Everybody knows the old adage about how to eat an elephant. ICPDDs and their nuances certainly qualify, and the goal here is to present information in easily digestible bits. This is the first bite.
While the reason/reasons (“etiology” it doctor-speak) a person develops IIH/PTC/etc. remain unknown and controversial, the physiology of how the pressure develops is actually quite straightforward: ICPDDs reflect an increase in the pressure of fluid in and around the brain. Thus, they are a result of a disruption in the dynamics of the fluids in and around the brain. While CSF is most commonly discussed and treated, there is another fluid that actually rules everything, including CSF:
Blood is the 800 lb Gorilla of the brain. It is pumped into the brain primarily through the
carotid arteries at a rate of 750 to 1000 cc/minute when we are at rest; it courses through an estimated 100,000 miles of blood vessels delivering nutrients and oxygen to brain tissue. Depleted blood is drained via the venous sinuses, a network of veins around the brain, and eventually through the jugular veins back to the heart.
Blood enters the brain at an average pressure (“Mean Arterial Pressure”, or MAP) of 90mm Hg. It drains from the veins at a pressure of 15-20 mm Hg. Because the same amount of blood that flows into the brain has to flow out, and because venous pressure is lower than arterial pressure, the blood flow in veins must be higher to compensate (fluid dynamics, Bernoulli, a couple other Italian guys in there). Suffice it to say that veins have to be able to carry away ALL blood that is pumped into the brain. In addition to that important necessity, there are TWO factors of blood that must be considered in ICPDD: pressure (as expressed by MAP), and flow. For the sake
of simplicity we will use heart rate as an indicator of blood flow.
CSF is a plasma like fluid that is made from blood in an interconnected network of small hollow areas in the brain known as ventricles. It is produced at a very low
rate of ~0.35 cc/minute from arterial blood. Its rate of production is dependent on the am
ount of blood flowing into the brain. CSF circulates through the ventricles and around the brain; this circulation is slow, with pulsations from blood vessels and body movement being the primary motivators. It is primarily absorbed back into the blood circulation in the venous sinuses.
CSF is something of a Mystery Fluid. Aside from cushioning and supporting the brain, it is thought to play a role in delivering nutrients and cleansing dead cells from the outer surface of the brain. Deficiencies in CSF are associated with accumulations of these dead cells, known as plaques, which are themselves associated with dementias such as Alzheimers and Lewy Body. Anyone with an ICPDD can tell you that “overdrainage”, ie, not enough ICP and/or CSF makes them feel weak and lethargic. Again, suffice it to say that CSF is Very Important; otherwise, it would not be in our heads.
SUMMARY, PART ONE: THE TWO FLUID POSTULATE
The dynamics of blood and CSF lay the groundwork for understanding ICPDDs. ICP is a result of the interaction of these fluids inside the watertight, airtight, non-expandable skull. These facts lead to what I call the Two Fluid Postulate:
“There are only two fluids inside the skull: blood and cerebrospinal fluid (CSF). Both are present in approximately equal volumes, 150cc. Only one flows in and out of the brain: blood. CSF is made from blood that enters the brain and absorbed back into the veins that drain blood away from the brain. Intracranial pressure is the sum of the forces exerted within the skull by these two fluids. Changes in the balance of CSF production/absorption and/or blood flow into and out of the brain will necessarily affect ICP. Most critically, because the brain is encased in a rigid, non-expanding skull, ANY INCREASE IN ICP TRANSLATES INTO INCREASED PRESSURE DIRECTLY ON THE BRAIN.”
This is just a restatement of what is known as the Monro-Kellie Hypothesis from the 1890s by two Scottish physicians to explain what goes on inside the skull.
CSF, while the most often discussed fluid in ICPDDs, is not the only fluid in the brain. Blood, in fact, rules everything inside the skull, including CSF. Because the volume inside the skull is fixed, and because there practically zero extra space in the skull, any net accumulation of CSF OR Blood will cause increased ICPs, and those increased ICPs exert direct pressure on the brain itself.
In Part 2, we will look at how the dynamics of blood and CSF become disrupted.