USMLE

Duchenne and Becker Muscular Dystrophy

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Duchenne and Becker Muscular Dystrophy are X-linked recessive diseases characterized by progressive muscle degeneration and weakness.

They are both caused by mutations in the DMD gene, which encodes the protein dystrophin. Dystrophin anchors muscle fibers (e.g. actin) to the extracellular matrix, so a loss of function causes muscle degeneration.

Duchenne muscular dystrophy is caused by a frameshift mutation, which leads to a non-functional protein. Contrastingly, Becker muscular dystrophy is caused by non-frameshift mutations, resulting in a partially functional protein. This difference helps explain why Duchenne is more severe than Becker, also presenting at an earlier age.

Early in the clinical course, muscle is replaced by fat and fibrous tissue, often appearing larger than usual in what is termed pseudohypertrophy. Pseudohypertrophy of the calf is extremely common. The degeneration of muscle also leads to weakness and difficulty standing up, so patients will use their upper extremities (e.g. arms) to help push their bodies up, in a series of actions called the Gower sign.

Complications of cardiomyocyte degeneration include dilated cardiomyopathy, while damage to muscles of respiration can lead to respiratory failure. Labs may reveal an elevated CK and elevated aldolase, indicative of ongoing muscle damage. Diagnosis is confirmed by genetic testing.

Find this mnemonic on Duchenne and Becker Muscular Dystrophy and other mnemonics for Musculoskeletal topics among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.