Communicating hydrocephalus occurs when the flow of cerebrospinal fluid (CSF) is blocked after it exits the ventricles.
This type of hydrocephalus may result from a thickening of the arachnoid around the base of the brain, which blocks the flow of CSF from the spinal to the cortical subarachnoid spaces. CSF normally flows unrestricted through the ventricles and into the subarachnoid space. A thickening of the arachnoid layer around the outside of the brain may prevent the free flow of CSF through this pathway.
The word “communicating” refers to the fact that CSF can still flow between the ventricles, which remain open.
Non-communicating hydrocephalus, also known as obstructive hydrocephalus, occurs when the flow of CSF is blocked along one of or more of the narrow passages connecting the ventricles.
One of the most common causes is aqueductal stenosis, a narrowing of the aqueduct of Sylvius, a small passage between the third and fourth ventricles of the brain.
Some individuals with non-communicating (obstructive) hydrocephalus can have a surgical procedure called an endoscopic third ventriculostomy (ETV). This surgery creates an opening to allow CSF to flow in and around the brain as it would do under normal circumstances.