Factors influencing optic nerve sheath diameter in patients undergoing prone-position spinal surgery: A prospective observational study

Background and Aim: The prone position in spinal surgery can alter intracranial pressure and optic nerve sheath diameter (ONSD) and potentially lead to visual complications. This prospective observational study aimed to identify factors influencing ultrasonographic ONSD changes and visual acuity in patients undergoing spinal surgery. Methods: Total 65 patients were included and ONSD measured in the supine, prone, and end-of-surgery in the prone positions. Anesthesia type, use of a pinned head holder and intraoperative head position were documented. After prone positioning and at the end of surgery in prone position, arterial blood gases, vital signs, EtCO2 and PEEP values were recorded. Duration of surgery, crystalloid administration, estimated blood loss, duration of hypotension and blood product use were also documented. Visual acuity was assessed preoperatively and on postoperative day 1 using the Snellen chart and ocular complications were noted. Results: Right, left and mean ONSD values significantly increased from supine to the end-of-surgery prone measurements. This increase was significantly greater in patients without the pinned head holder compared to the with pinned head holder (p<0.001, p=0.002). A mild but statistically significant decline in visual acuity was observed (right p=0.024, left p<0.001), with a weak positive correlation to the duration of prone positioning and blood loss (r=0.292; p=0.018, r=0.296; p=0.017). However, ONSD elevation did not correlate with right or left visual acuity (r=-0.058; p=0.645, r=0.038; p=0.762). Conclusion: The placement of pinned head holder may prevent further increase in ONSD in the prone position. The postoperative minor visual decline cannot be fully explained by ONSD changes alone. Keywords: visual acuity, logMAR, spinal surgery, pinned head holder