Myasthenia Gravis

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Diagnosis

Diagnosis of myasthenia gravis includes the following:

• Blood tests
• Clinical and neurological examination
• Imaging tests (e.g., x-ray, CT scan)
• Intravenous anticholinesterase or Tensilon test
• Medical history
• Neurological tests (e.g., electromyography)

Blood tests are performed to determine serum levels of certain antibodies (e.g., AChR-binding antibodies, AChR-modulating antibodies, antistriational antibodies). High levels of these antibodies may indicate MG.

Neurological examination involves testing muscles and reflexes. MG may cause abnormal eye movements, inability to move the eyes normally, and drooping eyelids. To test arm and leg muscles, the patient may be instructed to maintain a position against resistance for a period of time. Weakness that occurs during this test is called fatigability.

Chest x-ray and computed tomography (CT scan) may be performed to detect enlarged thymus (thymoma), which is common in MG.

The Tensilon test is often used to diagnose MG. The enzyme acetylcholinesterase breaks down acetylcholine (ACh) after the muscle is stimulated, preventing prolonged muscle response to a single nerve impulse. Edrophonium chloride (Tensilon) is a drug that temporarily blocks the action of acetylcholinesterase.

In MG, there are few acetylcholine receptor sites (AChR) on the muscle and acetylcholine is broken down before it can fully stimulate the muscle, resulting in muscle weakness. By blocking the action of acetylcholinesterase, Tensilon prolongs muscle stimulation and temporarily improves strength.

In this test, Tensilon is administered intravenously (into a vein) and muscle response is evaluated. In cases of MG, muscle weakness improves within 1 minute. The Tensilon test is most effective when easily observed weakness is present, and is less useful for vague or fluctuating complaints. Side effects of the test include temporary abnormal heart rhythms such as rapid heart rate (atrial fibrillation) and slow heart rate (bradycardia).

Electromyography (EMG) uses electrodes to stimulate muscles and evaluate muscle function. Muscle contractions that become progressively weaker may indicate MG.

Differential Diagnosis
Disorders that may cause symptoms similar to MG include botulism and Lambert Eaton syndrome. Botulism results from Clostridium botulinum infection, which can also block AChR and cause muscle weakness.

Lambert Eaton syndrome is associated with certain types of lung cancer. This condition produces EMG results that are different than those produced by MG.

Medications can block the neuromuscular junction and cause symptoms similar to MG. These drugs include the following:

• Antibiotics (e.g., ciprofloxacin, erythromycin, ampicillin)
• Antispasmodic drugs (e.g., trihexyphenidyl; used to treat movement disorders)
• Beta-adrenergic receptor blocking agents (e.g., propranolol, timolol)
• Cardiac drugs (e.g., procainamide, verapamil, quinidine)
• Lithium (used to treat bipolar disorder)
• Penicillamine (penicillin product; used to treat metal poisoning)

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