Effects of Positioning and General Anesthesia on Intraocular Pressure Examined. Cheng et al. (page 1351)
Episodes of visual loss after spine surgery seem to be related to changes in hemodynamics affecting optic nerve perfusion. The balance between the opposing effects of general anesthesia and prone positioning may have a role in net ocular perfusion pressure, defined as the difference between mean arterial pressure and intraocular pressure (IOP). Cheng et al. studied 20 patients scheduled to undergo spine surgery in the prone position to understand the contributions of IOP to the “ocular perfusion pressure puzzle.”
Using a handheld tonometer, investigators obtained baseline IOP readings of patients in the supine position before premedication. Anesthesia was standardized for all patients. Mean arterial pressure was kept within 20% of awake value, and ventilation was adjusted to keep end-tidal carbon dioxide in the range of 30–35 mmHg throughout the intraoperative period. Ten minutes after intubation, the IOP was again measured, with patients in the supine position.
Patients were then turned to the prone position. Their heads were held in a neutral position using pin fixation to avoid extraocular pressure. Neck flexion and extension were limited to less than 15° from horizontal. IOP was then measured before incision in the prone position, at conclusion of surgery in the prone position, and in the supine position before reversal of muscle relaxants and emergence from anesthesia. In addition to standard monitoring, length of time in the prone position was recorded for each patient, as was hematocrit, preoperatively and postoperatively. Patients were asked about any vision changes or eye discomfort in the recovery room.
The authors found that the prone position increases IOP in anesthetized patients, suggesting that a concurrent decrease in mean arterial pressure could be deleterious to the eye. Because none of the study participants experienced postoperative visual loss, the authors were unable to draw conclusions about the possible role of IOP in intraoperative visual loss. There was a direct correlation between the amount of time spent in the prone position and the magnitude of the last prone IOP measurement, suggesting a linear relation. Hemodynamic and ventilatory parameters remained unchanged while patients were in the prone position.