Impact of Reverse Trendelenburg on ONSD in Obesity Surgery

Obesity is associated with physiological alterations that may affect intracranial dynamics during laparoscopic surgery. In bariatric procedures, the creation of pneumoperitoneum and the use of reverse Trendelenburg positioning are essential for optimal surgical exposure. However, pneumoperitoneum increases intra-abdominal pressure and may impair venous return from the brain, potentially leading to elevations in intracranial pressure (ICP). These effects may be particularly relevant in obese patients, who are more susceptible to hemodynamic and respiratory changes during surgery.

Direct measurement of ICP using intraventricular or intraparenchymal catheters remains the gold standard; however, these invasive techniques are not feasible in routine surgical practice because of the risks of hemorrhage, infection, and other complications. Ultrasonographic measurement of optic nerve sheath diameter (ONSD) has emerged as a reliable, noninvasive surrogate marker for assessing changes in ICP and has been widely used in perioperative and critical care settings.

Previous studies have demonstrated that pneumoperitoneum may increase ONSD during laparoscopic procedures. Reverse Trendelenburg positioning has been shown to facilitate cerebral venous drainage and may attenuate increases in intracranial pressure. However, the effect of the timing of reverse Trendelenburg positioning relative to pneumoperitoneum creation has not been adequately investigated, particularly in patients undergoing bariatric surgery.

This prospective randomized controlled trial aims to evaluate whether applying the reverse Trendelenburg position before pneumoperitoneum can reduce perioperative increases in ONSD during laparoscopic bariatric surgery. Eligible patients undergoing elective laparoscopic bariatric surgery will be randomly assigned to one of two groups. In the conventional group, pneumoperitoneum will be established before reverse Trendelenburg positioning. In the intervention group, reverse Trendelenburg positioning will be applied before pneumoperitoneum creation.

ONSD measurements will be performed using ocular ultrasonography at predefined perioperative time points. Hemodynamic parameters, including mean arterial pressure and heart rate, as well as respiratory variables including end-tidal carbon dioxide and peak airway pressure, will also be recorded.

• Pre-pneumoperitoneum Reverse Trendelenburg Group (Other): Patients are placed in the reverse Trendelenburg position before carbon dioxide insufflation and establishment of pneumoperitoneum.

Primary outcomes

• Change in Optic Nerve Sheath Diameter (ONSD) [Baseline (before induction of anesthesia), immediately after induction, 5 minutes after pneumoperitoneum and positioning, and at the end of surgery.]: Optic nerve sheath diameter will be measured ultrasonographically at predefined perioperative time points. The outcome measure is the change in ONSD from baseline and the difference between study groups.

Secondary outcomes

• Mean Arterial Pressure (MAP) [During the intraoperative period]: Mean arterial pressure measured during surgery
• Heart Rate [During the intraoperative period]: Heart rate measured during surgery.
• Peak Airway Pressure [During the intraoperative period]: Peak airway pressure measured during surgery

Sex: All, ages: 18 Years to 65 Years, accepts healthy volunteers

Inclusion Criteria:

* Age between 18 and 65 years.
* American Society of Anesthesiologists (ASA) physical status I-III.
* Body mass index (BMI) ≥ 40 kg/m² and scheduled for elective laparoscopic bariatric surgery.
* Ability to provide written informed consent.
* Willingness to participate in the study and comply with study procedures.

Exclusion Criteria:

* Age younger than 18 years or older than 65 years.
* Refusal or inability to provide written informed consent.
* History of ophthalmologic disease that may affect optic nerve sheath diameter (ONSD) measurements (e.g., glaucoma, optic neuritis, optic nerve tumors, severe ocular trauma, previous ocular surgery).
* Known intracranial pathology, including intracranial hypertension, hydrocephalus, intracranial mass lesions, or cerebrovascular disease.
* Neurological disorders that may influence intracranial pressure measurements.
* Emergency surgery.
* Pregnancy.
* Severe cardiopulmonary disease (e.g., decompensated heart failure, severe pulmonary hypertension, severe chronic obstructive pulmonary disease).
* Inability to obtain adequate ocular ultrasonography images for ONSD measurement.
* Conversion from laparoscopic to open surgery. Intraoperative events requiring deviation from the study protocol.

• SEZEN KUMAS SOLAK, MD, Study director, Bagcılar Training Research Hospital