Dermatomyositis is a non-purulent inflammatory lesion involving mainly the transverse muscles with a lymphocytic infiltrate and may or may not be associated with a variety of skin lesions. It is characterized by symmetrical weakness of the limb girdle, cervical and pharyngeal muscles, often involving multiple organs, and may be associated with tumors and other connective tissue diseases. 1. symmetric muscle weakness Limb-girdle and cervical anterior flexor muscle weakness that progresses over weeks to months with or without dysphagia or respiratory muscle involvement 2. muscle biopsy abnormalities Type I and II muscle fiber necrosis, phagocytosis, regeneration, basophilia, large membranous vesicles and enlarged nucleoli in the nucleus, perimysial atrophy, muscle fibers of unequal size, and perivascular inflammatory cell infiltration. Elevated serum skeletal muscle enzyme levels, especially creatine phosphokinase, followed by aldolase, serum aminotransferase and lactate dehydrogenase. 4. EMG evidence EMG shows short duration, low amplitude, polyphasic waves of motor unit potentials, increased fibrillation waves, positive sharp waves and insertional activity, repeated abnormal high frequency discharges. 5. Skin manifestations Mauve spots on the eyelids (sunny rash) and periorbital edema, red rash on the back of the hands. A flaky rash (especially on the metacarpophalangeal and proximal interphalangeal joints, called Gottron’s sign) that may involve the knees, elbows, and ankles, as well as the face, neck, and upper chest. The limitations of the diagnosis using this criterion are defined below. Three of the four criteria plus a rash must be met to confirm a diagnosis of dermatomyositis; four criteria must be met without a rash to confirm a diagnosis of polymyositis. If two criteria are met and a rash is present, dermatomyositis is likely; if three criteria are met and no rash is present, polymyositis is likely. If one criterion is met and a rash is present, dermatomyositis is likely; if two criteria are met and no rash is present, polymyositis is likely. Evidence of central or peripheral neuropathy, including abnormalities of motor neurons leading to spontaneous contraction of muscle fibers or long tract signs, sensory changes, shortened nerve conduction times, and muscle biopsies revealing fiber atrophy and aggregation in clusters. 2, Slow progression and persistence of myasthenia gravis, positive family history or calf thickening suggest the presence of myotonic dystrophy. 3, Biopsy evidence of sarcoidosis myositis, such as nodular disease. 4, Infections, including trichinosis, schistosomiasis, trypanosomiasis, staphylococcal disease and toxoplasmosis. 5, Recent use of various drugs and poisons, such as clofibrate and alcohol. 6.Rhabdomyolysis mainly manifests as sarcoid myoglobinuria, which is seen after stressful exercise, infection, crush injury, occlusion of major arteries in the limbs, prolonged coma or spasm, high voltage electric shock accident, heat stroke, malignant hyperthermia syndrome and bite by some kind of sea snake. 7, metabolic abnormalities such as McArdle’s syndrome (McArdle’s) 8, endocrine diseases such as hyperthyroidism, mucinous edema, hyperparathyroidism, hypoparathyroidism, diabetes mellitus or Cushing’s syndrome. 9, response to acetylcholine, and diminished response to repetitive neurological stimulation in d-cyltropine sensitive myasthenia gravis.