Strictures of the aqueduct of Sylvius (the iter) are congenital maldevelopments, characterized by a replacement of this channel with glial tissue. Histologically remnants of this epithelial-lined channel are present (fig. 5 A and B). At times the stricture is not complete but is so tiny that fluid can pass only intermittently, or at least incompletely.1 Occasionally a stricture of the iter may follow a healed infection within the ventricles, but with these rare exceptions the strictures are of congenital origin.
Since the aqueduct of Sylvius is the only channel for passage of the cerebrospinal fluid that arises from the choroid plexuses of the third andboth lateral ventricles, hydrocephalus involving these ventricles is a necessary and unfailing sequel when this channel is closed. There are no other collateral channels through which the fluid can be sidetracked spontaneously. When the occlusion is complete, the hydrocephalus is fulminating and produces rapid destruction