FIG 14.

Midline sagittal (A) and reformatted axial (B) CISS sequence images in a patient with chronic extraventricular intracisternal obstructive hydrocephalus. A, Marked hydrocephalus with inferior bowing of the floor of the third ventricle (hollow arrow) and a widely patent cerebral aqueduct. B, The anterior pontine membranes are displaced medially (straight arrows) with the left anterior pontine membrane visibly thickened. The left abducens nerve is displaced medially (curved arrow). The displacement of the anterior pontine membranes may be due to altered CSF flow dynamics and/or tethering from other arachnoid membranes. The displacement and thickening of the membrane are likely the consequence of previous infection, inflammation, or hemorrhage, but they subsequently cause obstruction to CSF flow.