In this study, we evaluated the effect of the Trendelenburg position applied during laparoscopic hysterectomy on the increase in ICP by measuring ONSD. In the results of our study, we found that the ONSD value increased as the Trendelenburg position duration increased. This suggests that as the duration of Trendelenburg position and pneumoperitoneum increases, the sustainability of the mechanisms that balance the increase in ICP becomes insufficient.

Ultrasonographic ONSD measurement, which has seen a surge in popularity in recent years, is a faster, less harmful, portable, and more practical method compared to computed tomography and magnetic resonance imaging. Anatomically, the optic nerve is an extension of the central nervous system. It is surrounded by the arachnoid, dura mater, and CSF. The optic nerve sheath is adjacent to the dura mater, and its contents are closely associated with the subarachnoid space. The intra-orbital segment of the optic nerve is surrounded by the subarachnoid space and dura, extending from the intracranial space to the globe [10]. The average diameter of the optic nerve is approximately 4 mm when including the sheath and 3 mm without the sheath. The subarachnoid space contains approximately 0.1 ml of CSF. Cases where ONSD is larger than 5 mm are consistent with increased ICP. Kimberly et al. showed that ICP was > 20 mmHg when ONSD was above 5 mm [6]. Similarly, Soldatos et al. determined that the optimal cut-off value of ONSD that accurately predicted intracranial hypertension (ICP > 20 mmHg) was 5.7–6.0 mm [5]. In another study, it was reported that during laparoscopic prostatectomy, the ONSD increased to approximately 5.0 mm after the patients were placed in the isolated upright Trendelenburg position, and this reflected a high ICP defined as 20 mmHg. However, it was observed that the upright Trendelenburg position did not accurately reflect increased ICP, despite the presence of a possible increase in ICP that could reach critical levels in some patients [11]. In the current study, we found the ONSD value to be 5.08 mm (p = 0.024) for both eyes in the first measurement taken 30 min after the patients were placed in the Trendelenburg position. The ONSD value was above 5.08 mm for both eyes in all measurements performed in the Trendelenburg position during surgery.

In our study, it was observed that ONSD measured at the fifth minute after intubation increased by 13.4% compared to the first value measured when the patients were awake. This increase in ONSD can be attributed to the early post-intubation phase as well as the use of mechanical ventilation and PEEP.

According to the Monro-Kellie doctrine, in the confined and rigid structure of the brain, every increase in pressure or volume in one area is counterbalanced by a reduction in another region, in order to maintain equilibrium. Thus, an increase in intracranial blood volume caused by the head-down position is attempted to be balanced with the outflow of CSF [12]. To assess the adequacy of these mechanisms, Verdonck et al. used ONSD in their study to detect increased ICP in patients undergoing laparoscopic radical prostatectomy. The authors reported no significant change in ONSD during the 190-minute Trendelenburg position period, nor did they detect any significant change in cerebral perfusion pressure with the increase in central venous pressure [9]. In contrast, in our study, we found that the ONSD value measured at the 30th minute after pneumoperitoneum and Trendelenburg position was found to be 21% higher than the initial value. As the Trendelenburg position duration increased, the increase in ONSD continued. When the Trendelenburg position duration reached 90 min, the increase in ONSD was 32.8% when compared to the initial value and 7.3% compared to the measurement performed 30 min after Trendelenburg positioning.

Schramm et al. conducted a study to investigate the time-dependent effect of the extreme Trendelenburg position applied during robotic prostate surgery on cerebral autoregulation. The authors calculated cerebral autoregulation using the cerebral blood flow rate and MAP values ​​and reported that cerebral autoregulation was impaired after the 160th minute but improved when the posture was corrected. It was also reported that keeping MAP pressure at high values during the operation could trigger or aggravate the formation of brain edema [13]. In our study, ONSD started to increase after the 30th minute of the Trendelenburg positioning, increased further as the duration of this position increased, and started to decrease upon returning to the neutral position. However, throughout the Trendelenburg position, the MAP value was maintained within the range of 80–100 mmHg.

NIRS is a noninvasive monitoring method used to measure cerebral oxygen saturation. It has been reported that the rSO2 value related to cerebral oxygen saturation does not change when the Trendelenburg position and pneumoperitoneum are applied to patients during surgery, and that there is a low possibility of a decrease in oxygen saturation in the frontal cortex for a duration of up to 265 min [14,15,16]. Consistently, in our study, we did not observe any deterioration in the rSO2 values ​​of any of the patients during surgery. Although ONSD increased during the Trendelenburg position period, the rSO2 values ​​remained stable. Matsuoka et al. reported that MAP was the main factor affecting rSO2 during the Trendelenburg position and pneumoperitoneum [14]. The preservation of rSO2 in our patients can be attributed to the MAP exceeding 80 mmHg. The Trendelenburg position may cause venous stasis and affect the arterial-venous content of the blood flowing to the forehead skin. Davie et al. reported that extracranial contamination potentially affected rSO2 [17]. In addition, it is important to remember that NIRS only reflects the status of the frontal region. We did not observe any postoperative neurological complications and/or cognitive impairment in any of our patients. Similarly, Kim et al. reported that while they observed a 12.5% ​​increase in ONSD in patients who underwent laparoscopic radical prostatectomy, there was no deterioration in rSO2 or neurological complications [18].

Our study has certain limitations. First, we did not perform central venous monitoring on our patients because we did not deem it necessary for a surgical procedure. Second, we did not perform cerebral perfusion pressure measurements. Lastly, although NIRS monitoring was undertaken to monitor cerebral oxygenation, extracranial contamination was not taken into account.

In conclusion, when the ONSD measurement is used to assess the increase in ICP during the Trendelenburg position and pneumoperitoneum applied for laparoscopic hysterectomy, the ONSD value increases as the duration of these applications increases. To prevent postoperative neurological complications, it is recommended to limit the duration of the Trendelenburg position and pneumoperitoneum.