Aqueductal stenosis (HP_0002410, a Human Phenotype Ontology entry) is stenosis of the cerebral aqueduct (also known as the mesencephalic duct, aqueductus mesencephali, or aqueduct of Sylvius), which connects the third cerebral ventricle in the diencephalon to the fourth ventricle, which is between the pons and cerebellum. Also known as: Aqueduct of Sylvius stenosis, Aqueduct stenosis, Narrowing of aqueduct of Sylvius.