What is IH?


Intracranial hypertension literally means that the pressure of cerebrospinal fluid (CSF) within the skull is too high. “Intracranial” means “within the skull.” “Hypertension” means “high fluid pressure.” To understand how this happens, it’s helpful to look at the basic anatomy of the brain and skull, as well as the process in which cerebrospinal fluid is created and absorbed.

Cerebrospinal fluid is one of three major components inside the skull; the other two are the blood supply (the arteries and veins known as the vasculature) that the brain requires to function and the brain itself. Under normal circumstances, these components work together in a delicate balance. A pressure and volume relationship exists between CSF, the brain and the vasculature. But since the skull is made of bone and cannot expand, an increase in the volume of any one component is at the expense of the other two components. For example, if the brain swells and becomes enlarged, it simultaneously compresses blood vessels, causing the sub-arachnoid space to fill with more spinal fluid. This results in an increase in intracranial pressure (i.e. cerebrospinal fluid pressure), as well as a decrease in blood flow.

CSF has several important functions. It cushions the brain within the skull, transports nutrients to brain tissue and carries waste away. CSF is produced at a site within the brain called the choroid plexus, which generates about 400-500 ml. (one pint) of the fluid each day or approximately 0.3 cc per minute. (The total volume of CSF in the skull at any given time is around 140 ml. That means the body produces, absorbs and replenishes the total volume of CSF about 3-4 times daily.)

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Schematic Drawing of Cerebrospinal Fluid Circulation.
Elevated CSF pressure is transmitted around the brain and along the optic nerve (sub-arachnoid space) producing papilledema.

Cerebrospinal fluid flows from the choroid plexus through the brain’s four, interconnecting ventricles before finally entering the sub-arachnoid space, which surrounds the brain and spinal cord. The fluid then flows over the brain and spinal cord and is eventually absorbed into the venous blood system through tiny, one-way channels called arachnoid granulations or villi.

When this continuous cycle of CSF production, circulation and absorption functions normally, it regulates the volume of CSF in the skull and the fluid pressure remains at a constant level. In other words, the CSF production rate remains equal to the CSF absorption rate.

But when the body cannot effectively absorb or drain CSF, intracranial pressure increases within the fixed space of the skull. And since the brain and the vasculature can only be compressed so far, intracranial pressure must rise. Intracranial hypertension in adults is generally defined as intracranial pressure that reaches 250mmH2O or above.