Becker Muscular Dystrophy – Risk Factors, Causes, and Diagnosis.

What is Becker muscular dystrophy?

Becker muscular dystrophy (BMD) is one of nine types of muscular dystrophies, a group of genetic, degenerative diseases primarily affecting voluntary muscles. BMD belongs to a group of dystrophinopathies including Duchenne muscular dystrophy (DMD) and an intermediate form between DMD and BMD. The disease is named after German doctor Peter Emil Becker, who first described this variant of DMD in the 1950s. BMD is similar to DMD but allows the voluntary muscles to function better than they do in DMD. BMD has a later onset and milder symptoms compared to DMD. The heart muscle, however, can be affected similarly to the way it can be in DMD.

Pathophysiology

Advancements in the diagnosis of genetic conditions have revealed that BMD is a type of recessive, X-linked dystrophinopathy. Exon deletions exist in the dystrophin gene Xp21 (X-chromosome, short arm p, region 2, band 1). Affected males in approximately 30% of known cases of BMD phenotype do not have a demonstrable mutation/deletion. A reading frame or in-frame mutation hypothesis has been proposed to explain the abnormal translation of the dystrophin gene. Abnormal but functional dystrophin may be produced, in contrast to the pathology in DMD, in which a frame-shift mutation essentially leads to failure to produce dystrophin. Dystrophin levels in BMD are generally 30-80% of normal, while in DMD, the levels are less than 5%.

Dilated cardiomyopathy with congestive heart failure presents in males between age 20 and 40 years, but in carrier female carriers it is found later in life. This possibly explains why, in comparison with females, males suffer a rapid progression to death.

A study by Nicolas et al suggested that clinical variations in patients with BMD are related to differences in dystrophin mutations, as derived from different in-frame exon deletions. For example, delayed onset of dilated cardiomyopathy seemed to be related to specific exon deletions, as did earlier wheelchair dependency.

What are the causes of Becker muscular dystrophy?

Becker muscular dystrophy is a genetic condition – it is caused by a mutation or mistake in the genetic code (DNA). In Becker muscular dystrophy, the mutation occurs in a gene called dystrophin, which is located on the X-chromosome (girls have two X-chromosomes and boys have only one).

In Becker muscular dystrophy, the genetic mutation causes a faulty (smaller or less abundant than normal) protein called dystrophin to be produced in the muscles. This protein is important to maintain the integrity of the muscles, so when it is smaller or less abundant than normal, the muscle fibers gradually break down and the muscles slowly become weaker.

Some mutations result in the complete absence of the dystrophin protein in muscles and this causes the more severe form of muscular dystrophy, called Duchenne muscular dystrophy.

In some cases, Becker muscular dystrophy is inherited from the mother who is a carrier, but it can also be caused by a new mutation in the child’s genes. Each son of a carrier mother has a 50:50 chance of being affected and each daughter has a 50:50 chance of being a carrier.

If a woman carries the gene mutation, then she is known as a ‘carrier’. Usually, female carriers are not affected because they have a second X-chromosome, which produces the dystrophin protein. A small number of female carriers have a degree of muscle weakness themselves and they are known as ‘manifesting carriers’ (please see our factsheet called Manifesting carriers).

Genetic advice (counseling) and testing for other family members at risk of being carriers should be provided as soon as possible following a diagnosis of Becker muscular dystrophy.

What are the risk factors of Becker muscular dystrophy?

Becker muscular dystrophy is a genetic disease caused by a gene on the X chromosome that mothers carrying the gene can pass to their sons.

What Are the Signs & Symptoms of Becker Muscular Dystrophy?

Becker MD symptoms can range from mild to almost as severe as the symptoms of Duchenne MD.

A child with Becker MD may start to walk later than most kids do. But the disease rarely causes health problems until a child has muscle weakness in the hips and pelvis. This usually happens when kids are 10–13 years old. Walking problems are usually noticed around age 15–16.

A child with Becker MD may:

• Have more difficulty with sports
• Have trouble climbing stairs
• Not be able to walk quickly, run smoothly, or maintain a running pace
• Have trouble lifting heavy loads
• Have calf muscles that look bigger than normal, even though they’re weaker

Becker MD affects the muscles of the hips, pelvis, thighs, and shoulders, as well as the heart. Changes in the heart muscle may happen faster than in other muscles. Children who first show symptoms of Becker MD at younger ages are more likely to have heart problems than kids whose symptoms start later.

Becker MD is progressive, meaning problems get worse with age. Symptoms get worse slowly compared with Duchenne MD. Breathing muscles tend to stay strong enough that a ventilator or other mechanical breathing help isn’t needed.

Possible Complications

• Children with BMD may have additional health problems, such as:
• Heart muscle weakening and not pumping blood well (dilated cardiomyopathy)
• Breathing and lung problems
• Scoliosis, a sideways curving of the backbones (vertebrae)
• Diet, nutrition, and digestive problems
• Trouble focusing, learning, or controlling emotions
• Complications from anesthesia, for which special care is needed

How is Becker muscular dystrophy diagnosed?

Diagnosing Becker muscular dystrophy is complicated since it shares so many symptoms with other conditions including Duchenne, limb-girdle muscular dystrophy and spinal muscular atrophy.

The challenge is to determine whether the weakness is originating in the muscles themselves or in the motor neurons (branching from the spinal cord), which control these muscles.

A careful physical and history of signs and symptoms is the first step so the doctor can note the pattern of progression. Diagnostic tests for Becker muscular dystrophy include:

Blood tests: Genetic blood tests can reveal the gene mutation responsible for Becker muscular dystrophy. They can also measure the presence of creatine kinase, an enzyme that forms when muscle tissue breaks down. This substance is elevated in muscular dystrophy and inflammatory conditions.

Muscle biopsy: For those children who have clinical evidence of Duchenne muscular dystrophy but who do not show one of the common mutations, a small sample of muscle tissue is taken and examined under a microscope to confirm the diagnosis.

Electromyogram: This test checks to see if muscle weakness is a result of the destruction of muscle tissue rather than nerve damage.

Electrocardiogram (ECG or EKG): A test that records the electrical activity of the heart, an ECG shows abnormal rhythms (arrhythmias or dysrhythmias) and detects heart muscle damage.

The heart comprises mostly muscle, and therefore it is affected by muscular dystrophy. Becker muscular dystrophy can cause cardiomyopathy, a weakening of the heart muscles, which, if unaddressed, can lead to heart failure and the need for a transplant.

Treatment

There is no known cure for Becker muscular dystrophy. However, there are many new drugs currently undergoing clinical testing that show significant promise in treating the disease. The current goal of treatment is to control symptoms to maximize a person’s quality of life. Some providers prescribe steroids to help keep a patient walking for as long as possible.

Activity is encouraged. Inactivity (such as bed rest) can make the muscle disease worse. Physical therapy may be helpful to maintain muscle strength. Orthopedic appliances such as braces and wheelchairs may improve movement and self-care.

Genetic counseling may be recommended. Daughters of a man with Becker muscular dystrophy will very likely carry the defective gene and could pass it on to their sons.

Rehabilitation Program

Physical Therapy

The role of physical therapy services is to address the functional needs of the patient as the disease progresses. Early interventions may focus on stretching tight muscles (which may initially be the only therapy goal). As the patient’s weakness progresses, appropriate equipment and assistive devices will be required to enable the individual to maintain functional mobility and independence in daily living activities. Educational objectives include teaching the patient techniques for energy conservation, joint protection, and the prevention of overuse fatigue.

Occupational Therapy

Activities of daily living skills are addressed, depending on the level of impairments, in occupational therapy. Specific adaptations (to aid, for instance, dressing and bathroom skills) may be provided. Such adaptations range from methods of buttoning and zippering clothes to grab bars and raised toilet seats in the bathroom. Mobility concerns are addressed, including the need for devices to assist with mobility, such as a scooter or a fully adapted wheelchair with a custom seat and back, custom supports, and electric power. Speech Therapy

Dysphagia concerns may be evaluated by a speech therapist. Progressive weakness toward the end of the disease process may lead to dysphagia and an increased risk of aspiration pneumonia. Clinical evaluation may result in the recommendation to avoid specific food textures and liquid viscosities, as well as to avoid certain positions during feeding. A videofluoroscopic evaluation may be performed to demonstrate the risk of aspiration.

Recreational Therapy

Specific planning for avocational needs and desires may be coordinated with a recreational therapist. Resources within the community, such as activity programs with the local parks and recreation department, may be explored. Educational institutions, from public schools to community colleges and universities, may have resources that can be utilized. Adaptive physical education programs and disabled student services are generally available for qualified individuals. Access and mobility concerns in the community invariably touch upon the adjustment issues faced by individuals with a progressive disability.

How can Becker Muscular Dystrophy be prevented?

• Currently, there are no specific methods or guidelines to prevent Becker’s Muscular Dystrophy genetic condition
• Genetic testing of the expecting parents (and related family members) and prenatal diagnosis (molecular testing of the fetus during pregnancy) may help in understanding the risks better during pregnancy
• If there is a family history of the condition, then genetic counseling will help assess risks, before planning for a child
• Active research is currently being performed to explore the possibilities for treatment and prevention of inherited and acquired genetic disorders