Parenchymal injuries include concussion, contusion, laceration and white matter injury. Contusions occur where the brain hits the bones of the skull following a decelerating injury. Contusions at the site of the initial impact represent a coup injury, those on the opposite site, a contre-coup injury. The contre-coup injury is often more extensive. The common sites of injury are the crests of gyri at the tips of the temporal lobes, the inferior frontal surfaces and the occipital poles.
Early lesions show oedema and haemorrhage.
Histologically, the lesions show gliosis and macrophages containing haemosiderin. White matter injury can take the form of diffuse axonal injury deep in the brain. Axonal retraction balls and focal haemorrhages are seen. It is believed mechanical forces damage the integrity of the axons at nodes of Ranvier. In patients who survive head injuries, frontal and temporal lobe lesions often result in severe permanent disability.