Surgical patient positioning and its effects on intracranial pressure

Supine, trendelenburg, prone, lithotomy, and beach-chair are the most commonly used surgical patient positions, and these positions have various physiological effects on patients. It is particularly important to understand the cerebral hemodynamic effects caused by positioning in patients with intracranial pathologies. We believe that intracranial pressure, which affects cerebral perfusion, varies depending on surgical patient positions. An increase in intracranial pressure is transmitted to the subarachnoid space around the optic nerve through cerebrospinal fluid flow, leading to expansion in the dura mater and an increase in the diameter of the optic nerve sheath. A diameter of 5 mm or more of the optic nerve sheath indicates that the intracranial pressure is above 20 mmHg. In our study, we aimed to evaluate the changes in intracranial pressure related to position by measuring the optic nerve sheath diameter (ONSD) under ultrasound guidance. Ethical approval for this prospective study was obtained from the Tokat Gaziosmanpaşa University Clinical Research Ethics Committee (24-KAEK-004). Participants included individuals aged 18-45 who visited the outpatient clinics of our hospital and did not have known neurological, ocular or cardiovascular diseases, had not previously experienced eye infections or trauma, had not undergone eye or cranial surgery, were not pregnant, and provided written consent. After obtaining informed consent from all volunteers, demographic data were recorded. Electrocardiography (ECG), non-invasive blood pressure, and pulse oximetry were monitored, and measurements were recorded every 5 minutes. ONSD measurements via ultrasound were performed 5 minutes after each position was given, and volunteers were allowed to rest in the supine position for 5 minutes before each measurement. A disposable transparent cover was placed over the volunteer's eye, and a generous amount of gel was applied; the volunteer was instructed to look straight ahead with their eyes closed. Using a linear probe in transverse section, the optic nerve sheath diameter was measured three times 3 mm behind the optic disc, and the average value was recorded. The measurement taken in the supine position was considered as the baseline, and the values obtained from other surgical patient positions (trendelenburg, prone, lithotomy, beach-chair) were compared to those of the supine position. Statistically significant increases in optic nerve sheath diameter were found in trendelenburg, prone, and lithotomy positions; however, a statistically significant decrease was observed in the beach-chair position compared to the supine position. The study demonstrated that different surgical patient positions affect the optic nerve sheath diameter, as shown by ultrasound.