As a result of growth of the tectum and tegmentum, the lumen of the neural tube in the region of the mesencephalon becomes narrowed to form the aqueduct of Sylvius. Because of its small caliber, this area of the CSF pathway is vulnerable to obstruction by a number of congenital and acquired pathologies.
Aqueductal stenosis is responsible for approximately 10% of cases of hydrocephalus in childhood. Presentation may, however, occur at any time from birth to adulthood. In congenital forms of aqueduct "stenosis", the aqueduct, rather than being stenosed, is branched or forked into two or more channels. In some instances, the tectum is also deformed and it has been postulated that, here, the primary pathology is a communicating hydrocephalus in which external pressure on the mesencephalic structures has led to obliteration of the aqueduct secondarily.
Scarring or gliosis in the aqueduct following infection or hemorrhage may lead to an acquired aqueduct stenosis. Tumors arising in adjacent structures, such as the tectal plate, the rostral fourth ventricle, posterior thalamus or pineal region, may not be evident on CT but can similarly result in a picture of aqueduct stenosis. MRI is necessary in such cases.
Imaging reveals a tri-ventricular hydrocephalus, with a small or normal-sized fourth ventricle. In addition, MRI will often reveal a trumpeting of the proximal aqueduct and, moreover, will readily identify associated neo-plastic lesions that may not be seen on CT scanning. It is for this reason that MRI scanning should be performed routinely in cases of aqueduct stenosis.
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