Study Guide

Becker Muscular Dystrophy

Key Points:

Description:

Becker muscular dystrophy (BMD) is a heritable genetic disorder seen almost exclusively in males and characterized by a progressive loss of functional muscle mass. It is related to Duchenne muscular dystrophy (DMD), as the same dystrophin gene is affected in both disorders (Bradley, 1978). However, patients with BMD are less severely affected and therefore have a later onset and slower progression of symptoms (Flanigan, 2014). There is a wide range of severity in the clinical presentation for patients with BMD. Depending on the severity of the disease, muscles of the musculoskeletal, cardiac, respiratory, and gastrointestinal (GI) system can be affected. 
 

Epidemiology:

Becker muscular dystrophy is a rare disorder and is far less common than Duchenne muscular dystrophy. The disease is almost exclusively seen in males, though there are reports of female carriers demonstrating very mild symptoms of BMD.  The incidence of BMD is 1 in 18,518 (or 0.5 per 10,000) male births. The prevalence of BMD varies by country, ranging from 0.13 in South Africa to 7.29 per 100,000 in England. A review of multiple studies found the overall pooled prevalence of B MD among males worldwide was 1.53 per 100,000 (Mah, 2014).
 

Clinical Findings:

There is a wide range in the severity and clinical presentation of Becker muscular dystrophy, which can significantly affect the age of disease onset, rate of progression, and systems affected by the disease (Bradley, 1978; Morrone, 1997). The first symptoms can present between 3-21 years of age, and the age at death ranges from 21-89 years old with an average life expectancy of ~45 years. 

The most noticeably affected system is the musculoskeletal system. Children commonly present with new weakness after age 7, when parents may notice clumsiness or an abnormal waddling gait. This occurs because the hip and thigh muscles are affected the most. The weakness in these muscles causes other muscles to compensate to help the child remain ambulatory. This is visualized as ‘pseudohypertrophy’ of the calves, where the calf muscles appear significantly larger than the small, weak quadriceps muscles. Patients may also demonstrate the Gower maneuver to help themselves stand up, where they first get into a “downward dog” position on all fours, and then walk their hands up the thighs to bring their body upright. As the disease progresses, patients may develop tightness in the calves and begin to walk more on their toes. Further progression may lead to contractures of the hip and knee joints, and eventually, the patient will lose the ability to walk. It is difficult to predict when a child may lose the ability to walk, but physicians can use the 10-meter walk test in their office – if it takes a child greater than 12 seconds to walk 10 meters then he has a 100% chance of using a wheelchair within one year. Patients with BMD typically retain their ability to ambulate until late adolescence or early adulthood. Once a patient becomes wheelchair-bound, he has an increased risk of developing scoliosis, a curvature of the spine, due to weak trunk muscles. Scoliosis in a patient with BMD will gradually worsen with time. If the deformity becomes severe, it will eventually affect a patient’s ability to sit independently, may cause pressure sores, and may worsen respiratory function. While not a direct result of the disease process, patients with BMD are also at higher risk for fractures. Their bones become weaker primarily because they stop weight bearing, but weaker muscles and certain medications can contribute as well (Larson, 2000).

Other muscles that can be involved in the disease process include the heart, lungs, and gastrointestinal (GI) muscles. Cardiac involvement is very common in BMD, though symptoms are not always noticeable (Finsterer, 2008). When symptoms such as palpitations, dizziness, shortness of breath, or fainting, do occur, they usually present later in the disease process when the patient is a young adult. There are different manifestations of heart disease in BMD, which include electrical conduction abnormalities, abnormal heart rhythms, and hypertrophic or dilated heart muscles that can lead to heart failure (Mavrogeni, 2015). The degree of heart involvement does not correlate with the severity of the disease. Symptoms from the weakness of the respiratory muscles show up later in the disease course, commonly after a child stops walking. Typically, nighttime breathing will be affected first. This will present as snoring, morning headaches, or excessive daytime sleepiness. As the muscles become weaker, the child will have increasing difficulty breathing and be more susceptible to lung infections, which can be a significant cause of death. Weakness of the GI muscles can lead to symptoms of bloating, constipation, and gastric reflux (Barohn, 1988).
 

Imaging Studies:

The most common imaging studies obtained by orthopedic surgeons are x-rays of the spine to monitor for scoliosis, particularly once a child is no longer walking. While severe scoliosis is less common in BMD than in DMD, it is important to diagnose and monitor spinal deformities, as they will worsen with time. Orthopedic physicians may also order X-rays of stiff joints or any areas concerning osteopenic fractures.

Other physicians may order studies to evaluate the heart, lung, and GI function. These can include ECGs, Holter monitor tests, echocardiograms, pulmonary function tests, polysomnograms, or gastric emptying studies.
 

Treatment:

The management of Becker muscular dystrophy requires a multi-disciplinary approach, including orthopedics, physical medicine and rehabilitation, cardiology, pulmonology, nutrition, physical therapy, and orthotics and prosthetics. The needs of the child will depend on the severity and progression of the disease (Bushby, 2010b).

Corticosteroid medications, such as prednisone and deflazacort, are the mainstay of treatment in patients with Duchenne muscular dystrophy (Griggs, 1993; Johnsen, 2001). Their use improves muscle strength, helps maintain this strength, and slows the loss of strength over time. This may result in prolongation of ambulation for 1-3 years. In addition, steroids delay the decline of pulmonary function and reduce the need for scoliosis surgery. However, there are many side effects from steroid medications, including weight gain and increased osteoporosis. Deflazacort, recently approved by the FDA in 2017, has been shown to have fewer issues with weight gain than prednisone (Griggs, 2016). While commonly used in DMD, the use of steroids in BMD is far less frequent. Based on the severity and rate of disease progression, a conversation with the child’s physician regarding the risks and benefits of these drugs is warranted.

Treatment of orthopedic issues associated with BMD includes management of muscle tightness and joint contractures, fractures, bone health, and scoliosis. One of the first orthopedic issues to appear is tightness of the calf muscles, which may cause patients to walk more on their toes. Initially, this is treated with stretching and ankle-foot orthoses (AFOs) worn at nighttime. AFOs are typically not worn during the day as they can compromise a patient’s ability to walk. As the tightness becomes more severe and interferes with ambulation, surgery may be helpful to lengthen the calf muscles or heel cord. Transfer or lengthening of other tendons around the ankle may also be helpful in preventing the recurrence of the deformity. Flexion contractures of the knee may occur as muscle weakness progresses and make walking more difficult. Initial treatment again begins with stretching and nighttime bracing, but surgery to lengthen the hamstring muscles can be considered if the patient still has enough strength to walk. Because contractures often coincide with decreased muscle strength, careful attention should be paid to ambulatory potential before considering surgery for limb deformities (Sussman, 2002).

As a patient becomes less ambulatory, disuse osteopenia develops. This process may begin while children are still able to walk but significantly worsens after they are wheelchair dependent. Medications used to treat BMD, such as steroids, may also lead to the weakening of bones. This places patients at increased risk for fractures (James, 2015). Maintaining good nutrition with Vitamin D and calcium supplements may help promote improved bone health. If severe osteopenia or osteoporosis is present, treatment with bisphosphonate medications may be warranted. If a fracture does occur, treatment will depend partly on whether the patient is still able to walk. When a patient is ambulatory, the goal is to get the patient walking again as soon as possible to prevent further weakening of the bones, and this often requires surgery to repair the fracture. Once a patient is nonambulatory, the risk of surgery may be greater than the benefits, and so the fracture may be treated with splinting or casting. Significant deformities that may lead to unacceptable malunions, and the need for acute pain control, may warrant surgery for some fractures, even in nonambulatory patients.

Scoliosis is the biggest orthopedic concern in patients with BMD. Development of scoliosis usually begins after patients are wheelchair dependent. Once present, scoliosis in BMD typically worsens with time. As the curves get larger, they can lead to loss of sitting balance and decreased pulmonary function. Bracing or special seating systems may delay curve progression, but will not prevent it. Surgery, most commonly including a posterior spinal fusion with instrumentation, is typically recommended once a curve has reached 20-30° with consideration of pulmonary function test results. Because scoliosis is uniformly progression in muscular dystrophies, intervening early in the process will prevent the worsening of the deformity, decrease the risk of vertebral fractures, slow the rate of respiratory decline, and possibly increase the life span (Galasko, 1992). Patients are typically still healthy enough, with adequate pulmonary function, to withstand the stress of surgery earlier in the disease process.

Management of cardiac, pulmonary, gastrointestinal, and nutritional issues is typically done by a pediatrician or other medical specialist. In general, treatment of these issues will require regular testing to monitor for disease progression, specific medications to slow disease progression or manage symptoms, and assistive ventilation devices for worsening pulmonary function.

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References:

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