Skeletal Muscle PDF

## SKELETAL MUSCLE ### I DERMATOMYOSITIS - **A**. Inflammatory disorder of the skin and skeletal muscle - **B**. Unknown etiology; some cases are associated with carcinoma (e.g., gastric carcinoma). - **C**. Clinical features - Bilateral proximal muscle weakness; distal involvement can develo...

## SKELETAL MUSCLE ### I DERMATOMYOSITIS - **A**. Inflammatory disorder of the skin and skeletal muscle - **B**. Unknown etiology; some cases are associated with carcinoma (e.g., gastric carcinoma). - **C**. Clinical features - Bilateral proximal muscle weakness; distal involvement can develop late in disease. - Rash of the upper eyelids (heliotrope rash); malar rash may also be seen. - Red papules on the elbows, knuckles, and knees (Gottron papules) - **D**. Laboratory findings - Increased creatine kinase - Positive ANA and anti-Jo-1 antibody - Perimysial inflammation (CD4+ T cells) with perifascicular atrophy on biopsy - **E**. Treatment is corticosteroids. ### II POLYMYOSITIS - **A**. Inflammatory disorder of skeletal muscle - **B**. Resembles dermatomyositis clinically, but skin is not involved; endomysial inflammation (CD8+ T cells) with necrotic muscle fibers is seen on biopsy. ### III X-LINKED MUSCULAR DYSTROPHY - **A**. Degenerative disorder characterized by muscle wasting and replacement of skeletal muscle by adipose tissue - **B**. Due to defects of dystrophin gene - Dystrophin is important for anchoring the muscle cytoskeleton to the extracellular matrix. - Mutations are often spontaneous; large gene size predisposes to high rate of mutation. - **C**. Duchenne muscular dystrophy is due to deletion of dystrophin. - Presents as proximal muscle weakness at 1 year of age; progresses to involve distal muscles - Calf pseudohypertrophy is a characteristic finding. - Serum creatine kinase is elevated. - Death results from cardiac or respiratory failure; myocardium is commonly involved. - **D**. Becker muscular dystrophy is due to mutated dystrophin; clinically results in milder disease.