02/18/13
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2/6 Hydrocephalus Owner’s Manual

Hydrocephalus: An Owner’s Manual Part 2 of 6

A Few More Thoughts on Anatomy

Compartments

It turns out that the interior of the skull is divided into several areas, that is, it is compartmentalized. Essentially, there are three compartments: the supratentorial space, which is divided into right and left halves by a tough shelf of tissue called the falx; and the infratentorial space, which is not divided right from left but is separated from the supratentorial space by a second tough shelf of tissue call the tentorium.

The supratentorial space houses the cerebral hemispheres, within which are the right and left ventricles. We shall look at the ventricles in a moment, as they are crucial to the issue of hydrocephalus. The supratentorial space is located in the top half of the head, essentially above the level of the ear holes.

The infratentorial space is at the back of the head, below the ears. A small space, it houses the highest priced real estate in the brain: the so-called brainstem, which controls important but mundane things like breathing, swallowing, pulse, and blood pressure (to name but a few; there is also an area here seemingly devoted to vomiting—called the area postrema—and pressure here produces, you guessed it, vomiting).

 

ICP Revisited

We have already seen how the pressure inside the head, the ICP, must be at equilibrium. What this means in reality, is that pressure across the compartments mentioned above must be at equilibrium. Since every high school student knows that items move from an area of higher pressure to an area of lower pressure (this is why storms move across the atmosphere and forecasters and ship captains pay inordinate attention to barometric pressure), it stands to reason that if the pressure rises in one compartment more than another, shifts may occur inside the head. That is, the brain (or part of it), might move from one compartment to another!

In clinical terms, this is called herniation and it is deadly.

 

Herniation

As a neurosurgeon, everything I do inside the head must take into account the possibility of herniation. The last thing I want is the brain shifting around. Fortunately, it turns out one can predict these shifts fairly easily. And if one can predict them, one can prevent them. Usually.

Herniations (think of them as unwanted shifts of brain substance) occur when the ICP goes out of equilibrium because of an increase in one or more of the three important substances mentioned above (brain, blood, or CSF).

Increases in brain substance are represented by brain tumors, of which there are many kinds (some cancerous, some not).

Increases in blood substance are represented by bleeding inside the head. There are many different types of such hemorrhages, some requiring emergency surgery to remove.

Increases in CSF are represented exclusively by hydrocephalus. In fact, the definition of hydrocephalus is an unwanted and pathologic build-up of CSF within the skull, either inside or outside of the brain. The remainder of this monograph deals with hydrocephalus, a few of its variants, and how it is treated by modern neurosurgical techniques. Please note that what follows is not an exhaustive discussion but is for informational purposes only. Nothing here is meant to supersede or replace consultation with a competent expert, usually a neurosurgeon.

 

The Ventricles

The brain floats.

In the normal course of things, the brain floats in the liquor cerebrospinalis, CSF. The CSF is produced in the ventricles, which are four cavities deep within the substance of the brain, usually rather small and inconsequential.

Three of the ventricles, the right and left lateral ventricles and the IIIrd ventricle, are located in the supratentorial space. The right and left lateral ventricles are offset to the right and left of the body’s midline, and connect with the IIIrd ventricle through a small opening called the foramen of Munro. The foramen of Munro is the first choke point in the system. Choke points are areas small enough to be blocked, or at least partially obstructed, and so have the potential for trouble. The IIIrd ventricle is on the midline and is very close to the exact center of the head. It has a very small tail off of its back end, a narrow tube called the cerebral aquaduct (about the diameter of a pencil lead normally), through which every drop of CSF produced in the lateral and IIIrd ventricles must pass (a major choke point) on its way to the IVth ventricle, which is located on the midline in the infratentorial space. The IVth ventricle in turn opens into the wider spaces at the base of the brain through three openings, called foramina, which rarely cause problems.

CSF is actually absorbed into the venous system across the surface of the brain at the top of the head. Unfortunately, the absorption of CSF can fail following hemorrhage or infection (which perhaps gums up the works and thus prevents the reabsorption). This failure of absorption, combined with the continued production of CSF in the ventricles, leads to one common form of hydrocephalus called communicating hydrocephalus.

 open brain woodcut

Disclaimer: The information contained in this blog is simply that, information. I am not doling out specific medical advice. Nothing contained herein is meant to replace a complete evaluation by a qualified member of the medical establishment. This page is nonfiction.

02/17/13

1/6 Hydrocephalus Owner’s Manual

Hydrocephalus: An Owner’s Manual Part 1 of 6

A Few Thoughts on Anatomy

 

What is Hydrocephalus?

Loosely speaking, hydrocephalus is an abnormal build-up of spinal fluid within the head. This can happen for a number of different reasons. I will define hydrocephalus more fully in a moment, and we will look at a number of different ways it can come about, but first a quick look inside the box.

A Closed Box

The skull is, for all its complexity, essentially nothing more than a container, a closed box. Clinically speaking, that is looking at it from the neurosurgeon’s point of view, it has only three important openings: the paired openings for the eyes and a third opening through which the spinal cord exits at the base of the skull.

The two openings for the eyes allow the optic nerves to pass thru the skull to the eyeballs themselves. The optic nerves are actually not nerves at all, but true extensions of the brain itself—which makes them the only part of the brain visible from outside the body under normal circumstances. Every time you go to the doctor and he looks in your eyes with that bright light (an ophthalmoscope), you are having your brain examined. What he or she is actually looking for is the shape and color of the optic nerve head, that is, that portion of the nerve just as it enters the eye. Pressure inside the head, we will talk about this below, gives the nerve head a different appearance depending on whether it is high or low, normal or abnormal, old or new, etc. That is, the pressure inside the head is propagated along the nerve and causes visible changes in the eye when that pressure is abnormal.

So, the skull is, for our purposes, a closed box. One important caveat: a closed box has a fixed volume (think of your dishwasher—you can only fit so much inside it). The skull cannot expand. That is, there is only a certain amount of space inside the skull.

Contents of the Box

As far as the neurosurgeon is concerned, there are only three things inside the skull that matter and take up any space: the brain itself, the blood (which normally flows through the brain in blood vessels but sometimes hemorrhages and then is found outside the blood vessels, where it takes up space needed by the brain), and cerebral spinal fluid (CSF).

Brain, blood, and CSF.

Each of these substances takes up a certain amount of space. The key thing to remember is this: an increase in any one of these substances requires a decrease in the other two.

Why?

Because the skull is a closed box. And a closed box has a fixed volume.

Think once again of your dishwasher. Say it holds plates, glasses, and pots, but only a limited number of each. Of course, you can put in more glasses if you desire, but only at the expense of fewer plates and pots.

Intracranial Pressure

Let’s do a thought experiment. You are blowing up a balloon and watching it expand. It expands for two reasons: the walls are flexible and you are increasing the pressure inside the balloon.

Now imagine the walls are not flexible but semi-rigid. But you have superhuman strength and can still blow into the balloon and expand it, though now the balloon expands only very slowly. It enlarges in an attempt to keep the pressure inside the balloon equal to that outside. It will continue to enlarge until one of three things happens: the balloon pops, you can’t generate enough pressure to expand the balloon further (in this case the balloon is at equilibrium inside and out), or the walls encounter something external that is more rigid and prevents their further expansion.

Now imagine you are blowing up the balloon inside a closed box. And that box is the skull.

Now imagine the balloon to be any one or more of the three substances—brain, blood, or CSF.

You will see from this little thought experiment that the interaction of the three substances (brain, blood, CSF) inside the closed box of the skull creates a certain pressure, called the intracranial pressure (ICP). Normally, the ICP is in equilibrium across the three substances, but what happens if one of the substances grows in volume?

Well, as we have seen above, the other two must decrease. Or the pressure, the ICP, will rise.

It is this rise in the ICP under adverse conditions that is so dangerous inside the head. Why? We’ll get to that.

For now, just know that as a first approximation, if the ICP rises higher than the blood pressure, blood won’t be able to flow into the head and the brain will fail. This is what happens when a person suddenly faints. The two common reasons for fainting: the blood pressure suddenly falls or the ICP suddenly rises. Either way, the ICP eventually exceeds the blood pressure, blood flow to the brain fails, and the patient faints (if they are lucky, they will fall on flat ground and, with the brain and heart now level, blood flow to the brain resumes and the person wakes up).

Some causes of a sudden fall in blood pressure:

—A bad coughing fit, which actually reduces blood return to the heart and thus there is nothing to pump out to the brain;

—Sudden heart failure, such as a heart attack;

—A bad scare, in which case the blood vessels of the body sudden dilate wide open and the blood pressure falls as a result.

—Have you ever heard of micturition syncope? That’s the fancy medical term for what happens when a person (usually an older man) is straining to pee and suddenly faints. Why does this happen? Because, as any man past middle age knows, the enlarged prostate gland partially obstructs the flow of urine out of the bladder. The greater the obstruction, the harder a man has to push to pee. This pushing, or straining, effectively increases the pressure inside the belly, which in turn decreases the return of blood to the heart (though only transiently). Just as with a bad coughing fit, there’s nothing in the heart to pump and the brain fails. Result: a faint. Fortunately, the victim of such quickly awakens once he faints and thus stops straining to pee. Unfortunately, he may have cracked his head on the toilet…

Some causes of a sudden increase in the ICP:

—An acute obstruction to the flow of CSF, i.e., acute hydrocephalus (the usual cause of this is a shunt malfunction. A less common cause is a brain tumor);

—A spontaneous hemorrhage in the head, such as a ruptured aneurysm.

—A blow to the head, such as one might sustain from a simple fall (say, while straining to pee standing up).

A Recap

The skull is a closed box.

Only three important things take up space in the box: brain, blood, and CSF.

An increase in any one of these substances requires a compensatory decrease in the other two, or…

Or the ICP will rise, usually to the detriment of the patient.

Disclaimer: The information contained in this blog is simply that, information. I am not doling out specific medical advice. Nothing contained herein is meant to replace a complete evaluation by a qualified member of the medical establishment. This page is nonfiction.