The skull, when you think about it, isn’t very big. In those few square inches of hollowed bone is everything you remember, every quirky personality trait, every one of your special skills or talents, all held in a few precious pounds of brain.
As small as that space is, the brain has to share it. Cerebrospinal fluid (CSF) surrounds the brain, encasing and protecting the brain from bashing against the skull with every jolt of your head. Blood flows into that intracranial space as well, supplying the brain with the oxygen it craves and ridding the brain of toxic byproducts.
Other components of the body have to share spaces as well, but those areas are not enclosed quite so rigidly as the brain. There are just a few openings out of that skull through which nerves and the spinal cord can travel. Other than that, there’s no way in or out of that intracranial space.
All this is well enough in day-to-day life. It’s a good thing that our brain is so well protected with this kind of natural helmet, or we’d be prone to all kinds of brain injury. Problems arise, though, when one of the three main components of the intracranial space, either brain, blood, or CSF, requires more room. If something's going to take more space, something else has to give.
Sometimes, something is in the head that shouldn’t be there. Examples include a tumor, a hemorrhage, or an infection. Other times, the normal way fluids shift in and out of the intracranial space is interrupted. For example, CSF normally flows from the ventricles in the center of the brain through small openings known as foramina. If this CSF flow is blocked somehow, and CSF continues to be made, pressure builds.
A normal intracranial pressure is somewhere between 8 to 20 millimeters of mercury (mm Hg). Any more than this, and structures in the brain may begin to be impacted. One of the first structures to feel the strain are the tissues known as meninges that surround the brain. Whereas the brain itself lacks pain receptors, the meninges can fire off pain messages that result in a terrible headache. The classic headache caused by high intracranial pressure is improved by standing up and worsened by laying flat. Standing allows some of the pressure to descend down the spinal column, whereas laying flat evens out the pressure gradient, resulting in higher pressure surrounding the brain.
Another structure to be impacted relatively early as intracranial pressure rises are the optic nerves. People with high ICP may start to complain of blurring of their vision as the optic nerves are damaged. If the intracranial pressure isn’t fixed in a reasonable amount of time, permanent vision loss can result.
Even more concerning than optic nerve damage is how ICP can affect the brain itself. When pressures rise inside part of the skull, the brain can be pushed to an area of lower pressure. The lobes of the brain are divided by rigid slices of tissue. For example, the left hemisphere is separated from the right hemisphere at the top of the brain by the tissue called the falx cerebri. If a bleed in the left hemisphere creates enough pressure, it can push the brain of the left hemisphere under the falx cerebri, crushing brain tissue and blocking off blood vessels. Brain damage and stroke can result.
Similarly, the cerebellum is separated from the rest of the brain by the tentorial membrane. If pressure builds above that membrane, brain tissue can be pushed down through the small opening near the brainstem, causing irreparable brainstem damage. This can lead to paralysis, coma, and death.
It should now be clear that elevated intracranial pressure is bad news. So how can neurologists detect when ICP is high? More importantly, what do neurologists do when they find out the brain is under that kind of pressure?
Fortunately, neurologists have a few ways to manage high ICP. While nothing is guaranteed, timely intervention can prevent serious debilitation.
Allan H. Ropper, Daryl R. Gress, Michael .N Diringer, Deborah M. Green, Stephan A. Mayer, Thomas P. Bleck, Neurological and Neurosurgical Intensive Care, Fourth Edition, Lippicott Williams & Wilkins, 2004
Braunwald E, Fauci ES, et al. Harrison's Principles of Internal Medicine. 16th ed. 2005.