Diagnosis of Myasthenia Gravis

In patients with the characteristic clinical features and history, it may be fairly simple to make a diagnosis of myasthenia gravis on clinical grounds. However, several conditions may mimic autoimmune myasthenia gravis, and these include other neuromuscular junction disorders (Botulism, Eaton Lambert syndrome, drug induced myasthenia gravis etc), metabolic and toxic myopathies and brain stem diseases. As such, an accurate diagnosis prior to initiating long-term therapy for myasthenia gravis is crucial.

Serologic testing for specific antibodies, clinical electrophysiology and edrophonium chloride (Tensilon) tests are available. If doubt still remains or specialized facilities are not available, the clinical response to anticholinesterase medication may be adopted. The available diagnostic and other complementary techniques are further elaborated:

 

Anti-Acetylcholine Receptor Antibody Test.

 

This is a blood test that has a high specificity. It is also relatively sensitive with 85% sensitivity in generalized MG and 60% sensitivity in ocular MG. It may not be a readily available test at some centres.

Edrophonium (Tensilon) Test

 

This is a short acting drug that is administered intravenously. It is useful to assess objectively weak muscles which show a transient improvement following the drug adminstration. However, there are potential risks of hypotension and bradycardia.

 

Electrophysiological Tests

 

These are tests that assess the neuromuscular junction and include repetitive nerve stimulation and single fibre electromyography. The latter in particular is highly sensitive but can also be abnormal in other neuromuscular conditions. In addition, they may not be readily performed in some centres.

 

Ice Pack Test

 

This test involves the application of a cold pack on a weak muscle typically the droopy eyelids. Although sensitive (approximately 80%) for MG, the assessment is quite subjective.

Chest Imaging

This test is performed on all patients with MG to look for evidence of thymic enlargement which is sometimes associated in MG patients.

Pulmonary Function Testing

 

This test may be performed in MG patients who describe generalized symptoms, in particular any breathing symptoms. They may also be performed prior to any operative procedures to assess the lung function.

 

 4.1  Anti-Acetylcholine Receptor Antibody Test.

A special blood test that detect the presence of acetylcholine receptor antibodies. Most patients with myasthenia gravis have abnormally elevated levels of these antibodies, (approximately 85% of generalized myasthenia gravis and 60% of patients with ocular myasthenia gravis).This test is highly specific for MG.

 4.2  Edrophonium (Tensilon) Test

This approach requires the intravenous administration of edrophonium chloride or Tensilon(r), a drug that blocks the breakdown of acetylcholine and temporarily increases the levels of acetylcholine at the neuromuscular junction. The test is considered positive if there is a definite improvement in a documented weakness. However, the test can be positive in a variety of other conditions such as amyotrophic lateral sclerosis, poliomyelitis, peripheral neuropathies, brain stem lesions and even in normal people. In addition, there is a low risk of hypotension and bradycardia, and the test should only be performed when the diagnosis of myasthenia gravis is urgently needed.

 4.3  Repetitive Nerve Stimulation Test

This is an electrophysiological test that evaluates for specific muscle "fatigue" by repetitive nerve stimulation. This study records weakening muscle responses when the nerves are repetitively stimulated. Repetitive stimulation of a nerve may demonstrate decrements of the muscle action potential due to impaired nerve-to-muscle transmission. This test has a 65% sensitivity in generalized myasthenia gravis, but may be negative in ocular myasthenia gravis.

 4.4  Stimulated Single Fibre Electromyography

This is another form of an electrophysiological test, in which single muscle fibers are stimulated by electrical impulses and the action potentials generated are recorded with a fine electrode, allowing detection of impaired nerve-to-muscle transmissions. Muscle fibers in myasthenia gravis, as well as other neuromuscular disorders, do not respond as well to repeat electrical stimulation as compared to muscles from normal individuals. This is the most sensitive test for myasthenia gravis (approximately 95%) but false positives can occur, such as in peripheral neuropathies or motor neuron disease.

 4.5  Ice Pack Test

This is a test that can be utilized when ptosis is present, and consists of an application of an ice pack to the eyelid of the affected eyes.  Improvements in muscle weakness have been reported in 80% of cases. However, this test is somewhat subjective, and is rarely performed.

 4.6  Chest Imaging

Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) diagnostic imaging modalities might be utilized to identify an abnormal thymus gland or the presence of a thymoma in patients with myasthenia gravis, or used to assess for residual or recurrent growths in patients with prior thymectomy but who deteriorate unexpectedly. This in itself is not a modality for the evaluation for myasthenia gravis, but to look for associated thymic complications.

 4.7  Pulmonary Function Testing

Pulmonary function testing measures the breathing strength and helps to predict whether respiration may fail and lead to a myasthenic crisis.