Polymyositis and dermatomyositis PM/DM are autoimmune inflammatory myopathies, a group of connective tissue diseases with chronic, nonsuppurative inflammatory lesions of the transverse muscle or with characteristic skin changes. The main clinical manifestations are symmetrical muscle weakness of the extremities, increased serum muscle enzyme levels in laboratory tests, especially a significant increase in creatine kinase, myogenic damage in electromyography, and histopathology suggesting varying degrees of muscle inflammation and necrosis. Etiology The etiology of this disease is still unknown, but it is currently thought to be a group of autoimmune diseases induced by immune-mediated, infectious and non-infectious environmental factors in certain genetically susceptible individuals. (i) Genetic factors: HLA-BDR3, DR6, and DRW52 were found to occur more frequently in patients with polymyositis than in the control population, so the susceptibility of patients may be HLA-based and susceptible individuals develop myositis lesions in response to certain factors. (ii) Infectious factors: polymyositis/dermatomyositis may be a chronic inflammatory process resulting from an autoimmune response induced after certain viral infections. The viruses involved include influenza virus, coxsackievirus, poliovirus, and retrovirus. In addition, certain bacterial and parasitic infections are also associated with the development of myositis. (iii) Immune factors: Autoantibodies, including anti-nuclear, anti-Jo-1, anti-RNP and anti-PM-Scl antibodies, can be detected in the serum of most patients with polymyositis/dermatomyositis. In addition, immunoglobulin and complement deposits are seen in the muscle and skin vessel walls of patients with polymyositis/dermatomyositis, suggesting that immune factors are involved in the development of polymyositis/dermatomyositis. Clinical manifestations The disease has an insidious onset and mostly develops slowly with systemic manifestations of moderate or low-grade fever, malaise, lethargy and weight loss. (i) Myositis: Myopathy is one of the important clinical manifestations of the disease. Typical patients present with progressively increasing muscle weakness in the proximal muscles of the upper and lower extremities in a symmetrical manner. Myasthenia gravis mostly starts from the pelvic girdle and lower limb muscle groups, such as difficulty in going upstairs, difficulty in standing after squatting, slow gait, and swaying instability. When the upper extremities are involved, there is a decrease in grip strength, difficulty in raising the arms and difficulty in combing the hair. If the cervical muscles are involved, the patient’s head cannot be lifted off the pillow when lying down, and in severe cases, the patient cannot turn over, and the head cannot be held upright when sitting or standing. If the pharyngeal and esophageal muscles are involved, the patient may have hoarseness, slurred pronunciation and difficulty in swallowing. The measurement of muscle strength can help to determine the degree and extent of muscle damage, and can be used as an important clinical indicator to observe the disease development and treatment effect. Muscle strength is generally classified into 6 levels. level 0: complete paralysis; level 1: muscle contraction is possible, but no mobile movement can be produced; level 2: the limb can move in the plane, but cannot be lifted to overcome gravity; level 3: the limb can be lifted off the plane, but cannot resist resistance; level 4: the limb can resist resistance, but the muscle strength is weak; level 5: normal muscle strength. (ii) Skin lesions: Skin lesions may appear before or after muscle lesions, or simultaneously with muscle lesions, and skin lesions are often the first symptom in patients with dermatomyositis. The rash includes: 1. Gottron’s sign: a purplish-red maculopapular rash appearing on the extensor side of the metacarpophalangeal, proximal phalangeal and elbow joints, flattened on the top surface and partially scaly, with skin atrophy and hypopigmentation after a long period of time. 2. Zoster: a dark purplish-red edematous rash appearing on the upper eyelids and around the orbits bilaterally, which is a characteristic rash of dermatomyositis. 3. Cutaneous heterochromatosis: a rash on the back of the shoulders, neck The skin may be locally atrophic with capillary dilation, hyperpigmentation or hypopigmentation. 4. Mechanic’s hand: The skin on the lateral and palmar surfaces of the patient’s hands appears keratinized, cracked and desquamated, resembling hands that have been operated with oil for a long time. 5. Calcification: A few patients may have subcutaneous calcified spots or calcified plaques on the shoulders, elbows, thighs, knees and spine. Ulcers and sinus tracts may appear on the skin of the calcified surface.6. Other: Patients with dermatomyositis may also develop nail degeneration, fingertip ulcers and Raynaud’s phenomenon. (iii) Joint lesions: Some patients with polymyositis/dermatomyositis may present with arthralgia or arthritis, with proximal interphalangeal joints, metacarpophalangeal joints and wrist joints being most commonly involved, with occasional joint deformities, but no destruction of bony joints on X-ray. (iv) Pulmonary changes: Pulmonary lesions in patients with polymyositis/dermatomyositis are mainly manifested as interstitial lung lesions, aspiration pneumonia and pleurisy. Patients may present with cough, sputum and dyspnea. (v) Cardiovascular system changes: Some patients may present with palpitations, precordial discomfort and dyspnea. ST-T changes, arrhythmias, mitral valve prolapse and pericardial effusion can be seen on electrocardiogram and echocardiogram. (vi) Digestive changes: Patients with polymyositis/dermatomyositis may have dysphagia and esophageal reflux due to involvement of esophageal and pharyngeal muscles, and a few patients may develop peptic ulcers. (vii) Renal changes: The disease rarely involves the kidneys. Individuals may develop focal proliferative glomerulonephritis, but most have normal renal function. (viii) Other: 10-30% of patients develop malignant tumors, such as gastric cancer, lung cancer and breast cancer are common. In addition, myositis may be significantly relieved after removal of the tumor.