Postoperative delirium is a sudden and reversible disturbance of mental function, occurring after surgery, characterized by confusion, inattention, and fluctuating mental status, which can manifest as agitation or lethargy. It is a common post-operative complication, especially in older adults, and is associated with longer hospital stays and worse recovery outcomes. Validated delirium screening tools such as the Intensive Care Delirium Screening Checklist (ICDSC) and the Richmond Agitation and Sedation Scale (RASS) are widely used tools to assess delirium. However, the sensitivity of these screening tools can be variable when used in real-world practice and may miss early cases of delirium. The optic nerve sheath is a protective sheath that encloses part of the optic nerve that is located at the back of the eye. Measurement of the optic nerve sheath diameter is usually done using ultrasound to detect increased cranial pressure. Previous studies that have been conducted have shown that increased ONSD may be associated with occurrence of postoperative delirium. The purpose of this study is to determine if there is an association between optic nerve sheath diameter (ONSD) and the occurrence of delirium after surgery. This study will be conducted at the London Health Sciences Centre and will include 300 patients.

Delirium is one of the most frequent and serious complications after cardiac surgery, with reported incidence between 15-30% depending on patient population, procedure type, and diagnostic method. It is associated with prolonged ICU and hospital length of stay, increased risk of institutional discharge, higher readmission rates, long-term cognitive impairment, and mortality. Validated delirium screening tools such as the Confusion Assessment Method for the ICU (CAM-ICU) and the Intensive Care Delirium Screening Checklist (ICDSC) are widely used, but their sensitivity in real-world practice can be variable, sometimes missing early cases despite high specificity. This contributes to delayed recognition and potentially worse outcomes.

Ultrasound measurement of the optic nerve sheath diameter (ONSD) has emerged as a non-invasive surrogate marker for intracranial pressure (ICP). Several meta-analyses confirm good diagnostic accuracy of ONSD ultrasound compared to invasive ICP monitoring. Concordance studies also demonstrate strong agreement between ultrasound and MRI ONSD measurements, particularly when standardized measurement protocols are used (e.g., 3 mm posterior to the retina, bilateral averaging). These findings support the reliability of ONSD ultrasound as a bedside monitoring tool.

In critical care, early broadening of ONSD has been associated with delirium, coma, and death within 28 days. Importantly, the first cardiac surgery-specific study that prospectively examined patients undergoing open-heart surgery and found that increased peri-operative ONSD was independently associated with postoperative delirium. Their findings suggest that ONSD may serve not only as a marker of raised ICP but also as a predictive biomarker for delirium in the cardiac surgical population. Taken together, the burden of postoperative delirium in cardiac surgery, the limitations of current screening methods, and the growing evidence supporting ONSD as a surrogate of cerebral dynamics provide the rationale for this study. We hypothesize that perioperative or early ICU ONSD enlargement will be associated with the development of delirium, and that ONSD monitoring may help identify high-risk patients earlier, guiding preventive strategies.

Informed, written consent will be obtained prior to the start of surgery. The participant's surgical plan will not be altered in any way. Participants that are enrolled in this study will have their surgery proceed according to plan and will be placed under general anesthesia using standard of care practices. Cardiopulmonary bypass will be implemented in accordance with standard of care practices.

For the ONSD ultrasound, the ultrasound will be performed using a linear probe 7.5-13 MHz and will follow standard practice for this procedure. Patients will have closed eyelids covered with a Tegaderm adhesive for eye protection. Ultrasound gel will be applied to the patient's eyelids. The ultrasound probe will be gently placed on the patient's eyelids and to locate the optic nerve and measure ONSD. The ultrasound scan will be performed 3 mm posterior to retina with 4 measurements taken at each time point to allow for a mean result to be determined. Average time for each measurement is 1 minute. Each ultrasound will take 10-15 minutes to complete. This ONSD ultrasound will be performed at the following time points:

1. Prior to the start of surgery
2. At the start of cardiopulmonary bypass (CPB)
3. At the end of cardiopulmonary bypass
4. Admission to the ICU
5. 1 hour after ICU admission
6. 6 hours after ICU admission
7. 24 hours after ICU admission
8. Daily until hospital discharge for delirious patients or daily until discharge from the ICU for non-delirious patients.

Delirium will be assessed by administering the Intensive Care Delirium Screening Checklist (ICDSC) to patients twice daily until discharge from ICU. This ICDSC will be administered in person. The study team will also collect information such as the patient's age, biological sex, height, weight, medications, medical history, surgical details, results of preoperative and postoperative blood work, pain scores that are collected as part of standard of care practices, and details of their postoperative recovery.

Inclusion Criteria:

• A d u l t s ≥ 1 8 y e a r s .
• Scheduled for cardiac surgery requiring cardiopulmonary bypass (CPB) (e.g., Coronary artery bypass graft (CABG), valve, combined procedures, aortic surgery).
• Anticipated postoperative admission to the Cardiac ICU with expected stay >48 hours.
• Preoperative ability to undergo ocular ultrasound examination (closed-eyelid scanning).
• Provided informed consent to participate in this study.

Exclusion Criteria:

• Known or suspected open-globe injury, penetrating ocular trauma, or intraocular foreign body.
• Recent ocular surgery or intravitreal intervention within the past 6 weeks (if surgeon advises against periocular pressure).
• Ocular conditions that invalidate ONSD measurement (e.g., severe proptosis, advanced optic neuropathies, orbital tumors) or obstruct ultrasound window (extensive periocular dressings, severe periorbital edema).
• Known intracranial pathology expected to significantly alter baseline ONSD (e.g., large mass lesion with midline shift, obstructive hydrocephalus) or presence of external ventricular drain at baseline.
• Severe facial trauma or unstable cervical spine precluding safe positioning for ocular ultrasound.
• P r e g n a n c y .
• Inability to perform serial ONSD assessments at scheduled timepoints (e.g., continuous prone positioning) despite reasonable accommodations.
• Enrollment in a conflicting interventional trial that mandates deviations from delirium assessment or ONSD protocol.
• Inability to communicate in the English language.