A concussion is a mild traumatic brain injury (TBI) induced by biomechanical impulsive forces transmitted to the head, resulting in rapid (minutes to hours) onset of short-lived nonfocal motor or sensory neurologic impairment. Symptoms should not be explained by medication or substance use or by comorbid medical or psychological conditions. This can be the result of a sports-related injury (most common in children), motor vehicle collision (MVC), falls, or violence. A concussion may or may not involve a loss of consciousness.

Concussions result in cellular-level injury in the brain that then results in functional changes. Concussive symptoms may involve physical, emotional, cognitive, and behavioral symptoms. Symptoms may include confusion, amnesia, disorientation, concentration difficulties, headache, neck pain, nausea, vomiting, dizziness, vertigo, vision changes, photophobia, phonophobia, emotional and sleep changes, impaired balance, and slower reaction times. There are several different pediatric concussive guidelines published by the American Academy of Pediatrics (AAP), the US Centers for Disease Control and Prevention (CDC), and other organizations that can assist in diagnosis as well as recommendations regarding return to learning and return to play. The diagnosis of a concussive injury is a clinical diagnosis based on history and physical examination. The use of neuroimaging should be limited; see Best Tests for guidelines.

Concussions are common and have been diagnosed in over 3.2% of patients younger than 18 years. Sports-related concussions are more common in males, with football having the highest incidence of all high school sports.

Most concussive symptoms will resolve spontaneously in days to a few weeks, but they can last for months or years in some cases. Prolonged symptoms may indicate persisting symptoms after a concussion (a term that has replaced postconcussive syndrome). It is imperative to identify concussive injuries to prevent long-term complications as well as second impact syndrome and to facilitate return to activities (return to learning and return to play). Second impact syndrome is a rare condition where a second head injury occurs shortly after a previous head injury, before the first head injury symptoms have resolved. It can result in rapid and severe brain swelling that can lead to brain herniation and death.