musculoskeletal and connective tissue disorders
Autoimmune Myositis
Diagnosis of Autoimmune Myositis
Clinical criteria Muscle biopsy (definitive) Autoimmune myositis should be suspected in patients with proximal muscle weakness with or without muscle tenderness. Dermatomyositis should be suspected in patients with symptoms of myositis and skin findings compatible with dermatomyositis. Establishing the diagnosis of autoimmune myositis requires as many as possible of the following 5 criteria: Proximal muscle weakness Characteristic rash Elevated serum muscle enzymes (if creatine kinase [CK] is not elevated, aminotransferases or aldolase, which are far less specific than CK) Characteristic electromyographic or MRI muscle abnormalities Muscle biopsy changes (the definitive test) findings can vary, but chronic inflammation with muscle degeneration and some regeneration is typical. Polymyositis and dermatomyositis can often be distinguished by muscle biopsy. A definite diagnosis made by muscle biopsy is recommended before treatment of polymyositis to exclude other muscle disorders, such as those due to missing or defective enzymes, necrotizing myositis, and postviral Rhabdomyolysis . Muscle biopsy is not usually necessary when skin findings are characteristic for dermatomyositis. There is no pathognomic skin finding for dermatomyositis on biopsy, but the absence of direct immunofluorescence helps distinguish the rash from the rash in patients with To increase the sensitivity of the biopsy results, the biopsy sample should be obtained from a muscle that has one or more of the following characteristics: Weakness on clinical examination Muscle edema identified on MRI Contralateral pair of a muscle shown to be abnormal on electromyography Laboratory studiescan increase or decrease suspicion for the disorder, assess its severity, identify overlaps, and help detect complications. Autoantibodies should be tested. Antinuclear antibodies (ANA) are positive in up to 80% of patients with dermatomyositis and polymyositis. If the ANA test is positive, further testing for specific types of antibodies is important in increasing the suspicion for an overlap syndrome. Clinical course and manifestations are associated with particular antibodies as described in table. The relationship between these autoantibodies and disease pathogenesis remains unclear, although antibody to Jo-1 is a significant marker for fibrosing alveolitis, pulmonary fibrosis, arthritis, and Raynaud syndrome. There are no antibodies specific for polymyositis. Evidence for increased risk of cancer is relatively strong in dermatomyositis and less strong for polymyositis. Therefore, cancer screening should be considered for patients ≥ 40 years who have dermatomyositis or for patients≥60 years who have polymyositis because these patients often have unsuspected cancers. Screening should include at least a physical examination that includes the breasts, pelvis, and rectum (with occult blood testing); complete blood count (CBC); biochemical profile; mammogram; urinalysis; chest x-ray; and any other tests appropriate based on the patient’s age. Additional investigation should be based on history and physical examination findings. Some authorities recommend CT of the chest, abdomen, and pelvis as well as colonoscopy, particularly in patients who have dermatomyositis. Younger patients without symptoms of cancer need not undergo screening. 