Becker Muscular Dystrophy

Becker Muscular Dystrophy results from mutations in the dystrophin gene on the X chromosome that produce an internally deleted or abnormal but partially functional dystrophin protein. In contrast to DMD mutations causing complete dystrophin absence or non-functional protein, BMD mutations preserve the reading frame and allow production of shorter but partially active dystrophin. The amount of residual dystrophin function correlates with disease severity and age of onset. In-frame mutations generally produce milder phenotypes than out-of-frame mutations, though genotype-phenotype correlation is not absolute. Pathophysiology involves incomplete dystrophin deficiency leading to progressive muscle fiber damage, necrosis, inflammation, and replacement by fat and connective tissue. Muscles affected include skeletal muscles (progressive weakness), cardiac muscle (cardiomyopathy in about 15% of patients), and rarely respiratory muscles. BMD shows slower progression than DMD, with some patients retaining ambulation into the third to fourth decade of life. Serum creatine kinase levels are markedly elevated. Complications include respiratory insufficiency in later stages, cardiac arrhythmias and heart failure, rhabdomyolysis with myoglobinuria following intense activity or trauma, and contractures limiting mobility.