Question:
Regarding some of the recent threads on the way brain injury is perceived, I thought you all might find this of interest. Recently, waiting.com is receiving letters from writers asking for info like; How long is the longest coma from which someone has awakened from and had a full recovery? How badly can the brain be injured and still have a full recovery? Can you be in a coma for a long time and not have a brain injury? Just goes to show everyone wants the "long coma, person awakens, and all that's passed is time" plot. Don't worry, I set them straight, gently. Maybe I've headed off a few misconceptions? Hopefully. Actually, I told one of them, instead of putting his character in a coma, put him in a small midwestern town, its kind of like being in a coma.

Answer: The dilation of pupils is actually a middle event typically caused by the beginning of a type of upper brain herniation (an uncal herniation, if you really want to know) placing preasure on cranial nerve 3. Initially, this impingement will knock out the parasympathetic fibers leading to overwelming sympathetic tone and pupillary dilation. Later as the pressure increases and CN III is further damaged, the motor fibers will be affected resulting in a deviation of the eye 'down and out' on the side of the injury.

Eventually, if this increased pressure is allowed to progress, you will begin to herniate the brain stem. This will result in damage to the vasculature in the area, leading to small areas of hemorrhage (called Duret hemorrhages) in the brainstem. At this point coma and death will occur within a short period of time. This also explains why after some episodes of severe edema even if you manage to get the ICP and cerebral edema down the patient will often become locked into a persistant vegitative state. There is an area in the midbrain called the reticular activating system (RAS) which controls, for lack of a better word, 'wakefullness'. If this area is damaged (either by Duret hemorrhage or hypoxia), it becomes impossible for the patient to wake up and become alert.