The aqueduct of Sylvius, normally narrow to begin with, may be obstructed by a number of genetically or acquired lesions (e.g., atresia, ependymitis, hemorrhage, tumor) and lead to dilation of both lateral ventricles as well as the third ventricle.
Fourth ventricle obstruction will lead to dilatation of the aqueduct as well as the lateral and third ventricles (e.g., Chiari malformation).