Myasthenia gravis

Myasthenia gravis (MG, Latin: "grave muscle weakness") is a neuromuscular disease leading to fluctuating weakness and fatiguability. It is one of the best known autoimmune disorders and the antigens and disease mechanisms have well been identified. Weakness is caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppression and cholinesterase inhibitors.

Diagnosis

Myasthenia can be a difficult diagnosis, as the symptoms can be subtle and hard to distinguish from both normal variants and other neurological disorders (Scherer et al 2005).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A thorough physical examination can reveal easy fatiguability, with the weakness improving after rest and worsening again on repeat of the exertion testing. Applying ice to the weak muscle groups may characteristically improve the weakness.

Related Topics:
Physical examination - Ice

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Blood tests

If the diagnosis is suspected, serology can be performed in a blood test to identify antibodies against the acetylcholine receptor. The test has a reasonable sensitivity of 80-96%, but in MG limited to the eye muscles (ocular myasthenia) the test may be negative in up to 50% of the cases. Often, parallel testing is performed for Lambert-Eaton myasthenic syndrome, in which other antibodies (against a voltage-gated calcium channel) are frequently found.

Related Topics:
Serology - Blood test - Antibodies - Acetylcholine receptor - Sensitivity - Lambert-Eaton myasthenic syndrome - Calcium channel

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Edrophonium test

The "edrophonium test" is frequently performed to identify MG. This test requires the intravenous administration of edrophonium chloride (Tensilon®), a drug that blocks the breakdown of acetylcholine by cholinesterase and temporarily increases the levels of acetylcholine at the neuromuscular junction. In people with myasthenia gravis involving the eye muscles, edrophonium chloride will briefly relieve weakness. Other methods to confirm the diagnosis include a version of nerve conduction study which tests for specific muscle fatigue by repetitive nerve stimulation. This test records weakening muscle responses when the nerves are repetitively stimulated, and helps to differentiate nerve disorders from muscle disorders. Repetitive stimulation of a nerve during a nerve conduction study may demonstrate decrements of the muscle action potential due to impaired nerve-to-muscle transmission.

Related Topics:
Intravenous - Edrophonium chloride - Cholinesterase - Neuromuscular junction - Muscle fatigue

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Single-fiber electromyography

A different test called single fiber electromyography (EMG), in which single muscle fibers are stimulated by electrical impulses, can also detect impaired nerve-to-muscle transmission. EMG measures the electrical potential of muscle cells. Muscle fibers in myasthenia gravis, as well as other neuromuscular disorders, do not respond as well to repeated electrical stimulation compared to muscles from normal individuals.

Related Topics:
Electromyography - Electrical potential

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Imaging

A chest X-ray is frequently performed; it may point towards alternative diagnoses (e.g. Lambert-Eaton due to a lung tumor) and may also identify widening of the mediastinum suggestive of thymoma. Performing computed tomography (CT) or magnetic resonance imaging (MRI) is a more senstive way to identify thymomas.

Related Topics:
Chest X-ray - Mediastinum - Thymoma - Computed tomography - Magnetic resonance imaging

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Pulmonary function test

Spirometry (lung function testing) is performed to assess respiratory function. Severe myasthenia (myasthenic crisis) may cause respiratory failure due to exhaustion of the respiratory muscles.

Related Topics:
Spirometry - Respiratory failure

~ ~ ~ ~ ~ ~ ~ ~ ~ ~