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Published online by Cambridge University Press: 07 September 2009
Myasthenia gravis
Myasthenia gravis is a neuromuscular disorder characterised by weakness and fatigability of voluntary muscles. The weakness is exacerbated by effort and improved by rest and it affects, in order of decreasing frequency, the ocular, bulbar, neck, limb, girdle, distal limb and trunk muscles.
It is a classic autoimmune disease marked by the presence of heterogeneous acetylcholine receptor antibody (IgG) in approximately 90% of symptomatic patients. The antibodies react with the receptor, block its action and accelerate receptor degradation. As a result, fewer receptors can be activated causing muscle weakness.
Diagnosis
Clinical suspicion and the finding of skeletal-muscle fatigability with repetitive exercise will support the diagnosis. The diagnosis can be confirmed by complete reversibility of muscle fatigue after IV administration of the rapidly acting anticholinesterase drug edrophonium (5–10 mg IV over 1 minute, should produce an effect within 10 minutes).
Electrophysiological testing will demonstrate progressive decline in muscle action potentials with repetitive stimulation of a motor nerve.
Treatment
Definitive treatment aims to reduce antibody production and/or increase the effect of unaffected acetylcholine receptors.
Anticholinesterases
The longer-acting anticholinesterase, pyridostigmine, is titrated against patient response using a starting dosage of 60 mg orally four times daily. Excessive use of anticholinesterases can cause a cholinergic crisis, with progressive muscle weakness as well as muscarinic effects, such as abdominal colic, diarrhoea, small pupils, lachrymation and excessive salivation.
Thymectomy
Among all patients with myasthenia gravis, 75% have thymic abnormalities. Most have thymic hyperplasia, but up to 15% have thymomas.
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