Myasthenia gravis - Diagnosing myasthenia gravis

Confirming a diagnosis of myasthenia gravis can take a long time because the condition is rare and muscle weakness can be a symptom of many conditions.

Your GP will look at your medical history and your symptoms. They may suspect myasthenia gravis if your eye movements are impaired or you have muscle weakness but are still able to feel things.

However, as these symptoms are common in a number of different conditions, you will need to have some tests before a diagnosis of myasthenia gravis can be confirmed.

If your GP thinks you may have myasthenia gravis, they will refer you to a neurologist (specialist in nervous system disorders). They will carry out some tests that will confirm whether or not you have the condition.

Tests

There are a number of tests that can be used to help diagnose myasthenia gravis. 

Blood test

A special type of blood test can be used to detect the antibodies (proteins) that block or damage the muscle receptors. Most people with myasthenia gravis have an abnormally high number of these antibodies.

The antibody blood tests are quite specific, which means that these antibodies are almost never detected in patients who do not have myasthenia gravis.

However, in people whose symptoms are limited to the eyes, (ocular myasthenia), high levels of antibodies are not always present.

Edrophonium test

An edrophonium test involves having an injection of a type of medication called edrophonium chloride. Edrophonium chloride prevents the substance acetylcholine from being broken down, which temporarily increases the amount of acetylcholine around the muscle.

In people with myasthenia gravis, the increased amount of acetylcholine produces a sudden but temporary improvement in muscle power. However, this will not usually occur in people with other causes of muscle weakness.

There are significant side effects associated with the edrophonium test, such as heart rate and breathing problems, that may occur during the investigation.

Therefore, the test should only be carried out if myasthenia gravis is still suspected despite negative blood and electrical tests. The test should only be carried out by experienced neurology doctors in specialist centres.

Nerve conduction tests

Electromyography is a procedure that can be used to identify communication problems between the nerves and muscles. It involves inserting a needle electrode through the skin into the muscle.

This produces an electrical recording of the muscle activity. If you have myasthenia gravis, your muscles will not respond very well to the electrical stimulation.

A nerve conduction study can also be used, which uses repeated nerve stimulation to test for muscle fatigue (weakness). In myasthenia gravis, the transmission of signals between the nerve and muscle will be poor.

Single-fibre EMG is the most sensitive electrical test for detecting disruption of the signal between the nerve ending and the muscle membrane (as in myasthenia gravis). It usually involves taking a recording from a very small needle in one of the muscles around the eye, forehead, or sometimes in the forearm.