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Aqueductal Stenosis: a narrowing of the aqueduct of Sylvius. This is one cause of obstructive hydrocephalus; it may be treated using a CSF shunt or by a surgical procedure known as an endoscopic third ventriculostomy (ETV)

Arachnoid: the middle layer of meninges. It covers the brain and spinal cord smoothly without conforming to the irregularities of their surfaces; CSF flows through the arachnoid space