Depends in part on location of chiasm with respect to sella turcica: the chiasm is located above the sella in 79%, posterior to the sella turcica (postfixed chiasm) in 4%; in front of the sella (pre-fixed) in 5 %.
1. compression of the optic chiasm:
a) bitemporal hemianopsia that obeys the vertical meridian: classic visual field deficit associated with a pituitary tumor. Due to impingement on crossing nasal fibers in the chiasm
b) other reported patterns that occur rarely: monocular temporal hemianopsia.
2. optic nerve compression: more likely in patients with a postfixed chiasm.
a) loss of vision in the ipsilateral eye. If carefully sought, there is usually a superior outer (tem- poral) quadrantanopsia in the contralateral eye 1) (so-called junctional scotoma AKA “pie in the sky” defect) from compression of the anterior Wilbrand's knee; may also be an early finding even without a post-fixed chiasm
b) may produce central scotoma or monocular reduction in visual acuity
3. compression of the optic tract: may occur with a pre-fixed chiasm. Produces homonymous hemianopsia