Polymyositis (PM) is mainly seen in adults, but rare in children.PM often has a subacute onset, with symmetrical proximal muscle weakness of the extremities occurring within weeks to months, and only a few patients may have an acute onset of the disease. It is often accompanied by systemic manifestations such as malaise, anorexia, weight loss and fever. Symmetrical weakness of the proximal muscles of the extremities is the main feature of the disease. When the proximal muscles of the upper extremities are involved, there may be difficulty in lifting the arms, and inability to comb the hair and dress. When the proximal muscles of the lower limbs are involved, it is often manifested as difficulty in walking up stairs and steps, and difficulty in squatting or getting up from a seat.Distal muscle weakness is uncommon in patients with PM, and about 50% of the patients may be accompanied by myalgia or muscle tenderness at the same time. However, patients may have varying degrees of distal muscle weakness throughout the course of the disease. With the prolongation of the disease, myasthenia gravis may appear. About half of the patients have cervical flexor muscle weakness, which is characterized by difficulty in raising the head when lying down, and the head is often tilted back.