<unk>  is a narrowing of the aqueduct of Sylvius which blocks the flow of cerebrospinal fluid (CSF) in the ventricular system. Blockage of the aqueduct can lead to hydrocephalus, specifically as a common cause of congenital and/or obstructive hydrocephalus. The aqueduct of Sylvius is the channel which connects the third ventricle to the fourth ventricle and is the narrowest part of the CSF pathway with a mean cross-sectional area of 0.5 mm2 in children and 0.8 mm2 in adults. Because of its small size, the aqueduct is the most likely place for a blockage of CSF in the ventricular system. This blockage causes ventricle volume to increase because the CSF cannot flow out of the ventricles and cannot be effectively absorbed by the surrounding tissue of the ventricles. Increased volume of the ventricles will result in higher pressure within the ventricles, and cause higher pressure in the cortex from it being pushed into the skull. A person may have<unk>  for years without any symptoms, and a head trauma, hemorrhage, or infection could suddenly invoke those symptoms and worsen the blockage.
Aqueductal stenosis
Chiari malformation
Dandy–Walker malformation
Cranio-lenticulo-sutural dysplasia
Acrocallosal syndrome
Septo-optic dysplasia
Basilar invagination
Periventricular leukomalacia