A nonspecific term used to describe transient alterations or loss of consciousness following closed head injuries. The duration of unconsciousness generally lasts a few seconds, but may persist for several hours. Concussions may be classified as mild, intermediate, and severe. Prolonged periods of unconsciousness (often defined as greater than 6 hours in duration) may be referred to as post-traumatic coma (coma, post-head injury). (From Rowland, Merritt's Textbook of neurology, 9th ed, p418)