Epidural anesthesia and optic nerve sheath diameter in knee arthroplasty: a randomized study

Abstract

AimTotal knee replacement is a procedure that significantly improves quality of life but may involve anesthetic techniques that affect cerebrospinal fluid dynamics. This study aims to evaluate the effect of epidural volume on optic nerve sheath diameter (ONSD) as a non-invasive indicator of intracranial pressure (ICP).MethodsSixty patients (American Society Of Anesthesiologists (ASA) I-III) were randomized into Spinal (n = 30) and Epidural (n = 30) groups. ONSD measurements were performed ultrasonographically at baseline (t0), 10 minutes after anesthesia (t1), 40 minutes after tourniquet removal (t2), and 24 hours postoperatively (t3).ResultsThe Epidural Group showed a statistically significant increase in ONSD at t1 (p=0.044), t2 (p<0.001), and t3 (p<0.001) compared to baseline. No significant changes were observed in the Spinal Group. No clinical symptoms of increased ICP were observed in any patient.ConclusionEpidural anesthesia leads to a significant widening of the ONSD, suggesting a volumetric effect on ICP that remains clinically well-tolerated in patients with preserved intracranial compliance.

Keywords

Introduction

Total knee replacement surgery is a procedure that is increasingly performed in the aging population and significantly improves the quality of life of patients.¹ In such lower extremity surgeries, neuroaxial techniques such as spinal or epidural anesthesia are preferred as the gold standard due to the low number of systemic side effects and the effective analgesia they provide.^1-2 However, it is known that these techniques have direct physiological effects on cerebrospinal fluid (CSF) dynamics and therefore intracranial pressure (ICP).³
During neuroaxial anesthesia, injected volume may compress the dural sac and transiently increase intracranial pressure.³ Although invasive monitoring of ICP (intraventricular measurement, etc.) is considered the "gold standard," its use in routine surgical cases is limited due to complications such as bleeding and infection risks.⁴ In this context, the fact that the optic nerve is surrounded by CSF and directly reflects ICP changes has made ultrasonographic optic nerve sheath diameter (ONSD) measurement a reliable, non-invasive, and rapid alternative.^5-6
In light of all this information, the primary aim of our planned study is to examine the effect of the volume applied epidurally during neuroaxial block on ONSD (indirectly, ICP). The secondary aim of our study is to evaluate the implications of these pressure changes after epidural injection on postoperative complication rates and clinical safety profile.

Materials and Methods

Study DesignThis study was designed as a prospective, randomized study. All processes were carried out in accordance with the principles of the Helsinki Declaration. The study was registered at ClinicalTrials.gov following institutional administrative procedures. Clinical trial number: NCT07458321.Study Groups and Patient SelectionThe study included 60 patients aged 18-75 years with an ASA I-III risk group who were scheduled for total knee replacement and volunteered to participate. Patients with contraindications to regional anesthesia, hypersensitivity to the drugs used, pregnancy, current or past ocular disease/operation history, orbital trauma, optic nerve pathology, glaucoma history, increased intracranial pressure, pseudotumor cerebri, coagulopathy, and local infection; and those who refused to participate were excluded. Additionally, patients requiring conversion to general anesthesia for any reason, those experiencing dural puncture during epidural block, and those developing post-spinal headache were also excluded. Patients were randomized using a closed envelope method and divided into two groups: the control group received spinal anesthesia (Group 1: Spinal Anesthesia) and the study group received epidural anesthesia (Group 2: Epidural Anesthesia).ProcedureIn patients who did not receive premedication, electrocardiography (ECG), heart rate (HR), non-invasive arterial blood pressure, and peripheral oxygen saturation (SpO2) values were monitored in the operating room in the supine position. Intravenous access was established with a 16-18 Gauge(G) intracatheter.Spinal AnesthesiaPatients were placed in a sitting position for regional anesthesia. Spinal anesthesia was administered at the L4-5 level with a 25-G needle. After reaching the subarachnoid space and detecting CSF flow, 15 mg of 0.5% bupivacaine hydrochloride (Heavy Marcaine) was injected, and the patient was placed in the supine position.Epidural AnesthesiaTo administer epidural anesthesia, the needle insertion site was identified by inspection and palpation. Local anesthesia was achieved by injecting 3 ml of 2% lidocaine subcutaneously at the identified site. Using a Touhy needle, the needle was inserted at the L4-L5 level and advanced into the epidural space. Entry into the epidural space was confirmed by a decrease in air pressure. A test dose of 2 ml of 2% lidocaine and 1 ml of 1/200,000 adrenaline was administered to check for correct placement. After checking for blood and CSF leakage, 20 cc of fluid containing 0.25% bupivacaine was administered to the patients. The patient was placed in the supine position.
After administering an epidural or spinal anesthesia, the level of sensory block was assessed using a hot-cold test. The operation was initiated when the sensory block reached the T10 dermatome region.Ultrasonographic ONSD Measurement ProtocolONSD measurements were performed using a high-frequency (>7.5 MHz) linear transducer (Clarius scanner L7 HD3) via B-mode. Measurements were taken with patients in the supine position, with sterile gel applied to the closed eyelids. Measurements were taken 3 mm posterior to the papilla, and the average of the values obtained from both eyes was recorded.^4-5-6 The person performing the ONSD measurement did not apply a neuroaxial block.Perioperative and Postoperative MonitoringONSD measurements were performed and recorded ultrasonographically before anesthesia (t0), 10 minutes after the onset of anesthetic effect (t1), 40 minutes after tourniquet removal (t2), and 24 hours postoperatively (t3).
During the operation, all patients' heart rate (HR), mean arterial pressure (MAP), ECG, and peripheral oxygen saturation (SpO2) were continuously monitored. Maintenance fluid therapy was continued with crystalloid solutions at a rate of 10 mL/kg per hour. All patients received oxygen support at a rate of 2-4 L/min via nasal cannula.
Patients were monitored in the postoperative recovery unit with non-invasive blood pressure, HR, and SpO2 values. Close monitoring was performed for possible clinical manifestations of increased intracranial pressure, such as headache, nausea, vomiting, and blurred vision. Patients who did not develop any complications and whose vital signs remained stable were transferred to the orthopedics department.Postoperative AnalgesiaAt the end of the operation, all patients were transferred to the ward with intravenous patient-controlled analgesia initiated using a solution containing 1 mg/cc morphine. As a baseline analgesia plan in the postoperative period, all patients received a standard dose of paracetamol (1g, intravenous, 3 x 1). Meperidine (0.5 mg/kg, intravenous) was planned as a rescue analgesic for patients with a Visual Analog Scale (VAS) score of 4 or higher.Ethical ApprovalThe study was approved by the Ethics Committee of Bursa Health Sciences University City Hospital (Date: 06.12.2023, Decision No: 2023-20/3)Statistical AnalysisSample size was calculated using G*Power 3.1.9.4 based on the study by Tire et al. (2019). With an effect size of 0.96, alpha of 0.05, and power of 0.95, at least 23 patients per group were required. Considering a 30% dropout rate, 30 patients were included in each group (total n = 60).
Data were expressed as median (minimum–maximum) and n (%). Mann-Whitney U and Friedman tests were used for intergroup and intragroup comparisons, respectively. Categorical variables were analyzed using Pearson chi-square, Yates corrected chi-square, or Fisher-Freeman-Halton tests. Percentage changes from baseline measurements were used for intergroup comparisons and correlation analyses.
The study analyses were performed using SPSS 26.0 (IBM Corp., Armonk, NY, USA) and a significance level of p<0.05 was accepted.Reporting GuidelinesThis study was conducted and reported in accordance with the CONSORT guidelines for randomized controlled trials.

Results

Demographic and Operational DataInitially, 64 patients were included in the study; however, 4 patients were excluded due to failure of epidural catheter placement. Analyses were completed on a total of 60 patients (n = 30, for each group) (Figure 1). No statistically significant differences were found between the groups in terms of age, ASA score, height, weight, gender, and pre-anesthesia baseline heart rate values (p>0.05). The median operation time was 50 min (40–70) in the Epidural Group and 60 min (45–80) in the Spinal Group, and the difference was statistically significant (p=0.012). The groups showed similar results in terms of patient satisfaction (Supplementary Table 1).Hemodynamic DataWhen the hemodynamic data of the groups were analyzed, no significant difference was found between the baseline values in terms of heart rate (HR) changes and peripheral oxygen saturation (SpO2) values at all follow-up times in both groups (p>0.05). No statistically significant difference was found between the groups in terms of MAP values either. (Table 1).ONSD MeasurementsIn ONSD measurements, which are the primary endpoint of the study, no statistically significant difference was found between the groups in terms of baseline values before anesthesia (t0) (p=0.328) (Table 2).
In the Epidural Group, ONSD values showed a statistically significant increase compared to baseline values at 10 minutes after anesthesia (p=0.044), at 40 minutes (after tourniquet) (p<0.001), and at 24 hours postoperatively (p<0.001). On the other hand, a slight time-dependent decrease trend was observed in ONSD values in the Spinal Group, but this change did not reach the level of statistical significance (Table 2).Complications and Side EffectsPatients in both groups were closely monitored for possible clinical manifestations of increased ICP (severe headache, projectile vomiting, blurred vision, confusion) and other complications that may arise due to anesthesia techniques during the perioperative and postoperative 24 hours. Despite the statistically significant increase in ONSD values observed in the epidural group, no clinical symptoms or neurological complications associated with increased ICP were observed in any patient. Similarly, no headache or other minor side effects were observed after dural puncture in the spinal group. In both groups, the surgical procedure and recovery period were completed without complications.

Discussion

Epidural volume may increase ICP by causing cranial displacement of CSF. Previous studies have shown that even small volumes administered into the epidural space can significantly reduce dural sac volume and lead to transient increases in ICP.^7-8
Interestingly, it has been observed that in cases where baseline ICP was already high, epidural volume injection did not lead to an additional expansion of the ONSD.⁹
However, it should be noted that not every increase in ONSD may directly correlate with a clinically significant increase in ICP. For example, studies in patients followed up due to subarachnoid hemorrhage have shown that ultrasonographic ONSD measurements do not always coincide with invasive ICP values.¹⁰ This confirms that the clinical sensitivity of ONSD measurements may vary depending on the patient population, underlying pathology, and intracranial reserves.
In our study, despite the increase in ONSD detected in serial measurements in the epidural anesthesia group, no clinical neurological findings or symptoms were observed in any patient. In individuals with preserved intracranial compliance capacity, this pressure increase is thought to be balanced by compensatory mechanisms within the framework of the Monro-Kellie doctrine, such as a decrease in venous blood volume and the redistribution of CSF into the spinal subarachnoid space. Although it is reported in the literature that the increase in ICP due to epidural volume injection usually returns to baseline values in a short time ^8,11 ; in some studies, it has been found that the values remain higher than baseline even 40 minutes after injection.¹¹ Indeed, it has been reported that an epidural volume of 10 mL can cause acute increases in ICP ranging from 18.¹² mmHg to 39.5 mmHg.¹³
In our study, we believe that the persistent elevation observed in the optic nerve sheath, which was statistically significant, may be due to the sheath's elastic limits being strained by the sudden distension caused by the epidural volume, and its subsequent loss of sensitivity to pressure changes.
Tourniquet deflation has been associated with transient increases in ICP due to hypercapnia and increased cerebral blood flow.¹⁴ However, in our study, tourniquet durations remained below critical thresholds reported in the literature, and no significant ONSD change was observed in the spinal group. This strengthens the opinion that the significant increase in ONSD observed in the epidural anesthesia group is due to hydrostatic pressure transmission created by the high volume applied directly to the epidural space, rather than physiological changes due to tourniquet deflation. Thus, the neutral effect of low-volume spinal anesthesia on ICP has created a reliable control point that allows the isolation of the epidural volume effect. Although the choice of the spinal anesthesia group as the control group in our study differs technically from epidural anesthesia, it is based on a scientific basis consistent with the literature. In the literature, it has been reported that low-dose spinal anesthesia applications do not lead to a clinically significant change in ONSD.¹⁵
Although a correlation has been suggested between the development of post-dural puncture headache (PDPH) and a decrease in ONSD values, the same studies have shown that ONSD values remain stable in cases that do not develop PDPH.¹⁶ In light of these data, we aimed to minimize the possible effects of spinal anesthesia on ONSD by using low-dose spinal anesthesia and excluding patients who developed PDPH, thus creating a reliable control group. Indeed, our findings—no significant change in ONSD values observed in the spinal group—confirm that this methodological choice is in complete agreement with the expected physiological results.

Limitations

This study has several limitations. It was conducted in a single center with a relatively small sample size, which may limit generalizability. Additionally, the follow-up period was limited to 24 hours, preventing evaluation of long-term changes in ONSD. Future studies with larger populations and longer follow-up are needed to further clarify these findings.

Conclusion

It was determined that the volume applied to the epidural space led to a statistically significant widening of the ONSD, but this physiological response did not predispose to any clinical complications or neurological adverse events in the cases without intracranial pathology within the scope of our study. The clinical implications and long-term effects of this volumetric expansion should be investigated more thoroughly through prospective studies whose primary focus is to analyze this correlation. In particular, given the dynamic nature of the volume-pressure relationship in the epidural space, the intracranial repercussions triggered by these injections should be investigated for critical clinical significance in patient populations with limited cerebral reserve or borderline compensatory mechanisms.

Declarations

Abbreviations

ASA: American society of anesthesiologists
CONSORT: Consolidated standards of reporting trials
CSF: Cerebrospinal fluid
ECG: Electrocardiography
G: Gauge
HR: Heart rate
ICP: Intracranial pressure
MAP: Mean arterial pressure
MHz: Megahertz
ONSD: Optic nerve sheath diameter
PDPH: Post-dural puncture headache
SpO2: Peripheral oxygen saturation
VAS: Visual analog scale

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