With aqueductal stenosis, aqueductal refers to a channel in the brain that allows fluid to flow through, and stenosis refers to a narrowing.
So aqueductal stenosis is a problem where a channel in the brain’s ventricular system gets narrowed, and that makes it hard for cerebrospinal fluid to flow through.
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 produce 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.
Cerebral aqueduct stenosis develops when there’s a blockage of the cerebral aqueduct between the third and fourth ventricle, and most of the time this blockage is caused by a tumor.
Most often it’s a pineal gland tumor, which sits just dorsal to the aqueduct at the level of the midbrain.