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Myasthenia gravis |
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What is myasthenia gravis
Myasthenia gravis is a chronic autoimmune
neuromuscular disease characterized by varying degrees of weakness
of the skeletal (voluntary) muscles of the body. The name
myasthenia gravis, which is Latin and Greek in origin, literally
means "grave muscle weakness." With current therapies, however,
most cases of myasthenia gravis are not as "grave" as the name
implies. In fact, for the majority of individuals with myasthenia
gravis, life expectancy is not lessened by the disorder.
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 may also be
affected.
What causes myasthenia
gravis?
Myasthenia gravis is caused by a defect in the transmission of
nerve impulses to muscles. It occurs when normal communication
between the nerve and muscle is interrupted at the neuromuscular
junction - the place where nerve cells connect with the muscles
they control. Normally when impulses travel down the nerve, the
nerve endings release a neurotransmitter substance called
acetylcholine. Acetylcholine travels through the neuromuscular
junction and binds to acetylcholine receptors which are activated
and generate a muscle contraction.
In myasthenia gravis, antibodies block, alter, or destroy the
receptors for acetylcholine at the neuromuscular junction which
prevents the muscle contraction from occurring. Individuals with
seronegative myasthenia gravis have no antibodies at all to
receptors for acetylcholine and muscle-specific kinase, which is
involved in cell signaling and the formation of the neuromuscular
junction. These antibodies are produced by the body's own immune
system. Thus, myasthenia gravis is an autoimmune disease because
the immune system - which normally protects the body from foreign
organisms - mistakenly attacks itself.
How is myasthenia gravis
treated?
myasthenia gravis can be controlled. There are several therapies
available to help reduce and improve muscle weakness. Medications
used to treat the disorder include anticholinesterase agents such
as neostigmine and pyridostigmine, which help improve
neuromuscular transmission and increase muscle strength.
Immunosuppressive drugs such as prednisone, cyclosporine, and
azathioprine may also be used. These medications improve muscle
strength by suppressing the production of abnormal antibodies.
They must be used with careful medical follow up because they may
cause major side effects.
Thymectomy, the surgical removal of the thymus gland (which often
is abnormal in myasthenia gravis patients), reduces symptoms in
more than 70 percent of patients without thymoma and may cure some
individuals, possibly by re-balancing the immune system. Other
therapies used to treat myasthenia gravis include plasmapheresis,
a procedure in which abnormal antibodies are removed from the
blood, and high-dose intravenous immune globulin, which
temporarily modifies the immune system and provides the body with
normal antibodies from donated blood. These therapies may be used
to help individuals during especially difficult periods of
weakness. A neurologist will determine which treatment option is
best for each individual depending on the severity of the
weakness, which muscles are affected, and the individual's age and
other associated medical problems.
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