A person in a coma cannot hear the sounds of outside speech. Coma is a common type of disorder of consciousness and the most severe of all types of disorders of consciousness; it is classified as superficial, moderate, or deep coma. Regardless of the type of coma, the patient has a complete loss of voluntary consciousness and cannot be awakened by external stimuli, including loud talking, hard tapping, or severe painful stimuli, and the patient does not have the usual withdrawal response. Comatose patients are severely impaired in both level and content of consciousness, and most of them are caused by serious organic diseases of the brain, or serious systemic diseases. For example, massive cerebral hemorrhage, massive cerebral infarction, severe encephalitis, severe open cranial injury, advanced hepatic or pulmonary encephalopathy, diabetic ketoacidosis, hypertonic coma, etc.