How can a seemingly trivial head injury kill you?
To answer this, you need a little anatomy.
Your brain is a pretty important organ, and is well protected. It sits inside a thick armor (the skull) and floats cushioned in a bath of cerebral-spinal fluid. It’s surrounded by several layers of tissue, and its blood supply is kept relatively separate from the rest of the body (the “blood-brain barrier”). This separation helps keep out toxins and micro-organisms (but is imperfect). Just beneath the skull is a tough, leathery layer called the dura mater. This picture shows the skull cut away, and the dura peeled back by a forceps.
Ignoring the markedly abnormal brain underneath, you can see reasonably well how closely adherent the dura is to both the brain and the skull (you can see how close it is to the skull near the bottom of the picture).
Here is a diagram of the skull and dura to help you visualize the relationships.
Anyway, this whole set up usually works quite well—a nice hard skull keeps out sharp objects, and a nice cushion of fluid protects the brain from blunt trauma.
Except when it doesn’t. Below we have two CT images of the brain. The top one is normal. There is that nice white line down the middle, running straight an true. The black crescents in the middle are the ventricles, filled with cerebral-spinal fluid. Around the edge you can see the outline of the gyri and sulci. There aren’t any dark or light areas where they don’t belong.
Next is a brain with an epidural hematoma. This is a bad, bad thing. In this picture, the blood (the white blob on the left side of the picture) has compressed the brain toward the right side of the picture (a phenomenon called “midline shift”).
You see, the one way the anatomic protection of the brain can fail is by cranking up the pressure inside the skull. There is only one way out of the skull, and that is through the foramen magnum at the base—and there’s already something there: the medulla oblongata. The medulla contains all sorts of important stuff, including the breathing center. If you try to squeeze the entire brain out the foramen magnum, you will die. The only way to prevent this is to open the skull and let out the blood, thereby relieving the pressure.
Now, since the skull is pretty tough, it’s not all that easy to develop a hematoma like this. But the skull does have its weak points. One is the “pterion”, which corresponds to what is commonly called the “temple”. Just beneath this thin bit of skull is the middle meningeal artery, between the skull and the dura. If you get clonked in the temple, this artery can rupture. And where does that blood go? Nowhere. It can’t burst through the rigid skull, so it simply expands, pushing against the dura mater, and hence the brain, and gives you the above scenario. Clinically, the injury can be inapparent until the pressure is sufficiently high inside the skull. At first, the injured person may feel fine, but as the blood accumulates, the patient develops a headache, and rapidly becomes unconscious and dies, unless a neurosurgeon opens the skull.
This type of head injury may be the most feared, but it’s not the most common. Many other types of head trauma can lead to brain injuries, some of which can be subtle but disabling. When engaging in activities in which head trauma is a significant risk (biking, skiing, etc.) a helmet can absorb a good deal of the energy of a blow so that your brain doesn’t have to. It’s not OK to let your kids ride around the neighborhood without a helmet, and it’s not OK to go without yourself. Helmets are finally becoming commonplace on ski slopes, but as the recent news has shown us, they are not universal. Even a safe place like a bunny hill can be deadly without proper protection. Remember, without your brain, nothing else maters.