Encyclopedia of Health and Medicine

myopathy 339
cate muscle destruction. Each type of muscular
dystrophy has fairly characteristic patterns of
symptoms. Muscle biopsy shows damage to the
muscle cells.
Treatment Options and Outlook
Treatment for all types of muscular dystrophy is
primarily supportive. Physical therapy, braces,
orthotics, and mobility aids extend the ability to
walk and function independently. Corticosteroid
medications slow the progression of symptoms in
some types of muscular dystrophy, notably
Duchenne’s. Though all types of muscular dystrophy
are lifelong, muscular dystrophy is not necessarily
fatal. Many people with milder types of the
disease live normal life expectancy with relative
independence.
Risk Factors and Preventive Measures
Muscular dystrophy is always inherited, so the key
risk factor is family history. People who have muscular
dystrophy or family history of muscular dystrophy
should consider GENETIC COUNSELING to aid
in FAMILY PLANNING decisions. There are no measures
known to prevent muscular dystrophy,
though research holds hope for GENE THERAPY that
can someday correct the mutations that cause the
disease.
See also CATARACT; MYOPATHY.
myasthenia gravis A rare autoimmune disorder
in which the IMMUNE SYSTEM produces antibodies
that target acetylcholine receptors on the cell
membrane surfaces of MUSCLE cells. Acetylcholine
is a NEUROTRANSMITTER that carries NERVE impulses
from neurons to muscle cells to initiate movement.
Acetylcholine receptors are specialized molecules
that bind acetylcholine molecules, a process
analogous to plugging an electrical cord into an
outlet. The NEURON releases acetylcholine to carry
the impulse across the synapsis to the muscle cell.
Binding forms a complete circuit and the nerve
impulse passes from the neuron to the muscle cell.
The antibodies present in myasthenia gravis
attack and destroy acetylcholine receptors, reducing
the ability of acetylcholine to carry to completion
the nerve impulses that direct movement.
Because there are fewer acetylcholine receptors in
myasthenia gravis, the acetylcholine molecule the
neuron releases often dissipates before a receptor
becomes available. As a result, muscle contractions
are weak. Muscles that have the fewest
numbers of acetylcholine receptors to begin
with—the muscles of the eyelids, eyes, face,
MOUTH, and THROAT—are the most dramatically
affected. Muscle function worsens during activity
that uses affected muscles, such as chewing or
talking, and improves after rest.
Myasthenia gravis may develop at any age
though is most common in women under age 40
and men over age 60. Researchers do not know
what causes myasthenia gravis but suspect a dysfunction
of the THYMUS, a structure of the immune
system responsible for the maturation of T-cell
lymphocytes, may play a significant role. The thymus,
which normally has little function in adults,
is abnormally active in people who have myasthenia
gravis. Symptoms of myasthenia gravis relate
to the muscles affected and may include difficulty
focusing the eyes, slurred speech, or difficulty
swallowing. Involvement of peripheral muscles
may result in balance and gait dysfunctions.
Because myasthenia gravis is relatively rare,
doctors typically explore more common causes for
weak muscles before looking specifically for myasthenia
gravis. BLOOD tests can detect the presence of
the acetylcholine receptor antibodies in most people
who have myasthenia gravis. Other diagnostic
procedures that point to the disorder include specialized
electromyogram (EMG) and tests that
measure the muscle’s response to acetylcholine.
Treatment includes anticholinesterase medications,
which block the action of cholinesterase, an
enzyme that breaks down acetylcholine, and
IMMUNOSUPPRESSIVE MEDICATIONS, which interfere
with the release of antibodies to slow the destruction
of acetylcholine receptors. Many people experience
significant improvement in their symptoms
after THYMECTOMY (a surgical OPERATION to remove
the thymus). Most people who have myasthenia
gravis are able to enjoy relatively normal lives
with appropriate treatment.
See also ANTIBODY; AUTOIMMUNE DISORDERS;
IMMUNE RESPONSE; LYMPHOCYTE; SURGERY BENEFIT AND
RISK ASSESSMENT; T-CELL LYMPHOCYTE.
myopathy MUSCLE weakness that occurs when
muscle cells do not function properly. There are