Intraoperative patient positioning plays a crucial yet often under-recognized role in determining ocular perfusion dynamics and postoperative visual outcomes, especially in surgeries of prolonged duration or those requiring physiologic manipulation. Ocular perfusion pressure (OPP), defined as the difference between mean arterial pressure (MAP) and intraocular pressure (IOP), is a critical determinant of retinal and optic nerve head blood flow. Any factor that disrupts this balance may predispose patients to transient or permanent visual dysfunction. Among these factors, surgical positioning particularly prone versus supine alignment—has been increasingly associated with alterations in IOP, episcleral venous pressure, and cerebral hemodynamics. Neurosurgical procedures frequently require prone positioning to facilitate posterior fossa or spinal access, whereas general surgical procedures predominantly utilize the supine position. This inherent difference offers a unique platform for comparative assessment of OPP and its postoperative consequences.[1] A growing body of literature has demonstrated that prone positioning is associated with progressive elevation of IOP secondary to increased venous congestion, direct orbital pressure, and reduced aqueous outflow. Prolonged IOP elevation may reduce OPP, resulting in optic nerve ischemia or ischemic optic neuropathy a feared complication most commonly reported after extended spinal surgeries. In contrast, supine positioning is physiologically more neutral, often maintaining stable venous return and IOP levels. However, even in supine surgeries, factors such as anesthesia-induced hypotension, fluid shifts, and patient comorbidities may influence OPP. Despite these recognized associations, comparative studies specifically evaluating prone versus supine positioning in terms of real-time OPP measurement and subsequent visual function outcomes remain limited, particularly in mixed surgical populations.[2] Furthermore, postoperative visual function changes—ranging from mild visual blurring to severe vision loss can significantly impact a patient’s quality of life, pose medicolegal challenges, and complicate postoperative recovery. Early identification of intraoperative risk factors is therefore vital. Variations in OPP during surgery may serve as a predictive marker, enabling timely interventions such as blood pressure optimization, head elevation, protection of the eyes, and reduction of external orbital compression. Incorporating both neurosurgical and general surgical cohorts enhances the study’s generalizability by capturing diverse physiological responses associated with different surgical fields and durations.[3] Aim To compare the effect of prone versus supine intraoperative positioning on ocular perfusion pressure and postoperative visual function in neurosurgical and general surgical patients. Objectives 1. To measure and compare intraoperative ocular perfusion pressure in patients positioned prone versus supine during surgery. 2. To assess postoperative visual function changes in both positioning groups. 3. To correlate intraoperative OPP variations with postoperative visual outcomes within each surgical category.

Source of Data Data were obtained from patients undergoing elective neurosurgical and general surgical procedures requiring prone or supine positioning at a tertiary care teaching hospital. Study Design The study was conducted as a prospective, comparative, observational study. Study Location The research was carried out in the Departments of Anesthesiology, Neurosurgery, and General Surgery at a tertiary care hospital equipped with advanced intraoperative monitoring facilities. Study Duration The study was conducted over a period of 18 months. Sample Size A total of 100 patients were enrolled, with 50 undergoing surgery in the prone position and 50 in the supine position. Inclusion Criteria • Patients aged 18-65 years. • Undergoing elective neurosurgical (prone) or general surgical (supine) procedures under general anesthesia. • Duration of surgery expected to exceed 90 minutes. • Provided informed written consent. Exclusion Criteria • Pre-existing ocular disease (glaucoma, optic neuropathy, retinal pathology). • History of ocular trauma or surgery. • Uncontrolled hypertension or diabetes mellitus. • Patients with anticipated difficult airway requiring non-standard positioning. • Emergency surgeries or those converted intraoperatively to alternate positions. Procedure and Methodology Eligible patients were enrolled preoperatively, and detailed demographic, clinical, and ocular history was documented. Baseline intraocular pressure (IOP) was measured using a handheld tonometer, and mean arterial pressure (MAP) was recorded. Ocular perfusion pressure (OPP) was calculated as MAP-IOP. Patients were positioned intraoperatively as per surgical requirement—prone or supine with standardized anesthetic protocols. IOP and MAP were recorded at predetermined intervals: post-induction, post-positioning, hourly during surgery, and before extubation. Care was taken to avoid external ocular pressure in both groups using appropriate padding. Postoperative visual assessment, including visual acuity, color vision, and visual field screening, was performed at 24 hours and 72 hours post-surgery. Sample Processing All measurements including IOP, MAP, and visual assessments were documented in predefined case report forms. OPP was computed automatically from recorded values and tabulated for analysis. Statistical Methods Data were analyzed using SPSS software. Continuous variables (IOP, MAP, OPP) were expressed as mean ± SD and compared using independent t-tests or repeated-measures ANOVA. Categorical variables (visual impairment incidence) were analyzed using chi-square tests. Pearson correlation coefficients were used to assess associations between OPP changes and visual outcomes. A p-value <0.05 was considered statistically significant. Data Collection Data were collected prospectively during the perioperative period by trained anesthesia residents and validated by senior faculty. All patient information was anonymized and securely stored.

Table 1: Baseline Demographic & Preoperative Ocular Profile Comparison (N=100) Variable Prone (n=52) Supine (n=48) Test of Significance 95% CI of Difference p-value Age (years), Mean ± SD 54.3 ± 8.9 53.1 ± 9.4 t = 0.65 -2.43 to +4.83 0.51 Male sex, n (%) 33 (63.4%) 27 (56.3%) χ² = 0.49 - 0.46 BMI (kg/m²), Mean ± SD 26.9 ± 3.4 26.3 ± 3.1 t = 0.92 -0.69 to +1.89 0.36 Surgery duration (min) Mean ± SD 214.7 ± 41.3 178.6 ± 37.9 t = 4.56 +20.41 to +51.78 <0.001* Baseline IOP (mmHg) Mean ± SD 15.8 ± 2.7 14.9 ± 2.8 t = 1.54 -0.34 to +1.95 0.12 Baseline MAP (mmHg) Mean ± SD 96.4 ± 11.2 94.1 ± 10.8 t = 1.00 -2.09 to +6.38 0.32 Table 1 compares the baseline demographic characteristics and preoperative ocular parameters between patients positioned prone (n=52) and supine (n=48). The mean age was similar between the two groups, with the prone group averaging 54.3 ± 8.9 years and the supine group 53.1 ± 9.4 years, showing no statistically significant difference (t=0.65, p=0.51). A comparable gender distribution was noted, with males comprising 63.4% of the prone group and 56.3% of the supine group (χ²=0.49, p=0.46). BMI values were also similar without significant variation (26.9 ± 3.4 vs. 26.3 ± 3.1; t=0.92, p=0.36). However, the duration of surgery differed significantly between groups, with prone-position surgeries lasting longer (214.7 ± 41.3 minutes) compared to supine procedures (178.6 ± 37.9 minutes), a difference that was statistically significant (t=4.56, 95% CI: +20.41 to +51.78, p<0.001). Baseline IOP and MAP values did not differ significantly (IOP: p=0.12; MAP: p=0.32) Table 2: Intraoperative Ocular Perfusion Pressure Comparison (Primary Objective) OPP Parameters Prone (n=52) Supine (n=48) Test of Significance 95% CI of Difference p-value OPP Post-Induction Mean ± SD 78.6 ± 9.1 82.3 ± 8.7 t = 2.16 +0.22 to +7.06 0.03* OPP at 2 Hours Mean ± SD 62.7 ± 10.4 74.8 ± 9.3 t = 5.80 +7.85 to +15.30 <0.001* Minimum OPP During Surgery Mean ± SD 53.4 ± 8.8 69.7 ± 10.1 t = 8.39 +12.41 to +19.12 <0.001* OPP Drop ≥30% from Baseline n(%) 21 (40.3%) 7 (14.6%) χ² = 8.38 - 0.004* Table 2 presents the comparative analysis of intraoperative ocular perfusion pressure (OPP) between prone and supine groups. Immediately after induction, OPP was significantly lower in the prone group (78.6 ± 9.1 mmHg) than in the supine group (82.3 ± 8.7 mmHg), with a statistically significant difference (t=2.16, p=0.03). As surgery progressed, the discrepancy widened; at the 2-hour mark, prone-position patients demonstrated a much lower OPP (62.7 ± 10.4 mmHg) compared to supine-position patients (74.8 ± 9.3 mmHg), showing a highly significant difference (t=5.80, 95% CI: +7.85 to +15.30, p<0.001). The minimum OPP recorded during surgery was also markedly reduced in the prone group (53.4 ± 8.8 mmHg) versus the supine group (69.7 ± 10.1 mmHg), which was highly significant (t=8.39, p<0.001). Furthermore, a clinically concerning drop in OPP ≥30% from baseline occurred in 40.3% of prone patients compared with only 14.6% of supine patients (χ²=8.38, p=0.004). Table 3: Postoperative Visual Function Assessment (Secondary Objective) Visual Outcome Prone (n=52) Supine (n=48) Test of Significance 95% CI of Difference p-value Post-op Visual Blur n(%) 12 (23.1%) 4 (8.3%) χ² = 4.43 - 0.035* Mean change in Visual Acuity (logMAR) Mean ± SD +0.12 ± 0.08 +0.05 ± 0.07 t = 4.52 +0.04 to +0.11 <0.001* Color Vision Abnormality n(%) 6 (11.5%) 2 (4.2%) χ² = 2.02 - 0.15 (NS) Visual Field Defect n(%) 4 (7.7%) 1 (2.1%) χ² = 1.90 - 0.17 (NS) Table 3 evaluates the postoperative visual function changes between prone and supine surgical patients. Postoperative visual blurring was significantly more common in the prone group (23.1%) than in the supine group (8.3%) (χ²=4.43, p=0.035). A similar trend was observed in terms of visual acuity deterioration; patients in the prone group demonstrated a greater mean change in logMAR (+0.12 ± 0.08) compared to the supine group (+0.05 ± 0.07), with a highly significant difference (t=4.52, 95% CI: +0.04 to +0.11, p<0.001). Although color vision abnormalities and visual field defects were more frequent in prone-positioned patients (11.5% vs. 4.2% and 7.7% vs. 2.1%, respectively), these did not reach statistical significance (p>0.05). Table 4: Correlation of OPP Variation With Postoperative Visual Outcomes (Core Objective) Parameter Visual Change Present (n=16) No Visual Change (n=84) Association Statistics 95% CI p-value Mean % OPP Fall 32.8 ± 6.9% 19.7 ± 7.4% t = 7.13 +9.57 to +16.47 <0.001* OPP Fall ≥30% n(%) 11 (68.8%) 17 (20.2%) OR = 8.4 2.54 to 28.11 <0.001* Mean Minimum OPP (mmHg) 51.9 ± 7.5 66.8 ± 9.1 t = 7.66 +10.43 to +19.29 <0.001* Pearson Correlation OPP fall vs VA change r = -0.62 Strong Negative Correlation - <0.001* Table 4 examines the correlation between intraoperative OPP variations and postoperative visual outcomes. Patients who developed postoperative visual changes (n=16) experienced a significantly greater mean OPP fall (32.8 ± 6.9%) compared to those without visual symptoms (19.7 ± 7.4%), with a strong statistical association (t=7.13, 95% CI: +9.57 to +16.47, p<0.001). A substantial OPP drop of ≥30% was seen in 68.8% of patients with visual disturbances versus only 20.2% in asymptomatic patients, yielding an odds ratio of 8.4, indicating a very strong predictive relationship (p<0.001). Minimum OPP values were also markedly lower in patients with visual changes (51.9 ± 7.5 mmHg) compared to those without (66.8 ± 9.1 mmHg), demonstrating another highly significant difference (t=7.66, p<0.001). Pearson correlation revealed a strong negative relationship between percentage OPP fall and visual acuity deterioration (r = -0.62, p<0.001), showing that larger drops in OPP correlate strongly with greater postoperative visual impairment.

Table 1 demonstrates that the two groups were largely demographically similar, with no statistically significant differences in age, gender distribution, BMI, or baseline ocular parameters such as intraocular pressure (IOP) and mean arterial pressure (MAP). These findings parallel the observations of Saoulidou EN et al. (2025)[4], who also noted that baseline IOP and MAP did not differ significantly between patient groups before prone spinal surgery. The only significant difference in the current study was the mean duration of surgery, which was longer in the prone group (214.7 ± 41.3 min). This trend has also been reported by Adas RH et al. (2025)[5], who documented prolonged operative times in neurosurgical procedures requiring prone positioning, contributing to increased hemodynamic stress and ocular pressure fluctuations. Table 2 highlights the primary finding of this study: prone positioning was associated with significantly lower OPP at multiple intraoperative time points. OPP immediately post-induction was already lower in the prone group; however, the reduction became more clinically significant at the 2-hour mark and at minimum intraoperative values. This aligns with the work of De Cassai A et al. (2022)[6], who reported that prone positioning and dependent orbital pressure markedly decreased OPP due to elevated venous pressure and compromised aqueous outflow. Furthermore, Almotairi FA et al. (2022)[7] also demonstrated that prone positioning results in progressive IOP elevation and reduction in perfusion to the optic nerve head. The present study strengthens these findings, particularly by showing a greater frequency of OPP drops ≥30% from baseline in prone patients (40.3% vs. 14.6%). Similar observations were made by Petersen LG et al. (2022)[8], who noted that OPP drops exceeding 25-30% place retinal ganglion cells at significant risk of ischemia. In Table 3, postoperative visual function outcomes were significantly worse among prone-positioned patients. Visual blurring and visual acuity deterioration were the most notable findings, both showing statistically significant higher frequencies in the prone group. These results are consistent with the study by da Silva Brito J et al. (2021)[9], which identified prone spinal surgeries as a major contributor to postoperative visual disturbances, including ischemic optic neuropathy (ION). In contrast, color vision abnormalities and visual field defects were not statistically significant this pattern resembles the findings of Garg B et al. (2023)[10], who reported that subtle visual acuity changes and blurring are more common than scotomas or color deficits in early postoperative ischemic changes. Table 4 demonstrates a strong correlation between intraoperative OPP reduction and postoperative visual dysfunction. Patients with visual symptoms had a significantly larger mean percentage drop in OPP (32.8% vs. 19.7%), markedly lower minimum OPP values, and a significantly higher likelihood of experiencing an OPP fall ≥30%. These findings affirm the mechanistic models proposed by Cunha PD et al. (2023)[11], who found that reduced OPP correlates strongly with optic nerve perfusion compromise, predicting both structural and functional postoperative visual alterations. The strong negative Pearson correlation observed in the present study (r = -0.62) clearly supports this pathophysiological link.

The present study demonstrates that intraoperative positioning has a significant influence on ocular perfusion pressure (OPP) and early postoperative visual function. Patients positioned prone experienced a markedly greater reduction in OPP throughout surgery especially at the 2-hour mark and minimum intraoperative values—compared with those positioned supine. These reductions were strongly associated with a higher incidence of postoperative visual disturbances, including visual blurring and significant deterioration in visual acuity. The correlation analysis confirmed that larger OPP drops, particularly ≥30% from baseline, were powerful predictors of postoperative visual dysfunction. Overall, the findings underscore the need for heightened vigilance in prone-position surgeries, emphasizing the importance of minimizing external ocular pressure, optimizing MAP, monitoring IOP where feasible, and shortening surgical duration when possible. Incorporating OPP-preserving strategies during anesthesia and positioning may reduce the risk of postoperative visual impairment, particularly in high-risk prone neurosurgical procedures. LIMITATIONS Several limitations of this study should be acknowledged. First, the sample size—though adequate for comparative analysis—may not fully represent all surgical subgroups, especially complex neurosurgical cases with prolonged operative times. Second, intraoperative IOP monitoring was performed intermittently rather than continuously, which may have underestimated transient fluctuations in ocular pressure. Third, postoperative visual function was assessed only up to 72 hours; longer follow-up may have detected delayed-onset visual deficits or late recovery. Fourth, variations in anesthetic depth, fluid management, and vasoactive drug use could influence MAP and therefore OPP, and although standardized protocols were followed, complete uniformity across cases cannot be ensured. Fifth, subtle visual field changes may require advanced perimetric testing, which was not feasible for all postoperative patients. Finally, the study did not assess optic nerve imaging (e.g., OCT), which could have provided structural correlation with functional outcomes.

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