Walsh & Hoyt: Pathophysiology of the Optic Chiasmal Syndrome

The mechanism by which the chiasm is compressed from below was studied by Hedges, who used normal, adult, fresh necropsy material. He removed the pituitary gland and inserted the balloon tip of a Foley catheter into the sella turcica and then expanded it. As the balloon expanded and the optic chiasm was elevated, the crossing fibers were first affected, initially the lower nasal and later the upper nasal. The uncrossed upper temporal fibers were put ""on stretch,"" whereas the lower temporal fibers, subserving the upper nasal visual field, were relatively unaffected. Hedges concluded that the results of this experiment explained the relative sparing of the upper nasal visual field in even advanced optic chiasmal syndromes. However, Harrington disagreed with this theory and believed that the explanation for relative sparing of the superior nasal visual field in patients with severe damage to the optic chiasm was related in some way to the blood supply of the chiasm, although he gave no definitive evidence to support this contention.