Aqueductal stenosis is one of the most common specific causes of obstructive hydrocephalus, and it's a helpful example for understanding how a blockage in the fluid pathway leads to fluid buildup. The name sounds technical, but the concept is straightforward.
What it is The cerebral aqueduct (also called the aqueduct of Sylvius) is a very narrow channel connecting the third ventricle to the fourth ventricle, deep in the brain. It's one of the tightest points in the entire CSF pathway. Stenosis simply means narrowing. So aqueductal stenosis is a narrowing (or blockage) of this already-narrow channel.
Why it causes hydrocephalus Because the aqueduct is the main route for CSF to flow from the upper ventricles toward the spaces where it's absorbed, narrowing it creates a bottleneck. CSF produced in the lateral and third ventricles can't pass through efficiently, so it backs up, enlarging the lateral and third ventricles (the chambers "upstream" of the blockage). The fourth ventricle, downstream, often stays normal in size — a pattern that can help doctors identify the problem on imaging.
What causes the narrowing. Aqueductal stenosis can be:
- Congenital — present from birth, sometimes because the aqueduct simply didn't develop with a normal opening. This is a recognized cause of congenital hydrocephalus, and some forms have a genetic basis.
- Acquired — developing later from a tumor pressing on the aqueduct, scarring after bleeding or infection, or other changes.
Symptoms As with other obstructive hydrocephalus, symptoms come from rising pressure and depend on age — a rapidly enlarging head and bulging fontanelle in babies; headaches, nausea, vomiting, vision changes, and drowsiness in older children and adults. Congenital aqueductal stenosis sometimes causes hydrocephalus that's detected in infancy, but milder narrowing can occasionally go unnoticed until later in life.
Why it matters for treatment Aqueductal stenosis is important partly because it's often an ideal situation for an ETV (endoscopic third ventriculostomy). Since the problem is a discrete blockage at the aqueduct, an ETV can create a new opening in the floor of the third ventricle, allowing CSF to bypass the blockage entirely and flow to where it's absorbed — frequently without the need for a shunt. This makes aqueductal stenosis one of the classic indications for ETV, though a shunt remains an option and the choice is individualized.
The takeaway Aqueductal stenosis is a clear, almost "plumbing-like" example of obstructive hydrocephalus: a narrow channel becomes too narrow, fluid backs up, and the ventricles swell. Because the blockage is well-defined, it's also one of the situations where a shunt-free surgery (ETV) is most likely to succeed.