The Rational Clinical Examination
David L. Simel, Drummond Rennie
Myasthenia Gravis
Katalin Scherer, Richard S. Bedlack, David L. Simel
Although patients rarely complain of facial weakness, it is often found on examination. Severe...
Topics Discussed:
acetylcholine receptor antibody, asthenia, curtain sign, diagnostic process, edrophonium, myasthenia gravis, neuromuscular junction, peek sign
Excerpt:
"Myasthenia gravis is an autoimmune disease associated with circulating
acetylcholine receptor antibodies, modification of the synaptic
cleft, and destruction of the postsynaptic neuromuscular membrane.
The clinical hallmark of the disease is fatigable weakness. The
clinical severity ranges from mild, purely ocular, forms to severe
generalized weakness and respiratory failure. Myasthenia gravis
is a rare disease; its prevalence in the United States is reported
at 14.2 in 100 000. Prevalence rates have been increasing steadily
during the past decades, likely because of decreased mortality,
longer survival, and higher rates of diagnosis.1-3 Men
older than 50 years have the highest incidence in the population,
with the peak at approximately aged 70 years. Women have 2 incidence
peaks: one at approximately aged 20 to 40 years and one at approximately
aged 70 years.4,5In the normal neuromuscular junction, acetylcholine is released
into the synaptic cleft, diffuses to the postsynaptic membrane,
binds to ligand-sensitive ion channels (nicotinic acetylcholine
receptors), and causes an excitatory postsynaptic end-plate potential.
If the threshold depolarization is achieved, an action potential
will spread along the muscle fiber membrane, causing muscle contraction.
Acetylcholine is cleared from the synaptic cleft by presynaptic
reuptake and by the metabolic action of acetylcholinesterase (Figure 34-1)...."
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