Brain Squeeze: The Intracranial Pressure Monster
by Edison McDaniels, MD
The skull is a closed box. Think about it. They don’t call it the cranial vault for nothing. Once past the age of a toddler, the skull is fused—it continues to grow towards the normal adult size, but the bones of the skull are knitted together and there is no mechanism whereby they can open again (short of the surgeon’s knife, or a terrible fracture) to rapidly expand the volume of the vault.
The box is closed and the volume within is, for all intents and purposes, fixed.
This, of course, presents a curious problem. There’s only so much room inside a closed box. What happens when that room is exhausted, that is, over subscribed? What happens when there’s more stuff filling the space than it was designed for?
To understand this further, imagine a shoe box with an expanding balloon inside. With the lid off the box, the balloon simply expands outward, eventually expanding out of the box entirely. But with the lid on, the expanding balloon eventually destroys the box. Now imagine the box is not empty to begin with, that when the balloon is placed inside it, the box is already 90% full. What happens then? In that case, it’s the stuff filling the box that gets damaged first as the ballon expands to squeeze the contents of the box.
Now, imagine that box is the skull, already filled to near capacity by the brain. Picture the expanding balloon to be a tumor, or perhaps trapped spinal fluid (hydrocephalus), or maybe even a brain swelling out of control after a head injury.
Ouch. Not a pretty picture. Let’s look at this box and its contents more closely.
For starters, it turns out that for all practical purposes there are only three things inside this skull box: blood, brain, and spinal fluid (CSF).
CSF is a non-compressible, usually straw colored fluid that is largely water. It contains no cells under normal circumstances. It bathes the brain and spinal cord, providing a cushion in which these elements float.
Yes, the brain floats.
This is good and bad. It’s good in that under normal circumstances there is nothing pushing against the brain. It’s bad in that with a rapid acceleration or deceleration, like a fall or a blow to the head (think of a baseball bat, or a fist), the skull stops moving before the floating brain does. Or the skull starts moving while the brain is still standing still. Either way, the brain slams into the skull. As they say, it’s not the fall that gets you, nor even the sudden stop. It’s that you don’t stop all at once. Ouch again.
Back to the CSF. The liquor cerebri, as it has been called, is produced at a constant rate of about 500 cc per day come hell or high water. Under normal circumstances it is reabsorbed into the venous system through the large vein at the top of the brain. The production and reabsorption of CSF are independent of each other however, and uncoupling of this fine balance can lead to life threatening problems within a matter of hours. This buildup of CSF is called hydrocephalus.
You might think the brain itself is a fixed volume, but you’d be wrong. The substance of the brain is composed of cells interspersed in a watery milieu, the so-called interstitial fluid that exists between the tightly packed cells. Both the cells themselves and the interstitial space they sit in are capable of changes in volume relative to a given physiological stress.
What kind of stresses? Simple breathing for one. The volume of the brain is reduced slightly with hyperventilation, i.e., rapid breathing. As a caveat, hypoventilation (very slow breathing) can cause an increase in the volume of the brain. This happens every night when we sleep (sleeping produces a relative hypoventilation—shallow breathing—leading to a slight increase in brain volume. Not enough to cause a problem in the normal circumstance, but if there is something else taking up space inside the head (hydrocephalus, a tumor, or even a slowly expanding hemorrhage—the chronic subdural of the aged), this slight change in brain volume while sleeping can cause symptoms (this is partly why patients with brain tumors tend to have a headache in the morning when they first get up—later in the process they often vomit in the mornings as well, after which they feel better because the act of vomiting has pushed a good deal of CSF out of the head and into the spine, relieving pressure within the skull).
Gravity is another stress. The volume of the brain (as well as the amount of CSF within the head itself) tends to be lower during the day when we are upright. Most of us sleep lying flat, however, and lying flat takes gravity out of the picture and fluid tends to flow back into the head during the sleeping hours. Patients with untreated brain tumors or chronic hydrocephalus learn that sleeping upright feels better and causes less trouble. Neurosurgeons often elevate the head of their patients after surgery for the same reason.
A much more significant stress is head injury. Physicians often refer to closed head injury, CHI. CHI is generally a blunt force trauma, like a fall or striking the head in an auto accident. Being hit by a thrown ball is another example. Gunshot wounds and stabbings are examples of penetrating head injuries. All of these cause swelling in various degrees, i.e., an increase in the amount of water in the brain tissue (not to mention hemorrhage—this is the definition of contusion, which is bruising).
Uncontrolled brain swelling can cause shifts of brain tissue within the skull. It turns out the intracranial space is not a simple box with a brain within. Rather, it is a complicated 3-D space with various shelves, nooks, and crannies. These divide the space into various compartments, each called a fossa (anterior, middle, and posterior—front, middle, and back), as well a right and left half with a large shelf of tissue between. Neurosurgeons spend years learning how to get in and out of these various fossas with as little injury as possible.
Swelling represents an increase in the local pressure within the brain. Simple physics dictates that material moves from an area of higher pressure to an area of lower pressure. The brain is no different. With brain swelling, injured tissue has a higher pressure than surrounding normal brain. These gradients are generally gradual and so the shift is small at first. But when the gradient increases (the swelling rises), shifts of significant magnitude can occur between and across the various shelves and compartments. This not only squeezes and distorts good brain, it also squeezes blood vessels, which may lead to stroke. All of these things can potentially turn good brain into bad brain. Bad brain swells, and the situation is potentiated until either the surgeon intervenes successfully or the patient dies. This is malignant cerebral edema (AKA malignant brain swelling).
So that’s CSF and brain. What about the third substance normally present within the skull, blood? Does the volume of blood change? Yes. In fact, the hyperventilation/hypoventilation discussed above also affects the amount of blood in the cerebral vessels. Hyperventilation reduces any pooling of blood, which is a very quick and effective way of reducing increased pressure in the head related either to brain swelling or tumors. This effect is short lived, hours to a day or so, but is quite potent and is used often in emergencies to temporize on the way to the operating room. It is one of the reasons that head injured patients are intubated so quickly.
The caveat of the above paragraph is that hypoventilation (under ventilation or shallow breathing) can kill. Hypoventilation allows pooling of blood in the head and, in the setting of brain swelling, that is very dangerous. One place this can occur is in the CT scanner in the first moments evaluating a head injured patient. This is another reason such patients are intubated early in their care, not so much because hyperventilation is necessary, but because hypoventilation is so bad.
So, the three most frequent occasions when there is a problem with the volume of brain tissue itself are tumors, brain swelling from head injury, and aging.
A brain tumor may be thought of in terms of an expansion of brain volume or brain tissue. As the tumor enlarges, it takes up valuable space inside the so-called cranial vault, space normally occupied by the normal brain. As the tumor grows, it displaces normal brain (and spinal fluid). If the tumor gets large enough, this displacement leads to shifts in brain matter (for the same reasons as above). These shifts are known as herniation. Herniation is a situation in which brain tissue is displaced out of it’s normal position. Sometimes this herniation is relatively benign and serves as a marker that there is a problem. Though the herniation itself might be relatively benign, if allowed to continue it may lead to either stroke or death—so not so benign afterall. Other types of herniation are even more significant, leading to pressure on the centers of the brain controlling heart rate, blood pressure, and breathing. Obviously, such herniations are immediately life threatening. These are sometimes irreversible.
Another situation in which blood becomes a problem inside the head is with hemorrhage (intracranial bleeding). An expanding intracranial hematoma can rapidly become lethal. This may take the form of a post-traumatic hemorrhage (epidural or sudural hematoma, hemorrhages outside the brain proper) or a spontaneous hemorrhage, which is usually within the substance of the brain itself. This latter is a form of stroke and is caused by the rupture of a small vessel within the brain substance. Sometimes such a hematoma will be amenable to removal, sometimes it occurs in an area where it cannot be reached without leaving a person devastated from a brain function perspective (in such a case, surgery is usually deferred).
Not only is the size of an expanding mass inside the skull important, but the rate of expansion of the mass is equally important. I have seen a slow growing benign tumor the size of a softball cause little in the way of problems (the young woman presented with a lump on her head she was curious about, no headaches or other symptoms!). Benign tumors can grow very, very slowly, taking many years to achieve a clinically significant size. For this reason, when such a tumor is discovered in an elderly person (whose brains have lost volume due to aging and so have extra room already), neurosurgeons often chose not to remove the tumor unless it shows evidence of significant growth over time. Depending on a person’s age, a slow growing tumor may not grow fast enough to become a problem during a person’s life.
On the other hand, an expanding epidural hematoma after a fall or other head injury may cause life-threatening problems at a relatively small size of just 25-30 cc. Sometimes a lucid interval occurs after a head injury, often seemingly minor, during which blood is accumulating in the skull until it reaches a symptomatic size, by which time it may be too late. The classic case is an epidural hematoma (bleeding outside the brain and just under the skull). Natasha Richardson, the actress, was in a lucid interval when she declined medical aid after suffering what was thought to be a minor head injury while skiing. Somewhat later, perhaps an hour or two, she collapsed when the hematoma finally reached a size where it caused one of the above noted herniations.
So, to recap. First, the skull is a closed box. Second, there are only three things inside the box: blood, brain, and CSF. Third, an increase in the volume of any one of these three substances (blood, brain, CSF) leads to a compensatory decrease in the volume of the other two. And finally, once these compensatory decreases are exhausted, the intracranial pressure rises and shift and herniation is the result. If this badness cycle isn’t interrupted at some point, death or stroke is the result.
The last paragraph is a statement of the most fundamental doctrine in all of neurosurgery, the Munroe-Kellie Doctrine. This is learned by every neurosurgery resident during the first week of training and never forgotten. In fact, there is nothing a neurosurgeon does inside the head where he or she does not have to take the Monroe-Kellie Doctrine into account.
Stated another way, as a neurosurgeon, with every intervention I make involving the brain, I have to consider the effect on intracranial pressure. Not to do so invites catastrophe, and preventing catastrophe is what treating brain squeeze is all about.
Oh, one more thing. I am often asked what happens after we lose CSF in surgery. The short answer is, nothing. As I noted above, CSF is produced at a constant rate of about 500 cc a day, which is about the volume of CSF in the head. Thus, anything lost at surgery is replenished in a matter of hours. Until then, the brain sort of sits like a lump inside the head. This might well cause a headache, though it doesn’t seem to be too bad in most patients. Sometimes nausea as well, though it’s difficult to say if this comes from the anesthesia meds, the removal of the tumor itself, or the loss of spinal fluid. Either way, the situation resolves itself before the patient goes home. This is one reason most patients don’t go dancing the night of surgery, though many are able to walk the halls of the hospital by the morning after surgery.