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Becker muscular dystrophy
A rare, genetic muscular dystrophy characterized by progressive muscle wasting and weakness due to degeneration of skeletal, smooth and cardiac muscle.
ORPHA:98895Classification level: Disorder
Becker muscular dystrophy (BMD) primarily affects males; in Europe the estimated prevalence ranges between 1:16,700-1:18,500 male births.
Onset is usually in childhood, typically after 7 years of age, but can be later. Presenting features in children include toe walking gait and or exercise-related cramps with or without myoglobinuria. Some patients may present following anesthetic induced acute rhabdomyolysis. In older patients, cardiomyopathy may be the presenting feature. As the condition progresses, muscle weakness leads to functional difficulties (difficulty climbing stairs or rising from a chair). Rarely, cardiomyopathy may be the presenting feature. Clinical examination reveals muscle pseudohypertrophy of the calf muscles and there may be atrophy of more proximal muscles such as the quadriceps. There is symmetrical and proximal muscle weakness, with the lower limbs being more severely affected than the upper limbs. There may be joint contractures, especially of the tendo- Achilles. The facial, ophthalmic and bulbar muscles are not involved. The condition is slowly progressive and about 40% of affected patients will eventually become wheelchair-dependent. In wheelchair dependent patients, restrictive respiratory insufficiency occurs due to weakness of the intercostal muscles and diaphragm. Cardiac involvement leads to dilated cardiomyopathy, which can be disproportionate to skeletal muscle involvement.
BMD is caused by dystrophin deficiency due to in-frame deletions, mutations or duplications in the DMD gene (Xp21.2).
Diagnosis is suspected on the basis of the clinical picture, family history and laboratory findings (raised serum creatine kinase to 10-100 times the normal level). The diagnosis is confirmed with DNA testing for variants in the DMD gene. Muscle biopsy shows a dystrophic picture with reduced dystrophin staining.
Differential diagnosis includes the limb girdle muscular dystrophies, Duchenne muscular dystrophy, malignant hyperthermia and metabolic muscle diseases. In some cases the distinction between a mild variant of BMD and X-linked dilated cardiomyopathy can be difficult.
If the diagnosis has been confirmed by genetic testing of carriers, then antenatal, genetic diagnosis is possible.
BMD is an X-linked recessive disease and genetic counseling is recommended. Male siblings of a proband have a 50% risk of being affected; female siblings have a 50% risk of being carriers. Whilst carriers are typically asymptomatic, a small percentage manifest milder forms of the disease (symptomatic form of muscular dystrophy of Duchenne and Becker in female carriers). Male offspring of an affected male do not inherit the disease, female offspring are obligate carriers.
Management and treatment
Management includes multidisciplinary care with physiotherapy to reduce joint contractures and prolong walking. Night time ankle-foot orthoses are prescribed for children to reduce tendo- achilles contractures. Cardiac surveillance and monitoring of respiratory function are very important to improve outcome. Early treatment of cardiomyopathy with ACE (angiotensin-converting enzyme) inhibitors and or beta-blockers is recommended, and referral for cardiac transplantation is appropriate for severely affected patients. Patients with respiratory insufficiency should have pneumococcal and flu vaccines. Respiratory insufficiency should be treated with nocturnal BiPAP (bilevel positive airway pressure) and cough augmentation.
BMD is slowly progressive with wide phenotypic variability. Despite childhood onset, independent walking is never lost before 16 years of age. Life-expectancy for patients can be normal but may be significantly shortened by dilated cardiomyopathy or respiratory failure.