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Myasthenia Gravis

Diagnosis

Physicians may suspect myasthenia gravis in anyone with characteristic weakness. Once suspected, various tests may be ordered to confirm the diagnosis.

Ice test. This is a quick test that does not require special equipment and can be performed in the physician’s office. After covering the patient’s eye with an icepack for a couple of minutes, the physician will look for improvement in eyelid drooping. Any improvement may point toward a myasthenia gravis diagnosis.

Acetylcholinesterase inhibition. Because acetylcholine receptors are blocked in myasthenia gravis, drugs that increase the amount of acetylcholine can be used to test for the disease. Edrophonium is a fast-acting acetylcholinesterase inhibitor that, when administered intravenously, will produce immediate and temporary relief of muscle weakness in myasthenia gravis patients by sparing existing acetylcholine. Edrophonium acts quickly (within 30 seconds) and lasts for about five minutes.

Immunohistochemistry of blood. Antiacetylcholine receptor antibodies are detectable in the serum of about 85 percent of people with myasthenia gravis. However, they are present in only about 50 percent of people with symptoms confined to eye muscles (Kasper 2005). More recently, antibodies to muscle-specific kinase (MuSK) have been discovered in about 70 percent of patients who test negative for antiacetylcholine receptor antibodies, yet suffer from classic symptoms of myasthenia gravis (Hoch 2001). MuSK is a protein that helps organize acetylcholine receptors on the muscle cell surface, and this test is emerging as a helpful diagnostic tool if no autoantibodies are detected when symptoms of the disease are present.

Electrophysiological studies. Nerve conduction studies may be used to detect muscle responses to mild stimuli. Patients with myasthenia gravis will demonstrate progressively smaller or weaker responses. Although this is the most specific nerve test for myasthenia gravis, it is not indicative in all cases and can be performed only on certain muscles.

 

Thyroid tests and thymic imaging. A number of tests might be conducted to assess the health of the thyroid and thymus glands. These include a computed tomography chest scan to reveal a thymoma or enlarged thymus gland and thyroid function tests to detect hyperthyroidism.