Genetics Northern California

Becker Muscular Dystrophy

What are Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)?
What causes DMD and BMD?
How are DMD and BMD inherited?
How are DMD and BMD diagnosed?
How common are DMD and BMD?
How are DMD and BMD treated?
Where can I get more information?

What are Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)?
Duchenne Muscular Dystrophy (DMD) and Becker Muscular Dystrophy (BMD) are inherited conditions that cause weak muscles and muscle wasting (muscle loss). They affect the muscles that move the body, as well as the muscles in the heart and diaphragm (the main muscle for breathing). The muscle weakness gets worse as a person grows older. Although they are often described as separate conditions, DMD and BMD have the same genetic cause. BMD is a milder version of DMD. These two conditions mostly affect males, but in rare cases, females can also be affected. 

Boys with DMD and BMD are slow to start walking and have more trouble with physical activities (e.g., running, jumping, climbing stairs, throwing a ball). As the legs get weaker, a wheelchair is eventually needed. For boys with DMD, wheelchair use usually starts in early teenage years, but the exact age varies. Boys with BMD may need a wheelchair in their 20s-40s, but some adults with BMD never need a wheelchair. For DMD and BMD, muscle weakness also involves the arms, neck, trunk, and other areas. Over time, the heart and the breathing muscles become weaker. Better treatments are leading to longer survival; however, life expectancy is still reduced for both conditions. The most common cause of death is due to breathing problems (pulmonary involvement) or heart failure.

DMD and BMD do not change the way a person looks on the outside. Boys with these conditions look like their family. IQ scores often fall in the normal range. However, there is a higher chance of learning disabilities and some psychiatric disorders (autistic symptoms, anxiety, and depression). 

What causes DMD and BMD?
DMD and BMD are both caused by pathogenic variants (harmful changes) in the dystrophin (DMD) gene. This gene makes a protein called dystrophin, which is needed to keep muscles healthy. Without enough of this protein, muscles slowly break down. Those with DMD have a gene that makes almost no dystrophin, while those with BMD have a gene that makes a small amount of dystrophin. This is the main difference between the two conditions. When there is no dystrophin, muscle symptoms start in early childhood and progress more quickly. Having a small amount of dystrophin leads to milder symptoms that start later (mid-late childhood). 

How are DMD and BMD inherited?
These are X-linked conditions. That means the gene is located on the X chromosome. Boys inherit an X chromosome from their mother and a Y chromosome from their father.  In 2 of 3 cases, the faulty gene is inherited from a carrier mother. However, in 1 of 3 cases, the condition is not inherited. It happens due to a brand new (“de novo”) change in the DMD gene. 

How are DMD and BMD diagnosed?
If the diagnosis is suspected based on clinical symptoms, a blood test is done to measure levels of creatine kinase (CK).  CK is a product of muscle breakdown and levels can be 20-50 times higher than average in someone with DMD or BMD. Genetic testing is used to confirm the diagnosis. A sample of muscle (muscle biopsy) was once used to help make the diagnosis. Now, genetic testing has replaced the need for a muscle biopsy in almost all cases. In the future, it may be possible to screen newborn boys for DMD.

  • Genetic Testing –Nearly all males with DMD or BMD have a pathogenic variant in the DMD gene that can be found by genetic testing. Testing is usually done on a small blood sample. At this time, genetic testing cannot distinguish DMD from BMD. The specific diagnosis is based on when symptoms start and how severe they are.

    When testing finds a harmful DMD variant, others in the family can be tested. Females can be offered carrier testing and prenatal testing is possible for future pregnancies. Some female carriers have muscle weakness and/or heart disease. However, most female carriers have no medical problems. Periodic heart exams are recommended for female carriers starting in adulthood.

How common are DMD and BMD?
About 1 in every 3500 boys is born with DMD or BMD. Symptoms are not present at birth. Changes become noticeable in the first few years of life for DMD and later for BMD. This condition affects all racial and ethnic groups worldwide.   

How are DMD and BMD treated? 
There is currently no cure for DMD or BMD, but medical treatment and supportive care may help slow down the disease. Since most muscles of the body can be affected, it is important for boys to be routinely seen by a team of specialists, especially heart and lung specialists. Symptoms of DMD and BMD vary for each person and cannot be predicted based on their genetic test. A specialty team approach provides personalized care that identifies and treats symptoms as they come up. 

Many studies are being done to find better treatments. To learn more about research opportunities contact one of the support organizations below or go to Clinicaltrials.gov.

Where can I get more information?


Last reviewed: September 9, 2020
Reviewed by: Katie Braverman, MS, LCGC; Kimberly Barr, MS, LCCG; Mark Lipson, MD