Defined as the pressure-volume relationship of the brain.
The cranium (skull) consists of brain matter, cerebrospinal fluid, and blood (both venous and arterial). Once thought to be a mere hypothesis, but now proven by MRI and accepted as doctrine, the Monro-Kellie Doctrine states, “The sum of volumes of the brain, CSF, and intracranial blood is constant. An increase in one should cause a decrease in one or both of the remaining two.”
Therefore, if an abundance of cerebrospinal fluid exists, both cranial blood volume and brain matter will be forced to deplete. This depletion is usually directed in the path of least resistance – into the sella turcica and through the foramen magnum and into the spinal canal. When the brain matter closest to the bottom of the skull (cerebellar tonsils) is pushed through the foramen magnum and into the spinal canal (an Acquired Chiari Malformation), the tonsils act like a cork and blocks the flow of cerebrospinal fluid (regardless of the size of the tonsillar descent), which in turn, continues to raise intracranial pressure.[4]