The effects of prone position on optic nerve sheath diameter and intraocular pressure in elective lumbar disc surgery"

Objective: Increased intracranial pressure (ICP) is an important cause of secondary brain injury and its association with poor prognosis has been extensively demonstrated. The prone position given to the patient in posterior spine interventions increases intraabdominal pressure and thus central venous pressure. As a result of decreased venous drainage leads to both increased intracranial pressure (ICP) and increased intraocular pressure (IOP) by decreasing aqueous humor drainage. The sheath surrounding the optic nerve is continuous with the duramater and the subarachnoid space filled with cerebrospinal fluid (CSF). When IOP increases due to the prone position, the increased CSF pressure results in widening of the subarachnoid space surrounding the retrobulbar portion of the optic nerve, thus increasing the optic nerve sheath diameter (ONSD). IOP also increases due to the effect of increased CSF pressure on optic nerve and aqueous humor drainage. Therefore, IOP measurement, as a noninvasive method like ONSD, may be an indirect indicator for the determination of ICP. In our study, we aimed to investigate the effects of pronated position on ICP in lumbar disc herniation operations under general anesthesia by measuring ONSD and IOP changes. Patients and Methods: In our prospective observational study, after the approval of the Local Ethics Committee (date: 01.09.2021, no: 2021/155), between September 2021 and September 2022, patients from the ASA 1-2 risk group, aged 18-65 years, who were planned for elective surgery and who signed the informed consent form were included. ONSD, IOP, mean arterial pressure, SpO2, end-tidal carbon dioxide and heart rate values were recorded at 3 time points: 1st measurement at entry (T0), 2nd measurement at 10 min supine (T1), 3rd measurement at 45 min pronated (T2). ONSD and IOP measurements were performed with ultrasonography and Tonopen XL. Results: Data from 59 patients were analyzed. 47.5% (n=28) were female and 93.2% (n=55) were in the ASA II group. The mean age was 46.31±11.42 years and body mass index was 27.13±2.93 kg/m2. It was found statistically significant that T1 ONSD-right (median: 6.5 mm) and left (median: 6.1 mm) values were lower than T2 ONSD-right (median: 6.7 mm) and left (median: 6.4 mm) measurements (p=0.001; p<0.01). It was found statistically significant that T1 IOP-right (median: 14 mmHg) and left (median: 15 mmHg) values were lower than T2 IOP-right (median: 23 mmHg) and left (median: 23 mmHg) measurements (p=0.001; p<0.01). ONSD and IOP measurements were significantly higher in pron position (p=0.001; p<0.01). However, no correlation was found between ONSD and IOP measurements. Conclusion: Similar to other studies, we found that ONSD and IOP measurements increased in the prone position in our study, but these two measurements were not correlated. ONSD can be used in the diagnosis and follow-up of elevated IOP. Since there are many factors affecting IOP values, we concluded that it would not be effective in the diagnosis and follow-up of IOP. 2023, Key Words: Intracranial Pressure, İntraocular Pressure, Optic Nerve Sheath Diameter, Prone Position