Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are genetic disorders that cause progressive muscle weakness and degeneration. Both conditions impact movement, heart, and respiratory muscles and originate from mutations within the same gene responsible for producing a protein named dystrophin. Although they share this common genetic origin, the specific nature of the mutation dictates the severity and progression of the disease.
The Role of the Dystrophin Gene
Duchenne and Becker muscular dystrophies are caused by flaws in the DMD gene, which holds the instructions for building the dystrophin protein. This protein is part of a complex that strengthens muscle fibers, acting as a shock absorber to protect them from damage during contraction. Without enough functional dystrophin, muscle cells become fragile and are easily damaged.
The gene for dystrophin resides on the X chromosome, which is why these conditions are categorized as X-linked disorders that primarily affect males. Females, with two X chromosomes, are usually carriers of the genetic trait and often do not show symptoms, though some may experience milder muscle weakness. In about one-third of cases, the mutation occurs spontaneously and is not inherited.
The difference between DMD and BMD lies in how the genetic mutation affects the dystrophin protein. In Duchenne muscular dystrophy, an “out-of-frame” mutation leads to a complete stop in the production of the protein, resulting in a near-total absence of functional dystrophin. In contrast, Becker muscular dystrophy is caused by an “in-frame” mutation, which allows for the production of a shorter but still partially functional version of dystrophin. This presence of some working protein accounts for the milder symptoms seen in BMD.
Distinguishing Symptoms and Progression
The clinical differences between Duchenne and Becker muscular dystrophy are apparent from early life. Symptoms of DMD emerge in early childhood, often between the ages of two and five, with signs like frequent falls, a waddling walk, or difficulty running and climbing stairs. The onset of BMD is more variable and occurs later, with initial signs of weakness appearing anywhere from late childhood into early adulthood.
The rate of progression also differs significantly. DMD is characterized by a rapid decline in muscle function, and many affected individuals require a wheelchair by their early teenage years. The progression of BMD is much slower and more varied, with many people maintaining the ability to walk well into their 30s or 40s, sometimes with assistance.
Certain physical signs are common to both conditions. One is the Gowers’ sign, where a child uses their hands to push off their own legs to rise from a seated position due to weak hip and thigh muscles. Calf pseudohypertrophy, an enlargement of the calf muscles, is another shared feature caused by the replacement of muscle with fat and scar tissue.
Both disorders affect the heart and the muscles responsible for breathing, leading to serious health complications. Cardiomyopathy, a condition that weakens the heart muscle, is a concern in both DMD and BMD. Respiratory muscle weakness can also lead to breathing difficulties, and these complications tend to appear earlier and advance more quickly in individuals with DMD.
The Diagnostic Process
Diagnosing Duchenne or Becker muscular dystrophy begins with observing clinical symptoms, such as muscle weakness and developmental delays in motor skills. A blood test is then used to measure the level of creatine kinase (CK), an enzyme that leaks from damaged muscles into the bloodstream. In both conditions, CK levels are elevated to thousands of times higher than normal.
While high CK levels suggest muscle damage, a definitive diagnosis requires genetic testing. A blood sample is analyzed to detect specific mutations, such as deletions or duplications, within the dystrophin gene. This genetic analysis confirms the diagnosis and distinguishes between the two types.
In the past, a muscle biopsy was a common diagnostic procedure to reveal the amount of dystrophin protein present in the muscle cells. This involved surgically removing a small sample of muscle tissue for examination. However, with the high accuracy and less invasive nature of modern genetic testing, muscle biopsies are now performed much less frequently.
Management and Therapeutic Approaches
As there is no cure for Duchenne or Becker muscular dystrophy, treatment focuses on managing symptoms, slowing disease progression, and supporting quality of life. Care is delivered by a multidisciplinary team of specialists and involves physical therapies, medications, and careful monitoring of heart and lung function.
A primary treatment for slowing muscle deterioration, particularly in DMD, is the use of corticosteroids like prednisone and deflazacort. These medications can preserve muscle strength and delay the loss of ambulation for several years. Regular physical and occupational therapy also help maintain mobility, prevent joint contractures, and provide strategies to assist with daily activities.
Consistent monitoring of cardiac and respiratory health is a component of care for both DMD and BMD. Regular check-ups with cardiologists and pulmonologists help detect problems early. Medications may be prescribed to manage cardiomyopathy, and respiratory support, such as non-invasive ventilators, may be needed as breathing muscles weaken.
Newer therapeutic strategies target the underlying genetic cause of the disease. These treatments, known as exon-skipping therapies, are for individuals with specific types of mutations in the dystrophin gene. By “skipping” over a mutated section of the gene, these drugs can enable the production of a shorter, functional dystrophin protein, similar to what is seen in Becker muscular dystrophy.