This study will examine the abnormal immune response in patients with sporadic inclusion body myositis (s-IBM)-the most common inflammatory muscle disease in people over the age of 50. s-IBM progresses steadily and may lead to severe weakness and wasting of arm and leg muscles. Patients may become unable to perform daily living activities and be confined to wheelchairs. s-IBM is thought to be an autoimmune disease, in which the body's own immune system attacks healthy muscles. This study will explore the causes of the muscle tissue inflammation that is responsible for destruction of muscle fibers and weakness in this disease. Information from the study may help in the development of an effective treatment for this disease. Patients with s-IBM may be eligible for this study. Those who are unable to travel or who have severe cardiovascular, renal or other end-stage organ disease will be excluded. Candidates will be screened for eligibility with a medical history and physical and neurological examinations. Participants will be seen at the NIH Clinical Center every six months over a 12-month period (visits at enrollment, 6 months and 12 months) either on an inpatient or outpatient basis, depending on their disease severity. Each 2- to 3-day visit will involve the following tests and evaluations: * Blood samples for routine laboratory tests are collected at every visit. Additional blood for research studies is collected at 12 months. * Quantitative muscle strength testing is done at every visit. The patient pulls against straps connected to dynamometers (devices that measure muscle power) to evaluate strength of the main muscle groups in the arms and legs. * Lymphapheresis is done at enrollment and at 12 months. This is a procedure for collecting quantities of lymphocytes (white blood cells that are an important part of the immune system). Blood is collected through a needle placed in an arm vein and circulated through a machine that spins it, separating it into its components. The lymphocytes are removed and the rest of the blood (red cells, platelets and plasma) is returned to the body through the same needle or another needle placed in the other arm. * Electrophysiologic studies (electromyography and nerve conduction testing) are done at enrollment and 12 months. Electromyography evaluates the electrical activity of muscles. A small needle is inserted into the muscle and the patient is asked to relax or to contract the muscle. For nerve conduction testing, nerves are stimulated by electrodes (small wires taped to the skin over the muscle). * Muscle biopsy is done at enrollment and 12 months. A sample of muscle tissue (about the size of a lima bean) from an arm or leg is surgically removed to confirm the diagnosis of s-IBM and for analysis of proteins involved in the muscle inflammation process. A local anesthetic is used to numb the area before the surgery and the wound is closed with stitches.
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