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.