Muscular Dystrophy – Top USMLE Diseases

Becker muscular dystrophy (BMD) is an X-linked recessive genetic disorder that is caused by a mutation in the DMD gene. Abnormal, partially functional muscle dystrophin protein is produced, which leads to progressive muscle weakness and the eventual loss of ambulation. The clinical course is highly variable, but symptoms generally occur by adolescence. The diagnosis is based on muscle enzymes, genetic testing, and muscle biopsy (if necessary). Management of BMD is supportive and aimed at slowing disease progression and complications. Dilated cardiomyopathy is the leading cause of death.

Myotonic dystrophies are genetic disorders due to autosomal-dominant genetic mutations and have 2 major clinical forms: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2). Myotonic dystrophies are heterogeneous diseases primarily affecting the muscles, but, unlike other muscular dystrophies, also have multisystem effects. Both DM1 and DM2 present with myotonia, muscle weakness, and myalgias; however, DM1 is severe and carries a reduced life expectancy, whereas DM2 is mild with a normal life expectancy. Diagnosis is made clinically, with genetic testing, and by electromyography (EMG). Management is primarily supportive.

Ion channel myopathies are a heterogeneous group of disorders that are caused by genetic defects in sodium, chloride, and calcium ion channels in the myocyte membranes resulting in myotonia. These myopathies present clinically with symptoms of muscle pain, weakness, stiffness, cramps, and spasm. Diagnosis is made on the basis of clinical presentation, lab testing, electromyography, and genetic testing. Management involves both prevention with diet and exercise modifications and treatment with diuretics in some cases.

Duchenne muscular dystrophy (DMD) is an X-linked recessive genetic disorder that is caused by a mutation in the DMD gene. The mutation leads to the production of abnormal dystrophin, resulting in muscle-fiber destruction and replacement with fatty or fibrous tissue. Affected individuals present with progressive proximal muscle weakness leading to the eventual loss of ambulation, as well as contractures, scoliosis, cardiomyopathy, and respiratory failure. A marked elevation in CK may be observed. Genetic testing is used to confirm the diagnosis. Management is supportive and aimed at slowing disease progression and complications. Duchenne muscular dystrophy is fatal with a life expectancy of about 20 years of age.