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Movies and television teach us that being "knocked out" is a temporary event with no lasting effects. Even among Hollywood's pantheon of lies, this one ranks near the top.

Blunt head trauma, which may or may not involve a clinically or radiographically visible injury to the bones, can cause intracranial bleeding and/or traumatic brain injury. A bleed may be arterial or venous, with the chief clinical distinction being the pressure within the affected system. An arterial bleed (a "subarachnoid hematoma") often allows for a "lucid interval" of a few hours after which the pressure of the growing intracranial hematoma upon the brain grows to such an extent that rapid death will result without prompt craniotomy. CAVEMEN trepanned their compatriots when so injured because even THEY realized that the injury was certainly fatal if not treated. Ponder that- this is an injury so severe that people with no knowledge of anatomy, germ theory, antiseptics, or even metal tools still felt that the risk of neurosurgery was merited (medicine and archaeology proves that they were right). The bleeding injury could also be of a venous nature (a "subdural hematoma"), which will take longer to manifest and often presents as subtle personality changes; these injuries often do not require surgical treatment, but that distinction is made by neurosurgeons with the aid of CT scans. A venous head bleed is a comparatively minor problem TO ME, because I have instant recourse to CT scanners and in-house neurosurgeons; It will probably get you transferred out of a community hospital for a higher level of care.

The injury might also be to the brain parenchyma, in which case it is a traumatic brain injury. The brain is not a tough organ (strangely enough embalmed brains are- it always amazed me how solid those things were), having an unusually high concentration of lipids which essentially function as the insulators for billions of miniscule wires ("axons"). In addition, most of the activity that we associate with the brain occurs near the surface of the brain, particularly in the telencephalon, with thinking occurring in the prefrontal cortex (the area beneath your forehead). Abrupt acceleration deceleration injuries can cause undetectably tiny and numerous tears in the fabric of the brain which, in aggregate, can constitute terrible injuries. TBI ranges in severity from "overcome with therapy" to "PVS", and the mechanism of injury may not be a good guide to where on that spectrum any one injury might fall. Blunt trauma severity follows an unpredictable scale with regard to causative injury, with the effects of multiple insults being additive (possibly in a supra-linear manner, see Chronic Traumatic Encephalopathy).

Finally, lack of blood flow the brain can result in anoxic brain injury. This sort of injury need not involve trauma to the head. The brain uses 20% of the cardiac output at rest, and the body will sacrifice everything else to preserve it. Many hand to hand fighting techniques are based upon denying blood flow to the brain, thus incapacitating one's opponent through unconsciousness. The differences between "incapacitated", "dead", and "victim of irreversible cognitive damage" can be very slight amounts of time. An assailant may not care that you have gone limp and are suffering brain damage and/or dying. Injuries which affect your heart's ability to provide your brain with blood can cause terrible cognitive deficits even if prompt medical attention saves your life.

That may be more than any cared to read about a genuinely terrible class of injuries, but their importance simply cannot be overstated.