Charcot-Marie-Tooth Disease (CMT)
What is it?
Charcot-Marie-Tooth Disease (CMT) is an
inherited disorder that affects the nerves that
carry information to and from the spinal cord. It
causes symptoms of weakness and loss of sensation
in the limbs. CMT is named for the three
neurologists who first described it, Jean Martin
Charcot, Pierre Marie, and Howard Henry Tooth. It
is also known as hereditary motor and sensory
neuropathy, and peroneal muscular atrophy, because
the peroneal muscles in the lower legs are often
affected in the early stages of the disease.
Who gets it?
Charcot-Marie-Tooth Disease (CMT) is a
hereditary neuropathy, which means it is a
disorder of the nervous system passed from parent
to child. About 1 in 2,500 people in the United
States has a form of CMT. CMT is the most common
hereditary neuropathy.
What causes it?
CMT affects the groups of nerve cells that
carry information to and from the spinal cord,
called the peripheral nerves. The peripheral
nerves have cells in the brain and spinal cord and
nerve fibers that run from the brain and spinal
cord to the whole body. The signals the peripheral
nerves send control muscle movement, and the
signals they receive relay feelings of pain,
pressure, temperature and position back to the
brain and spinal cord. The genetic defect that
causes CMT affects the myelin sheath, which is the
coating that insulates the nerve fibers so nerve
impulses can travel freely over the fibers. If the
myelin is defective, the impulses either travel
more slowly or carry a weaker signal. The areas of
the body most affected are those farthest from the
spinal cord, the hands and feet. Most forms of CMT
are inherited as an autosomal dominant trait,
which means that a child need inherit only one
abnormal gene to have the disease. The chance of a
child inheriting this one abnormal gene is 50
percent with each conception. Scientists have
identified three chromosomes that, when defective,
result in some form of CMT. They are chromosome 1,
chromosome 17, and chromosome X. Defects in
chromosomes 1 and 17 affect the myelin sheath and
cause the nerve impulses to travel more slowly
than is normal. Defects in chromosome X are a
little different. These defects may affect the
myelin sheath and nerve fibers themselves. They
are inherited as an X-linked trait, which means
the gene is carried on the X chromosome. In
X-linked dominant conditions, males and females
need only one abnormal X chromosome gene to show
symptoms of the disease. Females with X-linked CMT
may have milder symptoms than men. This is because
women have two X chromosomes and the normal
chromosome can serve as backup for the abnormal
one. Men have one X and one Y chromosome. Because
of this, men can only pass this defective gene
onto their daughters, not their sons, who receive
the normal Y chromosome. A less common type of CMT,
called CMT type 3, or Dejerine-Sottas disease, is
inherited as an autosomal recessive trait. This
means both parents need to contribute a defective
gene for the child to have symptoms of the
disease.
What are the symptoms?
CMT is usually divided into three types,
determined by the way in which the peripheral
nerves are affected and the symptoms caused. Most
people (around two-thirds) have type 1 CMT, which
affects the myelin sheath. Symptoms of type 1
usually appear during late childhood or
adolescence and include weakness in the lower
legs. This weakness causes a condition known as
foot drop, where the foot tends to flop down, and
loss of muscle tone in the calves. The child may
have difficulty walking and running because of
foot deformities such as high arches and toes that
curl under (clawed toes). Because of this and foot
drop, he or she may trip over curbs, have to take
higher than normal steps, and walk slowly and
carefully. As the disease progresses, the muscle
weakness affects the hands, and the child
experiences mild to severe loss of sensation in
the hands and feet. Tremor, or shaking, may also
occur. The remaining one-third of patients have
type 2 CMT, which affects the nerve fibers.
Because it progresses more slowly, type 2 is
usually diagnosed a little later in life than type
1. Symptoms are similar to type 1. In types 1 and
2 CMT, the disease progresses slowly and doesnŐt
affect life expectancy. The most severe form of
CMT is type 3, also known as Dejerine-Sottas
disease. Children with type 3 CMT may not develop
early motor skills such as walking until they are
three or four years old. Other symptoms include
severe sensory problems, hearing loss, and the
loss of the ability to walk during adulthood.
How is it diagnosed?
Your doctor will diagnose CMT by taking a
complete medical history and performing a thorough
neurological exam. He or she will test the childŐs
reflexes and look for any shriveling or wasting
away (called atrophy) of the muscles. The doctor
will test the strength of the muscles by asking
the child to push or pull against resistance. He
or she will test whether the child can distinguish
between different sensations by gently applying
pressure, heat, and vibration to various areas of
the hands and legs. A test called electromyography
is used to check how the nerves are functioning.
Small needles are inserted into a muscle to record
the muscles electrical activity. The electrical
activity will be abnormal if there is damage to
the peripheral nerves. The doctor may also perform
similar tests called nerve conduction studies, or
nerve conduction velocity tests, which measure the
speed at which the motor nerves conduct impulses.
With this test, a small electrical charge is used
to stimulate a motor nerve. When the impulse
reaches the muscle, it should contract. By
measuring the time it takes the impulse to reach
the muscle as well as the amount of impulse that
reaches the muscle, your doctor can diagnose CMT
type 1 and 2. These procedures can cause some
minor discomfort, but may be necessary to make an
accurate diagnosis. Occasionally, doctors may
remove a small piece of the nerve, called a nerve
biopsy, to look for the swelling that is usually
seen with CMT type 1. DNA testing is available for
CMT type 1. DNA testing is usually used to
identify people at risk for the disease, and for
family planning purposes where there is a family
history of the disease.
What is the treatment?
While there is no treatment to prevent this
disease from getting worse, there are ways to
reduce the physical symptoms that cause problems
with movement and coordination Physical therapy
can help keep muscles more flexible by using range
of motion and balance exercises. Stretching the
muscles can also help reduce muscle shortening,
called contracture, which often occurs with this
disease. Some patients wear lightweight braces to
correct foot drop. Occupational therapy is also
recommended to help the patient perform daily
activities such as getting dressed, eating,
opening doors and bottles, and writing. If tremor
is a problem, the patient should avoid caffeine
and alcohol. In severe cases, drugs called
beta-blockers can help reduce tremor. Certain drug
combinations can also cause muscle damage, so it
is important that your doctors and pharmacist are
aware of your condition. Surgery is sometimes used
to correct severe foot deformities.
Self-care tips
There is no way to prevent CMT in people who
have inherited the gene or genes that cause this
disease. If your family has a history of this
disease, you may want to undergo genetic testing
for family planning purposes. If you or your child
have this disease, education is an important tool
in treatment. Learn all you can about the disease
and its treatment, and develop a good support
system so you or your child can live life to the
fullest.
This information has been designed as a comprehensive and quick reference
guide written by our health care reviewers. The health information written
by our authors is intended to be a supplement to the care provided by your
physician. It is not intended nor implied to be a substitute for
professional medical advice.
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