signaling molecule that triggers muscle contraction. By blocking acetylcholinesterase, more acetylcholine is present, increasing muscle contraction and easing eyelid droopiness caused by MG.

Tensilon test

The tensilon test, also called the edrophonium test, used to be a key diagnostic test for MG. The test involved injecting edrophonium chloride, a compound that inhibits the acetylcholinesterase enzyme, to elicit an improvement in muscle strength.

Similarly to the ice pack test, the brief increase in acetylcholine brought on by the injections may temporarily ease disease symptoms. Sudden, temporary improvements in muscle strength after an edrophonium chloride injection signal the presence of MG.

The tensilon test is now used far less frequently and is often replaced by other diagnostic tests that are considered to be safer, such as the ice pack test. Edrophonium can cause a series of side effects, including low heart rate and blood pressure, sweating, nausea, stomach cramping, and muscle twitching. The test has been discontinued in the U.S. since 2018 due to the high rate of false positives and potential side effects.

Blood test

If MG symptoms are present, a blood test to measure the levels of the self-reactive antibodies that cause the disease is the main way of definitively diagnosing the condition.

Antibodies targeting the acetylcholine receptor (AChR) — an important protein for the function of the neuromuscular junction — are the most common type of MG-causing antibodies. Anti-AChR antibodies are found in up to about 85% of patients with generalized MG and 50% of those with ocular MG.

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Blood tests also can detect antibodies against muscle-specific kinase, known as MuSK, or lipoprotein receptor-related protein 4, which are less common causes of MG. Both of these proteins are important for the formation and function of the neuromuscular junction.

If none of these antibodies are detected, but symptoms persist or get worse, a blood test may be repeated, at which time a person may test positive.

A small subset of patients never test positive for either antibody type, and are said to have seronegative MG. For these patients, reaching a definitive diagnosis may require additional nerve and muscle function tests. Seronegative MG is more common in patients whose symptoms are restricted to the eyes (ocular MG) than in those with more widespread muscle weakness (generalized MG).

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Repetitive nerve stimulation test

One way for doctors to assess a nerve cell’s ability to transmit a signal to a muscle is through a repetitive nerve stimulation (RNS) test. With this assessment, an electrode placed on the skin is used to repeatedly stimulate nerves near a specific muscle of interest.

When a nerve’s ability to send a signal to a muscle is diminished over multiple rounds of stimulation, it may indicate the presence of MG. This observation is consistent with the knowledge that MG symptoms tend to worsen with fatigue or activity, and improve after a period of rest.

Abnormal RNS results are more often found in people with generalized MG than in those with ocular MG. An abnormal finding in people with ocular MG may indicate that their disease will progress to a generalized form of MG.

Single fiber electromyography

Considered the most sensitive myasthenia gravis diagnostic test, single fiber electromyography measures an individual muscle fiber’s response to stimulation.

A fine wire electrode is inserted through the skin into a muscle — usually one in the forehead above the eyebrow — where it electrically stimulates a single muscle fiber. In MG, muscle fibers are not as responsive to repeated electrical stimulation as they are in people without the disease.

Because of its sensitivity, this approach can be helpful for diagnosing mild cases of MG when other tests have failed to detect abnormalities. This is particularly the case for patients with ocular MG who may not test positive for disease-causing antibodies.

Although it is very sensitive, abnormal findings on this test can be the result of other disorders involving the neuromuscular junction, and may not be specific to MG.

Imaging scans

Imaging scans, most commonly CTs or MRIs, may be used by clinicians to assess the presence of a tumor or other abnormalities in the thymus gland, which is located in the chest.

Part of the body’s immune system, the thymus is thought to contribute to the production of the harmful self-reactive antibodies that cause MG. Some MG patients have an unusually large thymus or a tumor in the gland, known as a thymoma. In some cases, doctors may recommend surgery to remove the entire gland, a procedure known as thymectomy. For certain patients, this surgery can ease MG symptoms.

An MRI scan also can be used to rule out neurological disorders that may cause similar symptoms to MG.

Imaging scans are noninvasive and can provide a detailed look at almost every internal structure in the human body.

Pulmonary function tests

MG sometimes affects the muscles of the respiratory system, leading to breathing difficulties. Assessments of breathing strength can help predict a person’s risk of respiratory failure and myasthenic crisis.

For some patients with more more severe disease, particularly those with MuSK-associated MG, respiratory symptoms may emerge early in the disease course. Thus, pulmonary function tests can also be used during the diagnostic process.

The most common way to monitor respiratory health is with spirometry, which uses a small device called a spirometer to measure how much air a person can breathe in and out, and how fast a person breathes.

Next steps after a myasthenia gravis diagnosis

After an MG diagnosis, patients will work with their doctors to develop a comprehensive treatment plan that will best suit their individual needs.

In most cases, determining a plan will involve deciding on an appropriate myasthenia gravis treatment, which may include immunosuppressants, steroids, immune complement inhibitors, acetylcholinesterase inhibitors, or a thymectomy.

Intravenous immunoglobulin infusions or plasmapheresis, known as plasma exchange, both aim to lower the levels of disease-causing antibodies in the bloodstream. These treatment strategies may be indicated for patients with more severe or rapidly progressing symptoms.

In addition to deciding with their healthcare providers on appropriate MG treatments, some patients may be encouraged to make certain lifestyle adjustments. Such alterations may include changing one’s diet if and when swallowing becomes more difficult, and developing a regular exercise plan to maintain muscle strength and make daily activities easier.

Learning about disease triggers, such as heat or stress — and how to avoid them — also may help patients better manage their condition.

Across the Myasthenia Gravis News website, information and resources can be found to help people become more familiar with the disease, and learn about available treatments and strategies for living with myasthenia gravis.

Myasthenia Gravis News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.