Glucocorticoids in high dosage is the accepted treatment for severe dermatomyositis-polymyositis, though there is no controlled trial to prove its effectiveness. Prednisone is generally started at a dose of 1 to 2 mg/kg body weight per day (60 to 100 mg/d for adults). Improvement may begin within 1 to 4 weeks, though in some patients treatment may need to be continued for 3 months before improvement occurs. When improvement is noted, the daily dose may be reduced by 5 mg every 4 weeks. Repeated manual muscle testing and serum CK determinations should be performed to ensure that the myositis does not relapse. At about 40 mg/d, the schedule is changed gradually to 80 mg every other day in order to reduce the incidence of side effects. There is some evidence that the use of alternate-day glucocorticoids from the outset may be effective, particularly in patients with milder disease. Adults with acute to subacute dermatomyositis-polymyositis tend to improve more rapidly than those with chronic polymyositis; children also respond in most cases. If the dose is reduced too rapidly, or to too low a level, relapse will occur, necessitating return to high dosage. Prednisone therapy may have to be continued for several years, but an attempt should be made every year to withdraw the therapy from patients who are clinically stable in order to determine if the disease is still active.
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Patients with dermatomyositis who have the characteristic skin rash, muscle weakness, EMG changes, and elevation of serum CK may not require a muscle biopsy to confirm the diagnosis. In the case of idiopathic polymyositis, however, a firm diagnosis must be based on the presence of a typical clinical picture, a typical EMG, elevation of serum CK, and a diagnostic muscle biopsy. All four of these criteria are required to be certain of the diagnosis, since inflammatory changes may occasionally occur in other myopathies (e.g., facioscapulohumeral muscular dystrophy) and in other connective tissue disorders without clear muscle weakness. However, in fewer than one-third of cases of polymyositis are ALL these criteria satisfied. It may be particularly difficult to obtain a diagnostic muscle biopsy because of the patchy nature of the disease. Thus, a therapeutic trial of glucocorticoids should be given when full investigation of a patient with significant disability leaves a diagnosis of “possible polymyositis,” usually because of a nondiagnostic muscle biopsy.
In all forms of polymyositis there may be elevated serum levels of the enzymes present in skeletal muscle, such as, aldolase, serum glutamic oxaloacetic transaminase, lactic acid dehydrogenase, and serum glutamic pyruvate transaminase. The degree of elevation decreases from the first to the last in this series of enzymes, and the pattern is the reverse of that seen in liver disease. The erythrocyte sedimentation rate is elevated in about two-thirds of cases. Tests for circulating rheumatoid factor are positive in less than one-half and for antinuclear antibodies in about three-quarters of the cases. Most other hematologic indexes are normal. Several autoantibodies seem to be associated with clinically distinct groups of patients. Anti-Jo-1 antibodies are more common in polymyositis, especially in patients with interstitial lung disease, and anti-nRNP antibodies are often associated with polymyositis seen in lupus erythematosus. Other antibodies seen in patients with dermatomyositis and polymyositis in association with connective tissue diseases include anti-Scl-70 (progressive systemic sclerosis), anti-Sm (lupus erythematosus), anti-Ro and anti-La (Sjцgren’s syndrome and lupus erythematosus), and anti-ENA (mixed connective tissue disease). Myoglobin can be found in the urine when muscle destruction is acute and extensive; rarely, acute polymyositis causes the full syndrome of rhabdomyolysis and myoglobinuria. In about 40 percent of cases EMG reveals a markedly increased insertional activity (muscle irritability), together with the typical myopathic triad of motor unit action potentials, which are of low amplitude, are polyphasic, and have an abnormally early recruitment. In a further 40 percent of the patients only myopathic changes are present. The ECG is abnormal in about 5 to 10 percent of the cases at presentation. Since the pathologic process in myositis is patchy, greater diagnostic yield is accomplished by obtaining a biopsy from two clinically affected muscles and by skip serial sectioning of all specimens. Magnetic resonance imaging may serve to identify sites of muscle involvement. Muscles recently used for EMG or intramuscular injection must be avoided as these procedures can produce inflammatory changes and muscle fiber damage, leading to false-positive results. In about two-thirds of cases, the biopsies will demonstrate the typical pathologic changes of myositis, but despite following the above recommendations, about 10 percent of cases have normal muscle biopsy. Read the rest of this entry »
Group I: Primary Idiopathic Polymyositis
This group comprises about one-third of all cases of inflammatory myopathy. It is usually insidiously progressive over weeks, months, or even years. Rarely the disease is acute, producing severe muscle weakness in a matter of days or even rhabdomyolysis. The disease may develop at any age. Affected females outnumber males 2:1.
Patients first become aware of weakness of the proximal limb muscles, especially the hips and thighs, and find difficulty in arising from the squatting or kneeling position and in climbing or descending stairs. When shoulder girdle muscles are involved, placing an object on a high shelf or combing the hair becomes difficult. Occasionally the disease is more restricted, affecting only the neck, the shoulder, or the quadriceps muscles. Aching pain in the buttocks, thighs, and calves is experienced in about 10 percent of the cases, and tenderness on palpation in another 20 percent. In the majority of patients the disorder is painless. Early symptoms of dysphagia and weakness of flexor muscles of the neck in a patient with a chronic myopathy suggest the diagnosis of polymyositis. Order Nexium only for: $1.12
When the patient is first seen, there may be weakness of the muscles of the trunk, the pectoral and pelvic girdles, the upper arms and thighs, the neck, and the pharynx. Ocular muscles are almost never affected except in a rare association with myasthenia gravis. The distal muscles are spared in about 75 percent of cases. Muscle atrophy, contractures, and diminished tendon reflexes are rare in early myositis and never as pronounced as in muscular dystrophies and denervating conditions. When the reflexes are disproportionately reduced, carcinoma with polymyositis and polyneuropathy or the Lambert-Eaton syndrome should be considered. Occasionally, the reflexes may be paradoxically brisk in dermatomyositis-polymyositis, perhaps due to irritation of muscle spindle receptors by the inflammation.
Dermatomyositis and polymyositis are autoimmune disorders in which the skeletal muscle is damaged by an inflammatory process dominated by lymphocytic infiltration. The term polymyositis is applied when the condition spares the skin, and the term dermatomyositis when polymyositis is associated with a characteristic skin rash. One-third of cases are associated with various connective tissue disorders, such as rheumatoid arthritis, lupus erythematosus, mixed connective tissue disorder, and progressive systemic sclerosis, and one-tenth with a malignancy. Inclusion body myositis is a distinct clinicopathologic entity characterized by the presence of vacuolated inclusions containing tubulofilaments in muscle.
The precise cause of these diseases is unknown, but interplay between host genetic factors, viral infection of muscle, and autoimmune mechanisms is probably contributory. Familial occurrence of these diseases, and the increased frequency of HLA-DR3 and -DRw52 antigens in patients, suggest an underlying genetic and immunologic predisposition. Experimental viral myositis can be induced in animals by coxsackievirus. A mild inflammatory myopathy can occur with influenza and coxsackieviruses in humans. However, the several electron-microscopic observations of virus-like particles in muscle fibers in dermatomyositis or polymyositis have not been confirmed by virus isolation or demonstration of rising viral antibody titers, and the disease has not been passed into animals by injection of extracts of skeletal muscles. Nevertheless, the presence of serum antibodies to several cytoplasmic ribonucleoproteins involved in translation [especially histidyl tRNA synthetase or Jo-1 and signal recognition particle (SRP)] may result from an immune response to an altered virus that serves as an immunogen in polymyositis. These antibodies probably represent a cross-reactive phenomenon.