A Prospective Observational Study on Factors Affecting Optic Nerve Sheath Diameter and Visual Acuity in Prone Spinal Surgery

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.

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)