Learn about the differences and similarities between Duchenne and Becker muscular dystrophy.
Muscular dystrophy is an inherited disease caused by abnormal levels of dystrophin in the body, a protein necessary for maintaining muscle integrity for movement and for heart functions. It is characterized by progressive muscle atrophy and is also linked to a heart condition called cardiomyopathy.
Two types of muscular dystrophy involving an abnormality in dystrophin production are Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD), which differ mainly in severity. Being an x-linked recessive disorder, it mainly affects males. This is because females have two X-chromosomes and may “turn off” an abnormal X-chromosome to suppress the disease. Only about 5-10% of female carriers exhibit progressive symptoms of muscle weakness. Though numbers may vary, the combined prevalence of muscular dystrophy is estimated to be 1 in 3500 to 1 in 5000 male births worldwide.
Differences in dystrophin production in DMD and BMD
The main difference between the two disorders lies within the protein called dystrophin. The DMD gene is responsible for the production of this protein, and genetic mutations in this gene can impede dystrophin production. In the case of DMD, almost no functional dystrophin is produced at all whereas in BMD, a shortened form of the protein is usually produced and retains some functionality.
For this reason, DMD exhibits symptoms that are more severe as compared to BMD, although their effects on the heart muscle can be similar. This, however, doesn’t mean that dystrophin levels are directly correlated with the severity of the disorder. Some research done on 33 BMD patients show that for dystrophin levels above 10% of the normal case, a patient's dystrophin level does not determine the severity of the symptoms. Below 10%, BMD is considered severe, and even lower than that at < 3%, it mostly manifests as DMD. A factor that contributes more to the severity of BMD is the functionality of the dystrophin protein; i.e., how abnormal it is, rather than its amount.
Differences and similarities in symptoms
As early as 3 years old, infants may start to exhibit symptoms of DMD, whereas BMD usually manifests during later childhood to adolescence. The onset of BMD is slower, less predictable, and has more varied symptoms, whereas DMD rapidly worsens and patients may be wheelchair-dependent by adolescence.
Both disorders usually begin with muscle weakness at the hips and pelvic area, the thighs, shoulders, and later, the skeletal muscles in the arms, legs, and trunk. They usually affect the person’s gait to compensate for this condition, pulling back their shoulders or sticking out the abdomen, and walking on toes. Both types of disorders may cause enlarged calf muscles called pseudohypertrophy due to abnormal muscle tissue that may also contain scar tissue.
For people with BMD, the rate of muscle degeneration varies and some may be dependent on wheelchairs as early as 30 years old. Others may manage with minor aids, such as walking canes, for years. In contrast, people suffering from DMD may experience loss of ambulation by adolescence or at around 15 years of age. They would also have difficulty raising their arms and moving their trunk.
Lack of dystrophin also causes heart muscle weakness in a condition called cardiomyopathy. This muscle weakness prevents the heart from pumping blood more efficiently. Symptoms typically appear during adolescence for both DMD and BMD patients and may later progress to dilated cardiomyopathy, wherein the heart becomes enlarged and causes heart arrhythmia, shortness of breath, severe fatigue, and swelling of legs and feet.
Heart conditions associated with BMD may be just as severe as those in DMD patients, despite having milder symptoms involving skeletal muscles. This condition may be life-threatening; thus, it is recommended for both BMD and DMD patients to regularly see cardiologists.
While the lungs are not directly affected by muscular dystrophy, other respiratory muscles, such as those of the diaphragm, may still be affected. This is more common and severe with DMD; in fact, most patients with BMD have an almost normal respiratory function, though they are more at risk when they get older.
DMD patients may have difficulty in breathing. This, in turn, causes headaches, cognitive dullness, and difficulty concentrating or staying awake. They may also experience difficulty with coughing, which may cause an increased risk of respiratory infections. It is important for these patients to take pneumococcal and influenza vaccines and seek immediate treatment, even for a common cold. Respiratory diseases in DMD patients are a major cause of mortality, but these are treatable, especially due to recent advancements in respiratory care.
While mental retardation among DMD patients is rare, they may experience more subtle cognitive and behavioral deficits due to abnormalities with dystrophin in the brain. Dystrophin in the brain is similar to its skeletal counterpart and is made with the same gene. DMD patients are also likely to have flawed protein, which causes learning disabilities.
While BMD effects on cognitive function have yet to be studied, it is believed that they also have more learning disabilities than the general public. DMD was found to cause inability to focus on patients, and while they usually don’t suffer from intellectual deterioration, this may affect their IQ scores. They may also have trouble communicating and have problems in certain areas of emotional interaction.
Treatment and life expectancy of Duchenne and Becker muscular dystrophy
In the past, DMD patients were not expected to survive beyond their teenage years; however, due to recent advancements in medicine, around 85% of patients survive up to their thirties. There are even some cases where DMD patients reach their forties and fifties. BMD patients, on the other hand, are expected to survive well into adulthood. Lifespan is often shortened by heart and respiratory problems, so these conditions must be carefully monitored.
A cure for DMD and BMD is yet to be found, so the focus of treatments is in delaying the progression of symptoms and improving the quality of life. Steroids are commonly used to slow down the worsening of symptoms and help patients hold on to their ability to walk for as long as possible. In particular, corticosteroids are found to be effective for this purpose.
Physiotherapy prevents contractures—fixations of the joints that cause discomfort and restrict mobility. While too much exercise can damage muscles, adequate amounts of exercise may help keep the cardiovascular system healthy. Swimming and water exercises are recommended by some experts, as water buoyancy reduces stress on the body. Braces, standing frames, wheelchairs, and other orthopedic appliances are also recommended for increased mobility. Due to the risk of cardiomyopathy associated with muscular dystrophy, it is highly recommended to have regular cardiac evaluations at least every other year. DMD patients also suffer from respiratory weakness, but this may also be the case with older BMD patients. When difficulties in breathing and coughing occur, a pulmonologist must be consulted. DMD patients should also watch out for spinal curvatures, especially for those who start to use wheelchairs frequently. Exercises and proper sitting and sleeping positions should be practiced according to the advice of a physical therapist.
Despite the rarity of these diseases, patients do not have to suffer them alone. It might be helpful to join support groups to ease stress and collect more relevant information. Some of these support groups are Parent Project Muscular Dystrophy, Muscular Dystrophy Association, Coalition Duchenne, and National Organization for Rare Disorders (NORD): Becker and Duchenne Muscular Dystrophy.