A concussion is the most common type of traumatic brain injury. It typically occurs after the head strikes some object, though concussions can be caused by whiplash. In most cases, the brain does not suffer any actual structural damage, but blood vessels may be stretched and some cranial nerves may be damaged. A concussion may be followed by brief unconsciousness, memory loss, loss of balance, depression, and difficulty concentrating. There are five grades of concussions:

• Grade I is characterized only by confusion
• Grade II involves confusion and anterograde amnesia (sometimes called short-term memory loss) lasting up to five minutes
• Grade III involves all of the characteristics of Grade II, as well as unconsciousness lasting up to five minutes and an inability to remember events that occurred just prior to the concussion (also called retrograde amnesia)
• Grade IV is characterized by all of the symptoms of Grade III, but unconsciousness can last up to 10 minutes
• Grade V is characterized by all of the symptoms of Grade IV, but unconsciousness can last up to twenty-five minutes

Depending on the severity of the trauma, post-concussion symptoms may last up to a few weeks.Complete recovery may occur in a few months or may take several years.