Optic Nerve Sheath Diameter Changes During Coronary Artery Bypass Graft Surgery: A Mini Review Volume 62- Issue 2

Yasin Tire^1,2*, Mustafa Aydemir¹ and Mehmet Akif Yazar^1,2

• ¹Anesthesiology and Reanimation, Department of Anesthesiology and Reanimation, Konya City Hospital, University of Health Science, Konya, Turkey
• ²Outcomes Research Consortium®, Houston, Texas, USA

Received: May 26, 2025; Published: June 02, 2025

*Corresponding author: Yasin Tire, Anesthesiology and Reanimation, Department of Anesthesiology and Reanimation, Konya City Hospital, University of Health Science, Konya, Turkey

DOI: 10.26717/BJSTR.2025.62.009703

Abstract PDF

Changes in intracranial pressure (ICP) are among the many major physiological effects of coronary artery bypass graft (CABG) surgery. To provide useful insights into cerebral haemodynamics during surgery, the optic nerve sheath diameter (ONSD) is used as a surrogate measure to assess fluctuations in intracranial pressure (ICP) in a non-invasive way. This narrative review looks into the effects of coronary artery bypass grafting (CABG) on open-labyrinth syndrome (ONSD), the possible consequences for clinical practice, and the function of ocular ultrasonography (OUS) during preoperative assessment. Neuromonitoring techniques, patient safety, and the avoidance of negative neurological outcomes following cardiac surgery can all be enhanced by a better understanding of the connection between CABG and ONSD.

Abbreviations: ONSD: Open-Labyrinth Syndrome; ICP: Intracranial Pressure; CABG: Coronary Artery Bypass Graft; OUS: Ocular Ultrasonography; CSF: Cerebrospinal Fluid; OUS: Ocular Ultrasonography; CPB: Cardiopulmonary Bypass; SIRS: Systemic Inflammatory Response Syndrome; PEEP: Positive End-Expiratory Pressure

Coronary artery bypass graft (CABG) surgery is a widely performed procedure for patients with severe coronary artery disease, improving myocardial perfusion and reducing the risk of major cardiac events. However, this procedure is associated with significant systemic changes, including fluctuations in blood pressure, cerebral perfusion, and oxygenation, which can impact intracranial pressure (ICP) and potentially lead to neurological complications [1,2]. The optic nerve sheath is continuous with the subarachnoid space, rendering it a significant marker for variations in intracranial pressure (ICP). The subarachnoid space, containing cerebrospinal fluid (CSF), means that any rise in intracranial pressure (ICP) immediately influences the optic nerve sheath, resulting in its quantifiable enlargement. The optic nerve sheath diameter (ONSD) correlates significantly with invasively determined intracranial pressure (ICP), establishing it as a crucial surrogate marker for non-invasive neuromonitoring [3]. ONSD can be quantified by ocular ultrasonography (OUS), offering a swift, bedside technique for the real-time evaluation of intracranial pressure fluctuations. In contrast to conventional ICP monitoring methods, such as intraventricular catheters or lumbar puncture, [4] OUS is non-invasive, secure, and reproducible, rendering it a compelling choice for perioperative application. Research has shown that alterations in ONSD occur in reaction to variations in ICP during significant surgical interventions, such as cardiac surgery [5]. Patel et al. [6] examined cognitive results of differing acid-base control strategies during cardiopulmonary bypass (CPB).

Seventy CABG patients were randomly assigned to pH-stat or alpha- stat care. At various stages of the process, cerebral blood flow, middle cerebral artery blood flow velocity, and cerebral oxygen metabolism were monitored. While pH-stat control increased cerebral blood flow during CPB, it also disrupted cerebral autoregulation. Psychological examination six weeks postoperatively showed that pH-stat patients had more cognitive damage. Alpha-stat treatment improved cerebral autoregulation and reduced postoperative cognitive impairment. Monitoring intracranial pressure for these and similar effects will also prevent possible neurological complications. This review analyses the impact of CABG on ONSD, emphasizing the relevance of intraoperative haemodynamic alterations, cardiopulmonary bypass (CPB), and fluid management techniques on ICP. Furthermore, it examines the prospective application of OUS in perioperative neuromonitoring, addressing its function in detecting at-risk patients, directing haemodynamic therapies, and enhancing postoperative neurological outcomes. Ultimately, measures to reduce neurological risks in patients having CABG will be examined, highlighting the necessity of incorporating multimodal neuromonitoring techniques to augment patient safety and increase overall surgical results.

Figure 1

During CABG surgery, there are many changes in blood flow and metabolism that can impact ICP and brain autoregulation. (Figure 1) The main things that cause changes in ICP during CABG are:

1. Cardiopulmonary Bypass (CPB): Because of changes in perfusion pressure, hemodilution, and inflammatory reactions, CPB can affect how the brain controls itself. This could cause short-term rises in ICP.

2. Hypervolemia and Fluid Shifts: Giving fluids and blood products through an IV during surgery can cause big changes in the fluid levels in the brain, which can affect blood flow and make the ICP go up.

3. Hypothermia and Rewarming: Hypothermia is used during CPB to lower metabolic demand. However, when the person is rewarming, there can be rapid changes in brain blood flow that raise ICP.

4. Systemic Inflammatory Response Syndrome (SIRS): Systemic inflammation caused by CABG can make endothelial cells not work properly, which can make the blood-brain barrier more permeable and cause ICP to rise.

5. Mechanical Ventilation Strategies: Ventilation settings, including positive end-expiratory pressure (PEEP) and oxygenation parameters, can affect cerebral venous drainage and ICP regulation.

6. Postoperative Cerebral Edema: Cerebral oedema can happen because of prolonged CPB, ischemia-reperfusion injury, or low blood pressure during surgery, which can cause ICP to stay high afterward.

The Diameter of the Optical Nerve Sheath as a Stand-in for ICP Since the subarachnoid space and the optic nerve sheath are closely related, increases in ICP cause quantifiable increases in ONSD. The use of OUS for non-invasive ICP monitoring is supported by studies showing a significant correlation between ONSD and invasively measured ICP. Because it offers a quick, bedside way to evaluate cerebral haemodynamics, OUS is becoming increasingly used in neurocritical care and perioperative settings. Real-time ICP variations during CABG can be detected with the aid of serial ONSD measurements, which can direct intraoperative and postoperative care plans to avoid neurological problems [7].

OUS in the Perioperative Environment OUS Provides a Number of Benefits [8,9] for Tracking ONSD and ICP Fluctuations in CABG Patients

• Non-invasive and real-time evaluation: OUS offers a radiation- free, bedside substitute for invasive ICP monitoring.

• High sensitivity for ICP elevations: Adults with elevated ICP are often identified by an ONSD threshold of >5.7 mm, which enables early management.

• Reliability and repeatability: ONSD can be measured several times during operation and afterward, which facilitates trend analysis.

• Integration potential with multimodal monitoring: To give a thorough evaluation of cerebral perfusion, OUS can be paired with transcranial Doppler, near-infrared spectroscopy, and other neuromonitoring instruments.

Clinical Consequences and Prospects there are Various Clinical Ramifications to Comprehending ONSD Differences During CABG, Including

• Recognizing individuals at risk for neurological complications: Elevated ONSD could be a precursor to postoperative delirium, cerebral oedema, or ischaemic damage.

• Directing therapeutic interventions: Modifications in ONSD can help guide the administration of osmotherapy to control ICP, haemodynamic management, and breathing changes.

• Improving postoperative care: By enabling prompt management in patients with persistent ICP increases, serial ONSD monitoring may enhance neurological outcomes.

Future Research Should Focus on

1. Standardizing OUS protocols for CABG patients: defining cutoff values, training requirements, and measuring methods for clinical judgement.

2. Examining the predictive usefulness of ONSD changes: determining whether intraoperative variations in ONSD are associated with neurological outcomes over the long run.

3. Including OUS in regular cardiac surgery monitoring: Assessing the viability and economic impact of adding ONSD evaluations to perioperative neuromonitoring regimens.

4. Creating automated ONSD measurement systems: Using machine learning and artificial intelligence to improve measurement precision and lower operator variability (Table 1).

Table 1: Current and Future ICP Changes During CABG.

Potential Challenges and Limitations [10,11] Despite its Promise, the Use of OUS for ONSD Monitoring has Certain Limitations

1. Operator dependency: Standardization and training are required because examiner expertise determines how accurate OUS measurements are.

2. Variability in threshold values: Although ONSD >5.7 mm is frequently used to denote elevated ICP, customized reference ranges may be necessary for each patient.

3. Absence of extensive validation studies: To make ONSD assessment a standard procedure for patients undergoing heart surgery, more investigation is required.

4. Possible interference from other factors: Anatomical differences, orbital damage, and optic neuritis can all have an impact on ONSD values, therefore cautious interpretation is required.

Significant cerebral haemodynamic problems are presented by CABG surgery, which may have an impact on the management of intracranial pressure. A promising non-invasive technique for tracking ICP changes in real time during and after CABG is ONSD as assessed by OUS. Clinicians can improve haemodynamic management techniques, lower neurological sequelae, and increase patient safety by integrating ONSD assessment into perioperative treatment. The use of OUS for routine neuromonitoring in heart surgery requires more investigation. To advance this technology and enhance patient outcomes, it will be crucial to standardize testing methods, improve threshold values, and carry out extensive research.

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