Skeletal Muscle & Neuromuscular Junction PDF

## SKELETAL MUSCLE ### DERMATOMYOSITIS - Inflammatory disorder of the skin and skeletal muscle - Unknown etiology; some cases are associated with carcinoma (e.g., gastric carcinoma). - Clinical features: - Bilateral proximal muscle weakness; distal involvement can develop late in disease....

## SKELETAL MUSCLE ### DERMATOMYOSITIS - Inflammatory disorder of the skin and skeletal muscle - Unknown etiology; some cases are associated with carcinoma (e.g., gastric carcinoma). - 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) - Laboratory findings: - Increased creatine kinase - Positive ANA and anti-Jo-1 antibody - Perimysial inflammation (CD4+ T cells) with perifascicular atrophy on biopsy (Fig. 18.10) - Treatment is corticosteroids. ### POLYMYOSITIS - Inflammatory disorder of skeletal muscle - Resembles dermatomyositis clinically, but skin is not involved; endomysial inflammation (CD8+ T cells) with necrotic muscle fibers is seen on biopsy. ### X-LINKED MUSCULAR DYSTROPHY - Degenerative disorder characterized by muscle wasting and replacement of skeletal muscle by adipose tissue - 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. - 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. - Becker muscular dystrophy is due to mutated dystrophin; clinically results in milder disease ## NEUROMUSCULAR JUNCTION ### MYASTHENIA GRAVIS - Autoantibodies against the postsynaptic acetylcholine receptor at the neuromuscular junction - More commonly seen in women - Clinical features: - Muscle weakness that worsens with use and improves with rest; classically involves the eyes, leading to ptosis and diplopia - Symptoms improve with anticholinesterase agents. - Associated with thymic hyperplasia or thymoma; thymectomy improves symptoms. ### LAMBERT-EATON SYNDROME - Antibodies against presynaptic calcium channels of the neuromuscular junction - Arises as a paraneoplastic syndrome, most commonly due to small cell carcinoma of the lung - Leads to impaired acetylcholine release - Firing of presynaptic calcium channels is required for acetylcholine release. - Clinical features: - Proximal muscle weakness that improves with use; eyes are usually spared. - Anticholinesterase agents do not improve symptoms. - Resolves with resection of the cancer ## SOFT TISSUE TUMORS ### LIPOMA - Benign tumor of adipose tissue - Most common benign soft tissue tumor in adults ### LIPOSARCOMA - Malignant tumor of adipose tissue - Most common malignant soft tissue tumor in adults - Lipoblast is the characteristic cell. ### RHABDOMYOMA - Benign tumor of skeletal muscle - Cardiac rhabdomyoma is associated with tuberous sclerosis. ### RHABDOMYOSARCOMA - Malignant tumor of skeletal muscle - Most common malignant soft tissue tumor in children - Rhabdomyoblast is the characteristic cell; desmin positive - Most common site is the head and neck; vagina is the classic site in young girls.