How Intracranial Pressure Becomes Elevated: Part 2, Short and Sweet

Intracranial Hypertension is the elevation of pressure  of fluids and tissue inside the fixed volume of the rigid intact skull.   This elevation of pressure has two basic mechanisms*:

  • Cerebrospinal Fluid Dynamic Imbalance: A net accumulation of of Cerebrospinal Fluid (CSF).
  • Cerebrovascular Dynamic Imbalance: A net accumulation of blood in the brain caused when veins are not capable of draining the volume of blood pumped into the brain by the heart (Monro-Kellie 2.0, Dr. Mark Wilson, Royal College of London, 2016)

*soft tissue tumors can also occupy space inside the skull and potentially raise ICP but are considered a separate causative mechanism from those that are considered in ICPDDs

The cause of these imbalances can be either genetic/developmental (primary) or related to trauma (secondary).  They are closely interlinked and may both be present to a certain degree.

One example of CSF Imbalance would be non-communicating hydrocephalus.  CSF is produced from arterial blood in small hollow areas of the brain called ventricles.  Ventricles are normally interconnected with one another and the space around the brain.   CSF circulates through these areas passively due to the pulsation of blood vessels and also likely movement of the body.  If one or more of these ventricles does not communicate with the rest of the system, the CSF it produces accumulates and displaces the brain outward.

Another exmaple of CSF Imbalance illustrates the interrelationship between CSF and Blood dynamics:  obstructed veins may not absorb enough CSF out of the skull to prevent a net accumulation of CSF, as well as leading to this example of:

Cerebrovascular Dynamic Imbalance: would be Chronic Cerebrovascular Venous Insufficiency, or CCVVI.  In simple terms, damage or constriction (stenosis) of one or more of the veins that drains blood from the brain compromises the ability of blood to leave the brain.  At a certain critical level of blood flow and pressure, blood begins to accumulate in the brain as it is pumped through arteries at a greater volume than it can drain.  This accumulation of blood causes the thin-walled veins to swell, which in turn pushes on brain tissue.  Brain tissue may become displaced as a result, being pushed into areas occupied by CSF.  Since an intact skull is rigid and does not expand, the pressure of the CSF becomes pressurized (Newtons’s Third Law)  while simultaneously trapping brain tissue between a vise of swelling veins.    As focal areas of brain tissue that control specific physiologic functions become stressed, their functions become altered.  This pressure on brain tissue is likely the cause of symptoms associated with IIH.

THE MECHANISMS OF ICPDD: HOW INTRACRANIAL PRESSURES BECOME ELEVATED: PART ONE

 

 

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.

FLUID DYNAMICS

Blood is the 800 lb Gorilla of the brain.  It is pumped into the brain primarily through the

Coronal view of Brain and Carotid arteries
The Carotid Arteries and their branches supply enormous volumes of blood to a nutrient-hungry brain. Note how close the brain is to the heart, which is just out of the bottom of the frame. Image courtesy of The Mayfield Brain Foundation.

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

venous sinuses
A very basic illustration of the venous sinuses responsible for carrying blood away from the brain and back to the heart.

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

Ventricles of the brain
Blue areas represent CSF filled spaces. The approximate volume of the ventricles is 30 cc; total CSF in and around the brain is ~150cc. Image courtesy of the Mayfield Brain Foundation

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.

THE TAKE-AWAY:

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.