Myositis is defined as inflammation of muscle tissue. The condition may be referred to as polymyositis with inflammation and deterioration in the muscles.

The exact cause is unknown. The condition may be caused by infection, trauma, diabetes mellitus, infestation by parasites, or an autoimmune reaction.

Signs and symptoms of the disease may include: an acute onset with or without notable signs and symptoms; various structural changes in the muscles such as wasting away and degeneration; muscle weakness; and muscle tissue necrosis. There may be difficulty in raising the arms above the shoulders, climbing steps, or getting out of a chair. The muscle weakness may start suddenly and progress over weeks to months. Complete muscle weakness indicates advanced myositis. There may also be pain in several joints (polyarthralgias); increased fluid in the joint; dysphagia or inability to swallow; fever; fatigue; and weight loss. Malnutrition may be present in chronic polymyositis. The disease may also be associated with inoperable tumors.

Diagnostic findings include a complete blood count (CBC) which shows phagocytosis, and an elevated erythrocyte sedimentation rate (ESR). Other tests performed include electromyography, and a muscle biopsy.

Treatment may include limitation of physical activities during the inflammatory stage. In the initial stage, corticosteroids are the drugs of choice. Immunosupressive drugs may also be used.

Residuals may include extreme weakness of the pelvic and shoulder muscles requiring the person to be wheelchair-bound or bedridden. There may be an inability to lift the head off the pillow, and contractures may occur. Additional residuals may include acute respiratory insufficiency, as well as dysphagia and regurgitation. The person may receive maintenance corticosteroid dosages indefinitely. The prognosis is poor if the person has co-existing conditions such as cardiac and respiratory involvement.

• The rating schedule for musculoskeletal was updated on February 7, 2021. Protection still does apply and should be considered with existing evaluations (38 CFR 3.951(a)).

• The provisions for 10 and 20 percent evaluations for arthritis with multiple joint involvement without limitation of motion do not apply to these conditions as is reflected in Note (2) under DC 5003.

• Since myositis is not always chronic, chronic residuals are required to establish service connection.

• May be entitled to special monthly compensation under 38 CFR 3.352 for advanced myositis. Also, consider entitlement to SMC under 38 CFR 3.350 as an issue in every case where there is a severe degree of disability involving the loss or loss of use of an extremity or sensory organ or any other functional loss providing entitlement to SMC.

• Evaluate the diseases under diagnostic codes 5013 through 5024 as degenerative arthritis, based on limitation of motion of affected parts.