| WHAT ARE DUCHENNE AND BECKER
MUSCULAR DYSTROPHIES?
In the early stages, Duchenne and Becker MD affect the
pectoral muscles (which draw back the shoulders), the trunk, and the upper and lower legs.
These weaknesses lead to difficulty in rising, climbing stairs and maintaining balance.
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Muscular dystrophies are genetic disorders characterized by progressive
muscle wasting and weakness that begin with microscopic changes in the muscle. As muscles
degenerate over time, the person's muscle strength declines.
Duchenne muscular dystrophy (DMD) was first described by the
French neurologist Guillaume Benjamin Amand Duchenne in the 1860s. Becker muscular
dystrophy (BMD) is named after the German doctor Peter Emil Becker, who first
described this variant of DMD in the 1950s.
In DMD, boys begin to show signs of muscle weakness as early as age 3.
The disease gradually weakens the skeletal or voluntary muscles, those in the arms,
legs and trunk. By the early teens or even earlier, the boy's heart and respiratory
muscles may also be affected.
BMD is a much milder version of DMD. Its onset is usually in the teens
or early adulthood, and the course is slower and far less predictable than that of DMD.
(Though DMD and BMD affect boys almost exclusively, in rare cases they
can affect girls. See "Does It Run in the Family?")
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WHAT CAUSES DUCHENNE AND BECKER MUSCULAR DYSTROPHIES?
Until the 1980s, little was known about the cause of any kind of
muscular dystrophy. In 1986, MDA-supported researchers identified the gene that, when
flawed - a problem known as a mutation - causes DMD. In 1987, the protein
associated with this gene was identified and named dystrophin.
Genes contain codes, or recipes, for proteins, which are
very important biological components in all forms of life. DMD occurs when a particular
gene on the X chromosome fails to make the protein dystrophin. BMD results from different
mutations in the same gene. People with BMD have some dystrophin, but it's not enough or
it's poor in quality. Having some dystrophin protects the muscles of those with Becker
from degenerating as badly or as quickly as those of people with Duchenne.
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Muscles are made up of bundles of fibers (cells). A
group of independent proteins along the membrane surrounding each fiber helps to keep
muscle cells working properly. When one of these proteins, dystrophin, is absent, the
result is Duchenne muscular dystrophy; poor or inadequate dystrophin results in Becker
muscular dystrophy. |
By the way, eating or not eating food with protein can't replace lost
dystrophin. For more about the way gene mutations cause Duchenne and Becker dystrophies,
see "Does It Run
in the Family?"
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WHAT HAPPENS TO THE VOLUNTARY MUSCLES OF SOMEONE WITH DMD OR BMD?
Duchenne
The course of DMD is fairly predictable. Children with the disorder are
often late in learning to walk. In toddlers, parents may notice enlarged calf muscles, or pseudohypertrophy.
A preschooler with DMD may seem clumsy and fall often. Soon, he has trouble climbing
stairs, getting up from the floor or running.
By school age, the child may walk on his toes or the balls of his feet,
with a slightly rolling gait. He has a waddling and unsteady gait and can easily fall
over. To try to keep his balance, he sticks his belly out and puts his shoulders back. He
also has difficulty raising his arms.
Nearly all children with DMD lose the ability to walk sometime between
ages 7 and 12. In the teen years, activities involving the arms, legs or trunk require
assistance or mechanical support.
Becker
Often, the diagnosis of Becker muscular dystrophy isn't made until
adolescence or even adulthood, possibly when a young man finds he can't keep up in
physical education classes or military training. To compensate for his weakening muscles,
the young man begins walking with a waddling gait, walking on his toes or sticking out his
abdomen.
As with Duchenne, the pattern of muscle loss in BMD usually begins with
the hips and pelvic area, the thighs and the shoulders. But in BMD, the rate of muscle
degeneration varies a great deal from one person to another. Some men require wheelchairs
by their 30s or later, while some manage for many years with minor aids, such as canes.
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WHAT TESTS ARE USED TO DIAGNOSE DMD AND BMD?
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Because of weakened leg muscles, boys with DMD have a
distinctive way of rising from the floor, called the Gowers' maneuver. They first get on
hands and knees, then elevate the posterior, then "walk" their hands up the legs
to raise their upper body. |
In diagnosing any form of muscular dystrophy, a doctor usually begins by
taking a patient and family history and performing a physical examination. Much can be
learned from these, including the pattern of weakness. The history and physical go a long
way toward making the diagnosis, even before any complicated diagnostic tests are done.
It's important to get a formal diagnosis because other diseases have
some of the same symptoms as DMD and BMD. Becker MD has often been overlooked or
misdiagnosed as limb-girdle muscular dystrophy or spinal muscular atrophy. For this
reason, it's important to have both genetic testing and a muscle biopsy before assuming
that the problem is actually BMD.
The doctor also wants to determine whether the patient's weakness
results from a problem in the muscles themselves or in the nerves that control them.
Muscle-controlling nerves, or motor neurons, originating in the spinal cord and
brain and reaching out to all the muscles, can cause weakness that looks like a muscle
problem but really isn't.
Usually, the origin of the weakness can be pinpointed by a physical
exam. Occasionally, special tests called electromyography or nerve
conduction studies are done. In these tests, the electrical activity of the muscles is
measured and nerves are stimulated to see whether the problem lies in the muscles or the
nerves.
Early in the diagnostic process doctors often order a special blood test
called a CK level. CK stands for creatine kinase, an enzyme that leaks out
of damaged muscle. When elevated CK levels are found in a blood sample, it usually means
muscle is being destroyed by some abnormal process, such as a muscular dystrophy or an
inflammation. Therefore, a high CK level suggests that the muscles themselves are the
likely cause of the weakness, but it doesn't tell exactly what the muscle disorder might
be.
To determine which disorder is causing a problem, a doctor may order a
muscle biopsy, the surgical removal of a small sample of muscle from the patient.
By examining this sample, doctors can tell a great deal about what's actually happening
inside the muscles. Modern techniques can use the biopsy to distinguish muscular
dystrophies from inflammatory and other disorders as well as between different forms of
dystrophy.
Other tests on the biopsy sample can provide information about which
muscle proteins are present in the muscle cells, and whether they're present in the normal
amounts and in the right locations. This can determine whether the disease is DMD (with no
dystrophin) or BMD (with some inadequate dystrophin). An MR (magnetic resonance) scan may
also be ordered. These painless scans allow doctors to visualize what's going on inside
weakening muscles.
The availability of DNA diagnostic tests, using either blood
cells or muscle cells to get precise genetic information, is expanding rapidly. You can
ask your MDA clinic physician or genetic counselor what tests are available. Since many
men with BMD (and some with DMD) become fathers, it's important to know for certain which
inherited disease an individual has. Sisters of people with DMD or BMD can also be tested
to find out whether they're carriers of the disease, meaning they could have children with
the disorder
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