Becker muscular dystrophy (BMD) is a genetic disorder that causes the gradual weakening and deterioration of muscles over time. While the onset of symptoms and rate of progression can vary significantly among individuals, it is considered a less severe form of muscular dystrophy than Duchenne muscular dystrophy.
Genetic Origins and Dystrophin’s Role
Becker muscular dystrophy is caused by a mutation in the DMD gene, located on the X chromosome. This gene holds the instructions for producing dystrophin, a protein that protects muscle cells from damage during movement. In individuals with BMD, the genetic mutation allows for the production of a shortened or partially functional version of dystrophin. This differs from Duchenne muscular dystrophy (DMD), where mutations prevent the production of any functional dystrophin.
Because the DMD gene is on the X chromosome, BMD primarily affects males, who have only one X chromosome. If a male inherits an X chromosome with the mutated gene, he will develop the disorder. Females have two X chromosomes, so a mutation on one is compensated for by the normal gene on the other. This makes them carriers who do not show symptoms but can pass the gene to their children. The presence of some functional dystrophin explains its later onset and slower progression compared to DMD.
Progression of Physical Symptoms
The initial symptoms of BMD appear between childhood and early adulthood as weakness in the muscles of the hips, pelvis, thighs, and shoulders. This weakness causes difficulty running, jumping, or climbing stairs. An early sign can be an enlargement of the calf muscles, which seem strong but are weakened and replaced by fat and scar tissue. Another indicator is the Gowers’ sign, where an individual uses their hands to push off their legs to rise from a seated or lying position.
As the condition progresses over several decades, muscle weakness extends to other parts of the body, including the arms and neck. Individuals may develop a waddling gait or walk on their toes to compensate for weakened leg muscles. While the progression is slow, many people with BMD eventually require mobility aids like a cane or wheelchair, often by age 30 or later.
Associated Health Complications
Beyond skeletal muscles, BMD can lead to health issues affecting internal organs, the most serious being cardiomyopathy. The dystrophin protein is also present in heart muscle, and its deficiency weakens the heart, making it harder to pump blood. This complication can develop before significant limb weakness is apparent, making regular cardiac monitoring a necessary part of care from the time of diagnosis.
In the later stages, the muscles involved in breathing can also weaken. This respiratory muscle weakness can lead to breathing difficulties, especially during sleep, and an increased risk of respiratory infections like pneumonia. Regular monitoring of lung function through tests such as forced vital capacity (FVC) is important to manage these potential complications.
Diagnosis and Management Approaches
Diagnosing Becker muscular dystrophy begins with a physical exam and a review of the patient’s and family’s medical history. The initial step is a blood test to measure levels of creatine kinase (CK), an enzyme that leaks out of damaged muscles. Elevated CK levels suggest a muscle disease, prompting further investigation through genetic testing. Genetic tests can identify a specific mutation in the DMD gene, confirming the diagnosis.
Since there is no cure for BMD, treatment focuses on managing symptoms, preserving mobility, and addressing complications. Physical and occupational therapy are important for management, using stretching and exercise to maintain muscle function and prevent joint contractures. To manage cardiac complications, doctors may prescribe medications like ACE inhibitors and beta-blockers. Corticosteroids may sometimes be used to slow muscle deterioration. Assistive devices, from braces to wheelchairs, are also integrated into care plans to help individuals maintain independence.