This study showed that ONSD was significantly increased during laparoscopic gynecological surgery in both obese and non-obese groups and was significantly higher in the obese group than in the non-obese group when measured after the induction of anesthesia, during and after the release of artificial pneumoperitoneum and the Trendelenburg position. Furthermore, the increased ONSD during laparoscopic gynecological surgery returned to baseline 24 h after desufflation and the return to the supine position in the non-obese group, even so did not return to baseline even 24 h after desufflation in the obese group. The incidence of PONV 1 h after surgery was notable higher in the obese group than in the non-obese group. However, the occurrence of PONV 24 h after surgery and postoperative headaches did not differ based on obesity.

Our findings of a significant increase in ONSD during laparoscopic gynecological surgery are consistent with those of previous studies [4, 6, 15]. These studies showed an increase in ONSD during pneumoperitoneum for laparoscopic gynecological surgery, mostly in non-obese patients. In the present study, we compared ONSD in both obese and non-obese patients during surgery and up to 24 h after desufflation and found that ONSD was significantly higher in obese patients than in non-obese patients throughout and after surgery. ONSD was significantly greater in the obese group than in the non-obese group from the baseline because obese patients tend to have higher thoracoabdominal pressure, which may affect ICP, thereby being reflected in ONSD from the baseline, although it is within the normal range. Our finding that obese patients had higher ONSD after the induction of anesthesia and during pneumoperitoneum than non-obese patients is consistent with the results of a study comparing the change in ONSD in obese and non-obese patients who in supine position underwent laparoscopic procedures [11]. In that study, the ONSD was assessed prior to pneumoperitoneum, during insufflation at 15 and 30 min, and immediately after deflation of pneumoperitoneum. The ONSD was significantly higher in obese patients at each time point; however, the mean body mass index of all patients included in that study was described as 30 kg m−2, but the criteria for obesity and the body mass index of each group were not clearly described. Furthermore, in that study, the Trendelenburg position was not applied during laparoscopic surgery, and the ONSD increased over time and returned to baseline levels following abdominal deflation. In our study, the increased ONSD during surgery returned to baseline 24 h after surgery in the non-obese group, but not in the obese group, which is consistent with a previous study showing that the ONSD remained elevated compared to the baseline value despite CO2 desufflation when measured 10 min after the release of pneumoperitoneum [6]. Most other studies observed the ONSD before, during, and after pneumoperitoneum during surgery, and did not conduct long-term follow-up [4,5,6,7, 11, 15]. To the best of our knowledge, this is the first study to observe the change in ONSD not only during surgery but also up to 24 h after desufflation.

Another important issue in interpreting our results is that the absolute ONSD value of obese patients was already elevated compared to those of non-obese patients at Tinduction. Furthermore, the observed increases in ONSD following their implementation seem to be similar between obese and non-obese patients. Therefore, some specific factors might have contributed to the ONSD expansion in obese patients during the Tinduction stage, although we did not evaluate them. Furthermore, ONSD values in obese patients had not returned to baseline even by TDesuff+Sup24h. It also remains uncertain whether this outcome is attributable to the effects of pneumoperitoneum and the Trendelenburg position or to other factors already present during induction of anesthesia.

In the present study, the incidence of PONV was significantly higher in obese patients 1 h after surgery but did not differ between the two groups 24 h after surgery [16, 17]. The higher incidence of PONV 1 h after surgery might be due to increased ICP, as indicated by the higher ONSD following the steep Trendelenburg position and pneumoperitoneum in the obese group. It has been reported that the extent of the increase in ONSD during the procedure is significantly correlated with PONV and headaches occurring within the first 3 h of recovery [6]. However, our study was not powered to detect the relationship of ONSD with PONV; therefore, further studies are needed to validate this hypothesis.

The ONSD is thought to be correlated with ICP levels, and the reported cutoff for detecting elevated ICP ranges from 4.8 to 5.6 mm [13, 18, 19]. Mean baseline ONSD after the induction of anesthesia was 4.11 mm in the non-obese group and 4.44 mm in the obese group. However, during the Trendelenburg position and pneumoperitoneum, the ONSD significantly increased, with the mean peak ONSD reaching 5.33 mm at TPneumo+Tren30m in the obese group and 4.90 mm at TPneumo+Tren60m in the non-obese group. In the present study, the obese group had a higher ONSD than the non-obese group at each time point from the baseline. Although the change in ONSD from the baseline to each time point may not be different between the two groups, we focused on the absolute value of ONSD at each time point between the two groups because the absolute value of ONSD rather than the degree of increase in ONSD is more meaningful to assess increased ICP. The obese group had a significantly higher ONSD, which may indicate an increased ICP. Therefore, although pneumoperitoneum and Trendelenburg positioning are required to secure the surgical field during laparoscopic gynecological surgery, excessive insufflation or a steep Trendelenburg position should be avoided because of the possibility of increased ICP, particularly in obese patients. Furthermore, low arterial pressure should be avoided to ensure adequate cerebral perfusion. Higher airway pressure may also affect ONSD although we did not evaluate it in this study. Therefore, it is important to suppress an increase in airway pressure for ICP management in obese patients, and further studies on the relationship between airway pressure affected by obesity and respiratory management, and the change of ONSD are required.

Additionally, a difference of less than 1 mm in ONSD between the two groups might be considered insignificant. However, the standard for increased ICP is divided in mm, and even a slight difference in ONSD may reflect increased ICP; therefore, our results cannot be ignored.

In the present study, a PEEP of 5 cmH2O was applied to both groups because PEEP increases intrathoracic pressure. However, obese patients may need higher PEEP to avoid lung collapse, although an adequate level of PEEP for obese patients has not been clearly determined [20, 21]. Therefore, if higher PEEP is applied to obese patients during gynecological laparoscopic surgery, the ONSD could be much higher than in non-obese patients.

This study had several limitations. First, the operator who measured ONSD was not blinded to the time points of ONSD assessment. Furthermore, the operator evaluated the ONSD without knowing the patient’s BMI for blindness, but might notice the obesity status of the patients although he or she could not accurately determine obesity levels. However, the operator tried to measure ONSD according to a standardized study protocol. Second, a single operator assessed the ONSD, which makes the interpretation of the results challenging. However, it would minimize inter-observer variability. Third, the mean body mass index of obese group in this study was 36 kg m−2, and the results may differ between obese and morbidly obese patients (body mass index ≥ 40 kg m−2). Fourth, this study was conducted on an Asian population. Therefore, our results may not be generalizable to other ethnic groups. Fifth, the baseline ONSD was assessed after the induction of anesthesia, according to previous studies [6, 8, 11, 15], but not in the awake state before the induction of anesthesia, and the recovery of ONSD after surgery was evaluated based on baseline ONSD. Anesthesia may affect ICP. Therefore, if the baseline ONSD was set before the induction of anesthesia, the time point of return to baseline ONSD after surgery might differ from the current results. Finally, the mean operation time, including pneumoperitoneum and Trendelenburg positioning, was approximately 120 min in both groups. According to a previous study [22], prolonged intracranial hypertension affects ONSD reversibility. Therefore, if the duration of pneumoperitoneum and Trendelenburg positioning were longer or shorter, the timing of recovery to baseline ONSD might vary.