Yes, It Is In Fact Called A "Lucid Interval"
Greetings whumplings, I think you may like this.
Here's the thing: There's only so much space in your skull. 99.9% of the time, that limitation isn't going to be a problem for you. The rest of the time, well, that's what we're talking about today.
The skull is full of 3 things. There's brain, there's cerbrospinal fluid (CSF, what surrounds and cushions your brain), and there's blood (in blood vessels, which is how your brain gets oxygen and nutrients). If there's something else in the skull (like swelling, or a tumor, or blood outside blood vessels), some of the stuff that's normally in the skull has to get pushed out to accommodate.
The first thing that gets pushed out is the CSF. You can live with a lot less CSF than you normally have, and it's usually not that much of a problem. But getting rid of it only creates so much space.
If whatever is taking up space gets even bigger, you need to get rid of some blood. The first blood you get rid of is the blood in the veins, or venous blood. All this means is that venous blood spends less time in the brain, which is fine, I guess.
Once the brain runs out of venous blood to squish out, though, you start getting confused and unconscious real quick.
The arterial blood, which has all the nice oxygen and glucose our brain needs to live, goes next. Except that the body REALLY DOESN'T WANT THIS TO HAPPEN. So as the brain starts realizing it's getting less oxygen and glucose, it jacks up the blood pressure to try to overcome the pressure accumulating in the skull. We're talking 230s over 150s here. Maybe higher. This keeps blood going in-for a while- but this will eventually fail, and the brain will start to starve and suffocate.
While it's dying, the last thing to get pushed out is the brain itself. The bottom of the brain gets squished out of the little hole the spinal cord usually comes out of. This called herniation and it is really bad. The part of the brain that's getting squished here controls heart rate and respiration, and both of these get slow and irregular. The three symptoms of high blood pressure, low heart rate, and irregular respirations are called Cushing's Triad.
This is also when you'll see one pupil get really big and stop responding to light. This is because the nerve that controls the pupil is getting pressed on by the brain that is herniating. It should go without saying but at this point there is generally irreparable brain damage.
What is the lucid interval, you may ask? Well, after a head injury there is usually a few minutes of unconsciousness or confusion, which is usually transient. The person comes around and starts talking normally. Then, later, if there's a bleed or swelling that starts taking up space, they get confused again when the arterial blood starts to squish out and stops bringing the oxygen and glucose. The period where the person is, well, lucid, is the lucid interval.
How long is the lucid interval? Well, that depends on what is causing the problem. There are 2 main head trauma-related conditions in which a lucid interval is common: an epidural hematoma and a subdural hematoma. In an epidural hematoma (a bleed from arteries just inside the skull but above the covering of the brain, usually from a skull fracture to the area around the ear) happens quick. Less than an hour, sometimes as little as 15 minutes. In a subdural hematoma (a bleed of venous blood under the layer covering the brain) it usually is less than 72 hours, but can be as long as 3 weeks.
