“…Most of these theories have had to be revised since the identification of the anatomical structure responsible for upward gaze, which is located in the periaqueductal gray matter ventral to the aqueduct, in the dorsal interstitial nucleus of the medial longitudinal fasciculus. 7,48 The region of the upper brainstem as well as all structures located in or around the notch of the tentorium are known to be subject to significant anatomical modifications due to pressure variations across the tentorium; 4,9,18,23,[29][30][31][32][33][37][38][39]42,43,49,58,60,62 this is typical in obstructive hydrocephalus due to aqueductal stenosis. All these changes can be explained by the existence of a long-standing pressure gradient across the tentorium, with higher pressure levels in the supratentorial compartment and lower pressure levels in the posterior fossa.…”