POVL is vanishingly rare but when it happens its a nightmare on (insert your hospital's street) street.
It occurs typically during prone surgery even when pins are applied and there is clearly no eyeball pressure.
Opth exam shows ischemia of either the retina or the nerve. predisposing factors include long case, hypotension, bleeding. notably with longer cases intraocular pressure has been noted to rise. I have always hunched that it was somehow related to high venous pressure too. One clue for this was a patient of a colleagues from a few yrs back during thoracic vertebral surgery from an anterior approach they got into bleeding and had to occlude the svc to stop the bleeding. after all the face swelling finally subsided the patient was quite blind with not even light perception. similarly cavernous carotid fistuas also produce blindness.
So based on the above physiologic considerations for prone cases my approach is to give just enough anesthesthetic to ensure unconsciousness and if that makes the MAP go less than 80 mmHg i run dilute neosynephrine to make it so. Use bis and keep it at 55. transfuse to Hb 10.