Pathophysiology

Though it is more likely to cause infection, penetrating trauma is similar to closed head injury such as cerebral contusion or intracranial hemorrhage in a number of ways. As in closed head injury, intracranial pressure is likely to increase due to swelling or bleeding, potentially crushing delicate brain tissue. Most deaths from penetrating trauma are caused by damage to blood vessels, which can lead to intracranial hematomas and ischemia, which can in turn lead to a biochemical cascade called the ischemic cascade. The injury in penetrating brain trauma is mostly focal (affects a specific area of tissue).[2]

Source: http://en.wikipedia.org/wiki/Penetrating_head_injury