The primary test to confirm myasthenia gravis is a blood test looking for specific antibodies that disrupt communication between nerves and muscles. However, other tests can also aid in diagnosis.
Here's a more detailed breakdown:
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Antibody Testing: This is the most common initial test.
- Acetylcholine receptor (AChR) antibodies: These antibodies are found in the majority (around 80-90%) of people with generalized myasthenia gravis. They block or destroy acetylcholine receptors at the neuromuscular junction, preventing muscle contraction.
- Muscle-Specific Kinase (MuSK) antibodies: If AChR antibodies are negative, a MuSK antibody test is performed. MuSK antibodies are found in a smaller percentage (around 40%) of people with generalized myasthenia gravis who are AChR antibody-negative. MuSK is crucial for the formation and maintenance of the neuromuscular junction.
- LRP4 Antibodies: In some cases where both AChR and MuSK antibodies are negative, testing for LRP4 antibodies may be considered.
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Edrophonium (Tensilon) Test: This test involves injecting edrophonium, a medication that temporarily blocks the breakdown of acetylcholine. If muscle weakness improves noticeably after the injection, it suggests myasthenia gravis. However, this test is used less frequently now due to potential side effects and the availability of more specific antibody tests.
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Nerve Conduction Studies and Repetitive Nerve Stimulation (RNS): These tests assess how well nerves transmit electrical signals to muscles. In myasthenia gravis, repetitive nerve stimulation often shows a characteristic decline in muscle response.
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Single-Fiber Electromyography (SFEMG): This is the most sensitive electrodiagnostic test for myasthenia gravis. It examines the electrical activity of individual muscle fibers and can detect subtle abnormalities in neuromuscular transmission.
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Imaging Studies (CT or MRI): These may be performed to evaluate the thymus gland, as thymoma (a tumor of the thymus) is associated with myasthenia gravis.
In summary, while a blood test for AChR or MuSK antibodies is often the first step, a combination of tests is usually required to definitively diagnose myasthenia gravis.
