The main difference between myasthenia gravis and polymyositis is the location of their lesions. Myasthenia gravis is a disease of the neuromuscular junction and the main site of lesion in patients is the acetylcholine receptors on the postsynaptic membrane of the neuromuscular junction, which are attacked due to the autoimmune inflammation present in patients. Clinical symptoms manifest as generalized or partial skeletal muscle weakness, characterized by pathological easy fatigue, light in the morning and heavy in the evening, aggravated by activity and reduced by rest, and effective with cholinesterase inhibitor therapy. Polymyositis is a muscle disease, which is also essentially an immune inflammatory disease, a diffuse inflammation of skeletal muscle caused by multiple causes. Patients mainly present with acute or subacute muscle weakness in the proximal extremities, often accompanied by pressure pain. Blood tests can reveal a significantly elevated serum muscle enzyme profile and electromyography suggests myogenic damage, and symptoms can be improved with glucocorticoid therapy.