The brain is protected by fluid that fills the space between it and the hard, bony skull. The fluid acts as a cushion. However, a forceful blow to the head can cause the brain to slam against the skull’s inner wall. This can cause bruising and damage nerves that carry signals to the brain. It can also tear blood vessels and cause bleeding. Sometimes the victim is knocked out and is no longer conscious—or mentally aware—of what’s going on.

Two common early symptoms—or signs—of concussion are confusion and amnesia (a loss of memory for what caused the injury). Days later, victims may have headaches and blurred vision and be bothered by bright lights and noise. They may find it hard to concentrate and sleep, become easily upset, and feel depressed.