Polymyositis and Dermatomyositis...major causes of elevated sed rate and muscle weakness
by Nathan Wei, MD, FACP, FACR
Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.
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These two conditions are inflammatory autoimmune diseases that affect muscle.
(Another type of inflammatory muscle problem is called inclusion body myositis. It is clinically similar to the other two but can be differentiated by differences in cell histology. It is less common).
The cause of these conditions is unknown.
The diagnosis is often made by muscle biopsy. The biopsy will show muscle fibers being invaded or surrounded by lymphocytes. Various degrees of muscle cell damage, death, and regeneration can be seen. Late findings will demonstrate replacement of normal muscle cells with fibrous tissue, fat, and atrophied cells.
Polymyositis may be distinguished from dermatomyositis by the type of invading lymphocyte. In polymyositis, the lymphocyte invader is the CD8+ T cell. With dermatomyositis, it is usually CD4+ T cells and B cells. Complement, a blood protein, may also deposit in muscle cells causing injury to blood vessels and further muscle damage.
This group of diseases is relatively rare occurring in a prevalence of approximately 0.05 per cent. PM is a bit more common than DM. Both diseases strike most often in the 5th through 7th decades.
Patients present with muscle weakness occurring in a symmetric fashion affecting large muscles in the arms and legs. Activities such as getting out of bed, into and out of a car, going up stairs, getting out of a chair, getting off the toilet, or brushing hair may be difficult. Neck weakness will present with the patient being unable to get their head off a pillow. Involvement of the throat muscles may cause difficulty with swallowing, regurgitation or aspiration of food/drink, or a nasal sounding voice. When respiratory muscles (the intercostals muscles between the ribs, or the diaphragm), patients will develop shortness of breath or respiratory failure.
The heart is involved at least half the time. Serious involvement of heart muscle leads to cardiomyopathy… often fatal. (One note... the great University of Southern California running back, Ricky Bell, died as a result of cardiac involvement from polymyositis.) Lung involvement may cause scarring.
The differentiating factor between polymyositis and dermatomyositis is the presence of skin involvement in the latter disease. Raised areas of rash on the knuckles or knees are seen in about 60-70 per cent of patients with DM. These are called Gottron’s papules. Rash affecting the upper chest, upper back, and neck is called “V-shaped” and is seen in areas exposed to sunlight. The small blood vessels in the nail beds (capillaries) will be abnormal. These abnormalities can be detected using an ophthalmascope.
Inflammatory muscle disease may be associated with malignancy. Nine per cent of patients with PM and 15 per cent of patients with DM will have a malignancy. The cancer can occur before, during, or after the muscle disease. The type of cancer is the same as for the general population, e.g. lung, breast, and colon. It is recommended that patient go through a thorough workup including careful history, physical exam, and laboratory workup.
Laboratory abnormalities that may be seen include an elevation of muscle enzymes such as creatine phosphokinase (CPK). The level of CPK elevation mirrors the severity of disease and the level of CPK tends to fluctuate with level of disease activity. Another muscle enzyme, aldolase, may also be abnormal. The erythrocyte sedimentation rate (ESR) is often elevated. The ANA is positive in up to 80 per cent of patients. Anti-Jo-1 is an antibody that is present in 25-30 per cent of patients with PM or DM.
Electromyography is abnormal in more than 90 per cent of patients. Electrical activity tends to parallel disease activity. EMG is also useful for determining whether patients treated with long term steroids have muscle weakness due to their muscle disease or due to the steroids used for treating the muscle disease.
Muscle biopsy is usually required for definitive diagnosis. Biopsies may be obtained either by using a muscle biopsy needle or by doing an open biopsy. Biopsies need to be prepared appropriately and read by a pathologist skilled and experienced in interpreting muscle pathology.
Magnetic resonance imaging is helpful in establishing diagnosis. T2 weighted images are particularly useful in detecting inflammation. Fat suppressed T2 (STIR) images are especially helpful.
Cardiac evaluation and pulmonary functions tests should be obtained.
PM or DM can be confused with many other conditions that cause muscle weakness and fatigue. Patients need to go through a careful workup. Examples of diseases include neurological conditions (e.g., myasthenia gravis), metabolic muscle disease (McArdle’s disease), endocrine disorders (thyroid disease), drugs (alcohol, colchicine, hydroxychloroquine), and infections (viruses, parasites).
The aims of therapy are to control symptoms and prevent progression of disease. Early diagnosis and treatment is critical. Patients are usually treated with corticosteroids in relatively high doses (1mg/kg of prednisone or equivalent). Steroids are then tapered over time very slowly and carefully. Steroid response may take anywhere from 6-8 weeks. Patients who do not respond to steroids will require either methotrexate or azathioprine. High dose intravenous IgG (IVIG) has also been used with success.
Long term steroid therapy can lead to a condition called steroid myopathy which causes muscle wasting and weakness. Differentiation between this condition and worsening of inflammatory muscle disease may be made through either EMG or muscle biopsy.
Physical therapy is often helpful.
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