A brain bruise (contusion) occurs when the head sustains a hard blow and the impact creates a sudden movement of the brain within the skull. The brain can strike the skull with enough force to cause a bruise in the brain tissue.
Brain tissue swells and bleeds when the brain is bruised. As the brain tissue swells and blood builds up, pressure within the skull increases and may damage brain tissue. Surgery may be required to relieve the pressure.
WebMD Medical Reference from Healthwise