Disorders of Skeletal Muscle
- A pathologic contracture is permanent muscle shortening caused by muscle spasticity, as seen in central nervous system (CNS) injury or sever muscle weakness.
- Stress-induced muscle tension is presumably caused by increased activity in the reticular activating system and gamma loop in the muscle fiber. The use of progressive relaxation training and biofeedback has been advocated to reduce muscle tension.
- Fibromyalgia is a chronic musculoskeletal syndrome characterized by diffuse pain and tender points. Unknown but suspected is that muscle is the end organ responsible for the pain and fatigue. Most cases are women, and the peak age is 30 to 50 years.
- Atrophy of muscle fibers and overall diminished size of the muscle are seen after prolonged inactivity. Isometric contractions and passive lengthening exercises decrease atrophy to some degree in immobilized patients.
- Hyperexcitable membranes cause the physical and electrical phenomenon of Myotonia. The disorder is treated with drugs that reduce muscle fiber excitability. Periodic paralysis is caused by an unresponsive muscle membrane and is accompanied by changes in serum potassium. The biochemical defect is possibly related to changes in the muscle membrane and sarcoplasmic reticulum.
- Metabolic muscle diseases are caused by endocrine disorders, glycogen storage disease, enzyme deficiencies, and abnormal lipid function. The muscle depends on a complex system of carbohydrates and fats converted by enzymes to produce energy for the muscle cell. Abnormalities in these pathways can inhibit function or cause damage to the muscle fiber. These illnesses are rare, yet they account for significant functional abnormalities.
- Viral, bacterial, and parasitic infections of muscles produce the characteristic clinical and pathologic changes associated with inflammation. These are usually treatable and self-limiting disorders.
- Polymyositis (generalized muscle inflammation) and dermatomyositis (polymyositis accompanied by skin rash) are characterized by inflammation of connective tissue and muscle fibers and muscle fiber necrosis. Cell-mediated and humoral immune factors have been implicated. Treatment with immunosuppressive agents is effective in many cases.
- The most common toxic myopathy is caused by alcohol abuse. Direct toxic effects of alcohol-producing necrosis of muscle fibers and nutritional deficiency have been suggested. The only treatment is abstinence and improved nutrition. The toxic effects of many drugs on muscle fibers cause local trauma to the muscle fibers caused by direct effects of the needle, secondary infection, and changes caused by non-physiologic acidity and alkalinity in the fibers.