Because the pressure within the eye is an important factor in determining the blood flow to the eye, prevention of a dramatic increase in IOP may make patients less vulnerable to peri-operative ischemic optic neuropathy and vision loss. Because permanent vision loss is such a rare event after surgery, this study will measure more subtle (and most likely, temporary) vision changes (subjective blurriness, visual field deficits, decreased acuity), which occur more frequently and are thus a more easily measured outcome.6