A Prospective Observational Study on Factors Affecting Optic Nerve Sheath Diameter and Visual Acuity in Prone Spinal Surgery (PRONE-ONSD)

Factors Affecting Optic Nerve Sheath Diameter and Visual Acuity Changes in Adult Patients Undergoing Prone Spine Surgery: A Prospective Observational Study

The prone position in spinal surgery can alter intracranial pressure and optic nerve sheath diameter (ONSD) and potentially lead to visual complications. This prospective observational study aimed to identify factors influencing ultrasonographic ONSD changes and visual acuity in patients undergoing spinal surgery.

Study Overview

• Status: Completed
• Conditions: Prone Position; Spinal Surgery

Detailed Description

During spine surgery, many patients are positioned face-down (prone position) for several hours. This position may influence blood flow to the head and increase pressure around the brain and eyes. One structure that can reflect these pressure changes is the optic nerve sheath, which surrounds the optic nerve at the back of the eye. When the pressure inside the skull rises, the optic nerve sheath may become wider. This change can be measured non-invasively using ultrasound and is referred to as optic nerve sheath diameter (ONSD).

This prospective observational study will evaluate whether ONSD changes occur in adult patients undergoing prone-position spine surgery and whether these changes are related to temporary alterations in vision. Visual acuity will be evaluated using a standard Snellen chart before surgery and on the first postoperative day to detect any change in vision after the procedure.

Ultrasound ONSD measurements will be performed in three stages: before the patient is placed in the prone position, shortly after the prone position is established, and at the end of the surgery while still in the same position. Clinical variables such as blood pressure, carbon dioxide levels, ventilation parameters, anesthesia technique, the use of a pinned head holder, and patient characteristics will also be recorded. By comparing these variables with ONSD and visual outcomes, the study aims to identify whether specific surgical or physiological factors contribute to increased optic nerve sheath width or temporary vision disturbances.

This research does not involve any experimental drug or device. All measurements are part of routine safe monitoring techniques used to assess patients' condition during surgery. The goal of this study is to help better understand factors that may increase the risk of eye-related complications during prone spine surgery and to contribute to safer surgical practices.

• Study Type: Observational
• Enrollment (Actual): 65

Contacts and Locations

Study Locations

• Turkey (Türkiye)
  • Istanbul, Turkey (Türkiye)
    • Istanbul University - Cerrahpaşa, Cerrahpaşa Faculty of Medicine Hospital

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

• Sampling Method: Non-Probability Sample

Study Population

Adult patients scheduled for elective spine surgery requiring prone positioning at a university hospital setting.

Description

Inclusion Criteria:

• Age ≥ 18 years
• Scheduled to undergo elective spine surgery in the prone position
• American Society of Anesthesiologists (ASA) physical status I-III
• Preoperative visual acuity measurable using a Snellen chart

Exclusion Criteria:

• Preexisting ophthalmologic diseases affecting optic nerve or visual acuity (e.g., glaucoma, optic neuropathy, diabetic retinopathy)
• Previous eye trauma or history of orbital surgery
• Inability to obtain reliable ultrasonographic ONSD measurements (e.g., orbital deformity, ocular infection)

Study Plan

How is the study designed?

Number of groups / cohorts

1

Cohorts and Interventions

Group / Cohort
Prone Spine Surgery Cohort; Adults undergoing prone-position spine surgery, evaluated with ultrasound ONSD measurements and visual acuity testing.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in ultrasonographic optic nerve sheath diameter (mm)	Optic nerve sheath diameter (ONSD) will be measured bilaterally using standardized transorbital ultrasonography with a high-frequency linear probe (6-13 MHz). Measurements will be obtained 3 mm posterior to the globe, in accordance with established sonographic guidelines. For each eye, three measurements will be recorded, and the mean value will be calculated. The average of both eyes will be used for statistical analysis. The primary outcome will be the absolute change in ONSD (ΔONSD, mm) between baseline and intraoperative prone measurements.	Baseline (preoperative, supine), after prone positioning (15 minutes after positioning), and at the end of surgery (intraoperative, before repositioning to supine).

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in visual acuity (logMAR)	Visual acuity will be initially recorded in decimal format using a standard Snellen chart and subsequently converted to logMAR units for statistical analysis. The outcome will be the absolute change in visual acuity (ΔlogMAR) from baseline to postoperative day 1 (postoperative logMAR minus baseline logMAR).	Baseline (preoperative) and postoperative day 1.
Correlation between change in ONSD and intraoperative end-tidal carbon dioxide (EtCO₂)	Correlation analysis will be performed between changes in ONSD (ΔONSD, mm) and intraoperative end-tidal carbon dioxide levels (EtCO₂, mmHg) recorded simultaneously with ONSD measurements.	Intraoperative period: baseline (supine), after prone positioning (15 minutes), and at the end of surgery.
Difference in ONSD change between pinned head holder and non-pinned positioning	Change in ONSD (mm) will be compared between patients positioned using a pinned head holder and those positioned without pin fixation. The outcome will be the between-group difference in ONSD change (mm).	ntraoperative period: baseline (supine), after prone positioning (15 minutes), and at the end of surgery.
Association between postoperative visual acuity change and intraoperative ONSD change	The association between postoperative change in visual acuity (ΔlogMAR) and intraoperative change in optic nerve sheath diameter (ΔONSD) will be evaluated using regression analysis. Visual acuity will be recorded in decimal format and converted to logMAR for statistical analysis. ONSD will be measured using standardized transorbital ultrasonography and expressed in millimeters.	Baseline (preoperative) and postoperative day 1.
Correlation between change in ONSD and intraoperative mean arterial pressure (MAP)	Correlation analysis will be performed between changes in ONSD (ΔONSD, mm) and intraoperative mean arterial pressure (MAP, mmHg) recorded simultaneously with ONSD measurements.	Intraoperative period: baseline (supine), after prone positioning (15 minutes), and at the end of surgery.
Correlation between change in ONSD and intraoperative arterial partial pressure of carbon dioxide (PaCO₂)	Correlation analysis will be performed between changes in ONSD (ΔONSD, mm) and arterial partial pressure of carbon dioxide (PaCO₂, mmHg) obtained from arterial blood gas analysis and recorded simultaneously with ONSD measurements.	Intraoperative period: baseline (supine), after prone positioning (15 minutes), and at the end of surgery.
Correlation between change in ONSD and intraoperative arterial partial pressure of oxygen (PaO₂)	Correlation analysis will be performed between changes in ONSD (ΔONSD, mm) and arterial partial pressure of oxygen (PaO₂, mmHg) obtained from arterial blood gas analysis and recorded simultaneously with ONSD measurements.	Time Frame: Intraoperative period: baseline (supine), after prone positioning (15 minutes), and at the end of surgery.
Correlation between change in ONSD and intraoperative positive end-expiratory pressure (PEEP)	Correlation analysis will be performed between changes in ONSD (ΔONSD, mm) and intraoperative positive end-expiratory pressure levels (PEEP, cmH₂O) recorded simultaneously with ONSD measurements.	Intraoperative period: baseline (supine), after prone positioning (15 minutes), and at the end of surgery.

Collaborators and Investigators

• Sponsor: Istanbul University - Cerrahpasa
• Investigators: Principal Investigator: Özlem Korkmaz Dilmen, MD, Prof, Istanbul University-Cerrahpasa, Cerrahpasa Medical Faculty; Study Director: Begüm Ramakan, MD, Anesthesiology Resident, Istanbul University - Cerrahpasa

Study record dates

Study Major Dates

• Study Start (Actual): December 3, 2024
• Primary Completion (Actual): June 4, 2025
• Study Completion (Actual): June 5, 2025

Study Registration Dates

• First Submitted: December 3, 2025
• First Submitted That Met QC Criteria: February 3, 2026
• First Posted (Actual): February 9, 2026

Study Record Updates

• Last Update Posted (Actual): February 9, 2026
• Last Update Submitted That Met QC Criteria: February 3, 2026
• Last Verified: February 1, 2026

More Information

Terms related to this study

• Keywords: visual acuity; ONSD; logMAR; Snellen
• Other Study ID Numbers: 2024/244

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: IPD will not be shared because individual-level data are not permitted to be disclosed under the current ethical approval.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No