Normal pressure hydrocephalus

In normal pressure hydrocephalus, hydrocephalus can be broken down into “hydro” which means water and “cephalus” which means head.

Hydrocephalus, which is also termed as “water on the brain”, is an excessive buildup of cerebrospinal fluid within the brain.

Even though the pressure on lumbar puncture is expected to be increased, in this case, it’s normal, which is why it’s termed as “normal pressure” hydrocephalus.

Let's start with some relevant anatomy. The brain has four interconnected cavities in the brain called ventricles, and each one contains a structure called a choroid plexus.

The choroid plexus is made up of ependymal cells which produces cerebrospinal fluid - a fluid that helps provide buoyancy and protection, as well as metabolic fuel for the brain.

Highest up, are two C-shaped lateral ventricles that lie deep in each cerebral hemisphere.

The two lateral ventricles drain their cerebrospinal fluid into the third ventricle, which is a narrow, funnel-shaped, cavity at the center of the brain.

The third ventricle makes a bit more cerebrospinal fluid and then sends all of the cerebrospinal fluid to the fourth ventricle via the cerebral aqueduct.

The fourth ventricle is a tent-shaped cavity located between the brainstem and the cerebellum.

After the fourth ventricle, the cerebrospinal fluid enters the subarachnoid space, which is the space between the two inner linings of the brain - the arachnoid and pia mater.

Now, the arachnoid mater protrudes into the outermost covering, called dura mater, at various points.

It looks a bit like little fingers poking through, and these protrusions are called arachnoid granulations.

Cerebrospinal fluid makes its way through these arachnoid granulations and gets into the dural sinus which is filled with venous blood and is on the other side of the arachnoid granulation.

The pressure within the subarachnoid space is higher than the pressure in the dural sinus, so CSF flows through the arachnoid granulations is only in one direction.

Normal pressure hydrocephalus develops when cerebrospinal fluid is unable to get through those arachnoid granulations normally.

This process develops gradually and most commonly affects the elderly.

There are two types of normal pressure hydrocephalus.

The first type is an primary normal pressure hydrocephalus, where the cause is idiopathic or unknown.

The second type is secondary normal pressure hydrocephalus, where the cause is damage to the arachnoid villi which disrupts CSF reabsorption.

Some known causes of arachnoid villi damage are a subarachnoid hemorrhage and meningitis, both of which cause a tremendous amount of inflammation to the protective layers of the brain.