Hydrocephalus: An Owner’s Manual Part 2 of 6
A Few More Thoughts on Anatomy
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).
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.
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 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.
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.