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. more...
Signs and symptoms
The hallmark of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Certain muscles such as those that control eye and eyelid movement, facial expression, chewing, talking, and swallowing are often, but not always, involved in the disorder. The muscles that control breathing and neck and limb movements can also be affected.
Although myasthenia gravis may affect any voluntary muscle, muscles that control eye and eyelid movement, facial expression, and swallowing are most frequently affected. The onset of the disorder may be sudden or rapid. Symptoms often are not immediately recognized as myasthenia gravis; a proportion only receives a diagnosis after more than a year.
In most cases, the first noticeable symptom is weakness of the eye muscles. In others, difficulty in swallowing and slurred speech may be the first signs. The degree of muscle weakness involved in myasthenia gravis varies greatly among patients, ranging from a localized form, limited to eye muscles (ocular myasthenia), to a severe or generalized form in which many muscles - sometimes including those that control breathing - are affected. Symptoms, which vary in type and severity, may include asymmetrical ptosis (a drooping of one or both eyelids), diplopia (blurred or double vision) due to weakness of the muscles that control eye movements, unstable or waddling gait, weakness in arms, hands, fingers, legs, and neck, a change in facial expression, dysphagia (difficulty in swallowing) and shortness of breath, and dysarthria (impaired speech, often nasal due to weakness of the pharyngeal muscles).
A myasthenic crisis may give rise to a generalized paralysis, including those of the respiratory muscles, and assisted ventilation may be required to sustain life. In patients whose respiratory muscles are already weak, crises may be triggered by infection, fever, an adverse reaction to medication, or emotional stress (Bedlack & Sanders 2000).
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.
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.
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