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ORIGINAL ARTICLE
Year : 2022  |  Volume : 70  |  Issue : 4  |  Page : 1460--1467

Perioperative Variation in Optic Nerve Sheath Diameter – A Prospective Observational Study of Traumatic Brain Injury Patients Undergoing Decompressive Craniectomy

Varun Suresh1, PR Ushakumari1, Anurag Aggarwal2, Arun Kumar1, Raja K Kutty3, Rajmohan B Prabhakar3, Anilkumar Peethambaran3,  
1 Department of Anaesthesiology, Government Medical College, Thiruvananthapuram, Kerala, India
2 Department of Neuroanesthesia and Pain Medicine, Fortis Hospital, Noida, Uttar Pradesh, India
3 Department of Neurosurgery, Government Medical College, Thiruvananthapuram, Kerala, India

Correspondence Address:
Varun Suresh
Department of Anaesthesiology, Government Medical College, Thiruvananthapuram, Kerala
India

Abstract

Background: Measuring optic nerve sheath diameter (ONSD) by transbulbar ultrasonography (TBUS) can suffice non-invasive ICP measurement with considerable accuracy. Objective: The primary objective of this study was to evaluate the perioperative variation in ONSD by TBUS in Traumatic Brain Injury (TBI) patients undergoing emergency craniectomy. Methods: We prospectively compared bilateral ONSD measurements in 45 consecutive TBI cases undergoing decompressive craniectomy under general anesthesia; before and after surgery. A total of 180 ONSD images were obtained and measurements were done by the same investigator blinded to the pre/postoperative nature of the image. Results: Based on preoperative Glasgow Coma Scores, 34 cases (75.5%) had severe TBI; 10 cases (22.2%) moderate TBI; and 1 case (2.2%) mild TBI. Preoperative ONSD in the study population were as 6.625 ± 0.414mm. Average ONSD reduced significantly by 0.249 ± 0.148 mm (P < 0.001) after craniectomy. On pooled analysis of cases undergoing right versus left sided craniectomy average ONSD reduced significantly by 0.252 ± 0.173 mm (P < 0.001) and 0.259 ± 0.139 mm (P < 0.001), respectively. ONSD of right eye with left eye and vice-versa were strongly correlated both pre/postoperatively with Pearson correlation coefficients (r)=0.879 (P < 0.001) and r = 0.827 (P < 0.001), respectively. Conclusions: In TBI cases undergoing decompressive craniectomy ONSD is bilaterally increased preoperatively. ONSD reduces significantly immediately after craniectomy; however, the diameters did not near the normal range. There hold a strong correlation between right/left ONSD measurements irrespective of the laterality of injury or side of surgery. Variable elastic properties of ONS in an injured brain can possibly explain our findings.



How to cite this article:
Suresh V, Ushakumari P R, Aggarwal A, Kumar A, Kutty RK, Prabhakar RB, Peethambaran A. Perioperative Variation in Optic Nerve Sheath Diameter – A Prospective Observational Study of Traumatic Brain Injury Patients Undergoing Decompressive Craniectomy.Neurol India 2022;70:1460-1467


How to cite this URL:
Suresh V, Ushakumari P R, Aggarwal A, Kumar A, Kutty RK, Prabhakar RB, Peethambaran A. Perioperative Variation in Optic Nerve Sheath Diameter – A Prospective Observational Study of Traumatic Brain Injury Patients Undergoing Decompressive Craniectomy. Neurol India [serial online] 2022 [cited 2022 Dec 5 ];70:1460-1467
Available from: https://www.neurologyindia.com/text.asp?2022/70/4/1460/355178


Full Text



Rapid industrialization, urbanization, and motorization have added to the momentum of global economic growth. Emerging non-communicable diseases and accidents are 'silent epidemic' to any community undergoing a rapid economic and social transition. Traumatic brain injuries (TBI) are associated with high mortality, morbidity, and socio-economic impact. The estimated all-cause incidence of TBI is 69 million (95% CI 64–74 million) individuals each year.[1] Central Asia, Eastern Europe and Central Europe regions experience the greatest overall burden of TBI. Over the last three decades age-standardized incidence, prevalence, and years of life lived with disability rates due to TBI has significantly increased by 3·6% (95% UI 1·8 to 5·5); 8·4% (95% UI 7·7 to 9·2); 8·5% (7·6–9·3) respectively per 1,00,000 population.[2] The deleterious effects of TBI are almost always attributed to raised intracranial pressure (ICP) and brain parenchymal injury. Measurement of ICP gains relevance in this context.

Various invasive and non-invasive methods are validated to measure ICP.[3] Invasive methods are the gold standard, but associated with procedure-related severe complications like infections, intra-ventricular bleeds and cerebrospinal fluid (CSF) leakage. Measurement of the optic nerve sheath diameter (ONSD) by transbulbar ultrasonography (TBUS) can suffice non-invasive ICP measurement with considerable accuracy.[4],[5]

Craniectomy is among the third tier and the highest intervention in treatment of raised ICP in severe TBI.[6] Neurosurgical procedures are associated with fluctuations in ICP due to various reasons like CSF loss through dural incisions, use of external ventricular/lumbar drains and/or patient surgical position. These changes can also affect the CSF flow dynamics over the optic nerve sheath (ONS). This can be non-invasively measured by TBUS. Till date no published studies have analyzed the pre/post-operative effect of decompressive craniectomy on CSF flow dynamics measured by TBUS. The primary objective of this study was to evaluate the perioperative variation in ONSD measured by TBUS in TBI patients undergoing emergency decompressive craniectomy for TBI. The secondary objectives were to find out whether there is an intraoperative variation in ONSD based on laterality of injury/side of craniectomy surgery.

 Methods



After obtaining Institutional Ethics Committee approval (HEC.No. 12/39/2019/MCT dated 20.12.2019 of Human Ethics Committee, Government Medical College, Thiruvananthapuram), we conducted a prospective observational study to evaluate the postoperative variation in ONSD compared to preoperative values, measured by TBUS, in patients undergoing decompressive craniectomy for TBI. The study was conducted from January 2020 to May 2020. Informed written consent was obtained from legal representative of study participants. The study was prospectively registered with the Clinical Trial Registry of India (CTRI/2020/01/022860 dated 20.01.2020). The study was conducted in the emergency neurosurgical operating rooms (OR) of our institution.

A total of 45 consecutive adult patients aged between 18 to 60 years undergoing emergency craniectomy surgery for TBI were recruited to the study. Patients with intraorbital injury, and/or preoperative disorders such as neoplasms, inflammatory diseases, sarcoidosis, pseudo-tumor cerebri, Grave's disease, and extrinsic compression of the optic nerve were excluded from the study. Patients undergoing craniectomy for stroke (ischemic/hemorrhagic) were excluded from recruitment.

Once in the OR, patients were monitored with electrocardiogram, pulse oximetry, non-invasive arterial pressure, and the bispectral index (BIS) monitor. General anesthesia (GA) was induced with intravenous (IV) doses of propofol (2 mg/kg), fentanyl 1–3 mcg/kg followed by administration of vecuronium bromide (0.1 mg/kg). After endotracheal intubation, mechanical ventilation was initiated using anesthesia work-station with 50% oxygen-air mixture, using a tidal volume of 8–10 ml/kg adjusted to a peak inspiratory pressure below 25 cm H2O. Respiratory rate was adjusted to maintain the end-tidal carbon dioxide (etCO2) levels between 30 and 40 mmHg during surgery. Continuous arterial blood pressures were monitored with invasive vascular access in the non-dependant radial artery.

Subsequently, patients underwent bilateral TBUS measurements for ONSD. A layer of sterile water-soluble jelly was applied over the closed upper eye lid of the patient and sonography performed with portable USG machine using high frequency (7-11MHz) linear probe; in transverse-plane [Figure 1] as per standard guidelines.[7] Sterile precautions were maintained during sonography and pressure on eye ball was avoided. The probe was angled medially and caudally to obtain optimal axial view of the optic nerve entering the eye ball, after appropriately adjusting depth and gain of the image. The images were saved from recording of each eye subsequent to which the patients underwent craniectomy surgery. After the conclusion of surgery and before the reversal of neuromuscular blockade second bilateral TBUS was done using the same preoperative protocol.{Figure 1}

Modality of maintenance anesthesia – total intravenous anesthesia (TIVA) or inhalational anesthesia or combined; adjusted to maintain a BIS within 40 to 60; was at the discretion of the anesthesiologist conducting the procedure. Also the choice of IV fluids and cerebral decongestants were with the anesthesiologist conducting the procedure.

All TBUS procedures (pre/post-operative) were conducted, identified using code and saved in the sonography machine by the principal investigator throughout the study. The ONSD measurements were done by another investigator blinded to the pre/post-operative nature of the saved image. The ONSD measurements were handed-over written to the principal investigator using the image identification code. Each ONSD measurement was done 3mm behind the eye-ball perpendicular to the line of transection to the eye-ball using electronic calliper[Figure 1]. The average of three recordings each pre/post-operative was analysed for each eye, and average of right and left eye was also calculated. Demographic parameters, intraoperative hemodynamic parameters, choice of anesthesia, cerebral decongestants used, total IV fluids/blood transfusion received and total duration of surgery/anesthesia/ONSD imaging were recorded.

No previous studies have evaluated the before and after effect of craniectomy on ONSD measured by TBUS. The effect size (0.5 mm) was calculated from a previous study measuring ONSD variation before and after ventriculo-peritoneal shunt surgery.[8] The sample size was calculated at two-tailed significance level with alpha error of 0.05, beta error of 20, and power of 80%. The population to be recruited was 40 patients. We recruited 45 cases so as to compensate for outliers and confounders.

Data collected in a prescribed proforma was entered in to a Microsoft Excel spreadsheet and analyzed using SPSS version. 25 (IBM, USA). Bee-swarm plots were constructed using Graph-pad prism software version 8.0.1 (San Diego, CA92018). Results on continuous measurements are presented as mean ± SD; and categorical variables as percentages. Student-t test was used to find the significance of pre and post-operative ONSD measurements. A P value of < 0.05 was considered significant. Correlations between ONSD's of either eye were derived using Pearson correlation coefficient. Bland-Altman plots were used to check the level of agreement between right and left ONSD, both pre/postoperatively.

 Results



This prospective observational study was conducted among 45 consecutive adult patients undergoing emergency craniectomy for TBI. Flow diagram showing enrolment, exclusion and analysis of participants through each stage of our study is shown in [Figure 2]. The patient and surgical case characteristics of the study population are as described in [Table 1]. With 4 ONSD measurements examined in each patient a total of 180 TBUS examinations were performed.{Figure 2}{Table 1}

Mean age of the study population was 37.76 ± 12.93 years. Eighteen patients underwent right-sided large fronto-temporo-parietal (FTP) craniectomy and 22 cases underwent similar left-sided craniectomy; whereas 5 cases underwent bifrontal craniectomy for non-diffuse primary brain injury (focal hematoma). Based on preoperative Glasgow Coma Scores (GCS), 34 cases (75.5%) had severe TBI (GCS 3-8); 10 cases (22.2%) moderate TBI (GCS 8-12); and 1 case (2.2%) mild TBI (GCS 13-15). Preoperative Marshall CT grade was grade 4; 3; 2; and 6 in 22 (48.8%); 9 (20%); 7 (15.5%) and 7 (15.5%) of the cases respectively. The cases were operated within a time frame of 11.98 ± 4.28 hours after the onset of injury with 34 (75.5%) cases on preoperative mechanical ventilation. Mannitol IV (dose 0.25-1g/kg) was used for osmotherapy in all the cases. All cases were operated in supine position with a mean operative duration of 192.18 ± 27.05 minutes and anesthesia time of 215.84 ± 29.70 minutes. The perioperative TBUS examinations were completed within 11.20 ± 0.93 minutes.

[Figure 3]a, [Figure 3]b, [Figure 3]c, [Figure 3]d shows the evolution of physiological and ventilation variables over the course of the surgery. Mean Cerebral Perfusion Pressure (CPP) derived using the ICP measurement obtained from pre/postoperative ONSD, as in the method described by Robba C et al.,[9] (non-invasive ICP = 5 × ONSD-14 [CPP = Mean blood pressure-ICP) is portrayed along with physiologic variables in [Figure 3]a.{Figure 3}

Comparison of participant ONSD before and after surgery, and a pooled comparison of ONSD variations in patients undergoing right versus left FTP craniectomy and bifrontal craniectomy are as described in [Table 2]. Average ONSD reduced significantly by 0.249 ± 0.148 mm (P < 0.001) after craniectomy[Figure 4]a and [Figure 4]b. On pooled analysis of cases undergoing right versus left-sided craniectomy average ONSD reduced significantly by 0.252 ± 0.173 mm (P < 0.001) and 0.259 ± 0.139 mm (P < 0.001) respectively [Table 2]. The ONSD reduced by 0.193 ± 0.092 mm (P = 0.009) in cases undergoing bifrontal craniectomy.{Table 2}{Figure 4}

On linear regression analysis, ONSD of right eye with left eye and vice-versa were strongly correlated both pre/postoperatively with Pearson correlation coefficients (r)=0.879 (P < 0.001) and r = 0.827 (P < 0.001), respectively [Figure 5]a and [Figure 5]b. Bland-Altman analysis of preoperative right versus left ONSD yielded a mean difference of 0.015 with limits of agreement as -0.401 to 0.432; and 4.4% (2/45) of the plots were outside the limits of agreement [Figure 6]a. Bland-Altman analysis of postoperative right versus left ONSD yielded a mean difference of 0.016. The limits of agreement were -0.532 and 0.500; with 1 (2.2%) plot outside the limits of agreement [Figure 6]b.{Figure 5}{Figure 6}

 Discussion



This study provides evidence that TBUS measured ONSD is bilaterally increased in patients with TBI undergoing craniectomy irrespective of the laterality of lesions in preoperative CT, and the subsequent side of surgery. There was a significant reduction in ONSD immediately after the surgery; nevertheless, the diameters did not near normal range irrespective of the side of surgery. To our knowledge, till date this is the first study to evaluate a pre-post comparison of TBUS measured ONSD in patients undergoing craniectomy, with pooled analysis based on laterality of surgery.

The CSF within ONS is in direct communication with subarachnoid space and chiasmatic cisterns. TBUS measured ONSD is a validated non-invasive tool to quantify ICP after TBI.[4],[5] There is a 2.0-22.7 fold increased risk of mortality in TBI patients with enlarged ONSD.[10],[11] Similarly enlarged ONSD can predict neurologic outcome on return of spontaneous circulation after cardiac arrest.[12] An ONSD of 5.8mm is often described as appropriate cut-off in TBI.[5] The point-of-care diagnostic benefit and feasibility of rapid interventions with TBUS is unmatched when compared to CT/MRI with comparable ONSD measurements.[13]

The advantage of CSF drainage and invasive ICP monitoring with ventricular catheters cannot be ignored in severe TBI cases with GCS≤6[6]; however not all patients of our study population were suitable candidates for invasive ICP monitoring. Hence, ONSD quantified ICP is salutary in such circumstances. We excluded patients with ischemic/hemorrhagic stroke undergoing craniectomy as multiple co-morbid illnesses in those predominant elderly populations can confound the study methodology. Only transverse TBUS measurements were done and sagittal TBUS avoided in our study considering the emergency nature of the surgery. Axial measurement of ONSD with the TBUS probe transverse to eyelid is most likely to be predictive of high first-measured ICP.[14] Interobserver variability was negated with the same investigator performing TBUS in all the study cases and another blinded investigator performed ONSD measurements of the electronically saved image. Previous studies have distinguished TBUS as a technique easy to learn[15] with low intra/inter-observer variability.[16],[17]

Our study suggests that neither the CSF loss associated with brain parenchymal dissection in diffuse injury nor the intracranial hypertension lowering effect of craniectomy did contribute to a normalizing ONSD. We postulate four possible reasons attributing to our findings. Firstly TBI represents a state of neurologic hyper-inflammation[18] along with raised ICP and cerebral edema. The linear increase of ONSD in TBI cases should also be attributed to this inflammatory response. ONSD being a papilledema corroborate the toxic milieu due to astrocyte damage in TBI can persist beyond the duration of craniectomy. Secondly, the CSF flow dynamics in small volume ONS (volume of 0.1-0.2 mm3) may not be linearly extrapolated to similar dynamics in ventricles and cisterns in the context of an injured brain. Dural lymphatic clefts in ONS can also alter CSF flow. Thirdly, a heterogeneous higher distribution of lipocalin-like prostaglandin-D synthase (L-PGDS) around the ONS can also contribute to persistent raised ONSD.[19] Fourthly, raised ICP in TBI can compromise pial-septal blood supply to the ONS causing hypoxia and metabolic toxicity that further trigger a vicious cycle of neuro-inflammation.[18] Moreover, the vasoconstrictive effect of L-PGDS[19] and cerebral auto-regulatory failure in TBI can worsen this hypoxia. All these factors could have together contributed to a partial compartmentalization of CSF space within ONS. Hence, the shrinkage of once enlarged ONSD after important increase in ICP in TBI is likely related to multiple factors and not just limited to mere displacement of CSF. In-vitro experiments by Hansen et al. showed that residual dilation of ONSD persisted on decompression from high ICP levels.[20] The modest reductions in ONSD among our patients who underwent bifrontal craniectomy compared to lateral craniectomy could also be attributed to hyperinflammation and partial ICP compartmentalization, with the ONS being more proximal to the site injury in this cohort.

The research till date provides variable evidence on a simultaneous change of TBUS measured ONSD with ICP and CSF drainage. CSF drainage with ventriculo-peritoneal shunt has shown to reduce TBUS measured ONSD immediately after 30 minutes in the study by Choi SH et al.,[8] and after 12 hours in the study by Bhandari D et al.[21] Schott CK et al.[22] did not detect any acute change in ONSD among ambulatory patients after elective lumbar puncture. Whereas, Wang L et al.[23] found a significant change in ONSD that correlated with ICP (r = 0.702, P < 0.001) among lumbar puncture cases at 1 month follow-up. Gao Y et al.[24] found no significant correlation (r = 0.205, P = 0.362) between ICP and ONSD performed at the 6th postoperative hour in their small cohort of 33 non-diffuse primary brain injury cases who underwent hemicraniectomy; however, a preoperative TBUS and pooled analysis based on laterality of surgery was not performed. Similarly, ONSD did not normalize after ICP reduction in sub-arachnoid hemorrhage patients.[25]

Outside the scenarios that involve direct drainage of CSF, a significant increase in ONSD has been found during CO2 pneumoperitoneum for laparoscopic pelvic/abdominal/prostate surgery[26] and during tracheal suction.[27] Dynamic alterations of PEEP and etCO2 in anesthetized patients can also alter ONSD.[28] Trendelenberg position has been found to be equivocal in its effects on ONSD.[29],[30]

Hence, as TBUS measured ONSD gains momentum in multiple clinical scenarios, a delayed return of ONSD to normal range in post-craniectomy patients can debate its diagnostic utility in this selected population. The underlying mechanistic underpinnings that determine the elastic properties of ONS and its subsequent return to normal range can be variable in cases with normal brain and an injured brain.

That we did not perform serial TBUS measurements and correlated it with outcome across mild, moderate and severe TBI is a limitation of our study. Correlation between invasive ICP and ONSD as well could have added further to the lucidity of our results, nevertheless not all cases in our study population qualified for invasive ICP monitoring. Studies in larger samples of TBI cases can validate our results further.

 Conclusions



We showed that TBUS measured ONSD is bilaterally increased preoperatively in TBI cases undergoing decompressive craniectomy. TBUS measured ONSD reduces significantly immediately after craniectomy; however, the diameters did not near the normal range. There hold a strong correlation between right/left side ONSD measurements in TBI irrespective of the laterality of injury or side of surgery.Variable elastic properties of the ONS in an injured brain can possibly explain our findings. Post-craniectomy ONSD measurements have many limitations in being truly representative of ICP, hence its value in comparison with ICP monitoring for prognostication need be addressed with caution.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1Dewan MC, Rattani A, Gupta S, Baticulon RE, Hung Y, Punchak M, et al. Estimating the global incidence of traumatic brain injury.J Neurosurg 2018:1-18. doi: 10.3171/2017.10.JNS17352.
2GBD 2016 Traumatic Brain Injury and Spinal Cord Injury Collaborators. Global, regional, and national burden of traumatic brain injury and spinal cord injury, 1990-2016: A systematic analysis for the Global Burden of Disease Study 2016. Lancet Neurol 2019;18:56-87.
3Zhang X, Medow JE, Iskandar BJ, Wang F, Shokoueinejad M, Joyce K, et al. Invasive and noninvasive means of measuri ng intracranial pressure: A review. Physiol Meas 2017;38:R143-82.
4Koziarz A, Sne N, Kegel F, Nath S, Badhiwala JH, Nassiri F, et al. Bedside optic nerve ultrasonography for diagnosing increased intracranial pressure: A systematic review and meta-analysis. Ann Intern Med 2019;171:896-905.
5Lee SH, Kim HS, Yun SJ. Optic nerve sheath diameter measurement for predicting raised intracranial pressure in adult patients with severe traumatic brain injury: A meta-analysis. J Crit Care 2020;56:182-7.
6Carney N, Totten AM, O'Reilly C, Ullman JS, Hawryluk GW, Bell MJ, et al. Guidelines for the management of severe traumatic brain injury, fourth edition. Neurosurgery 2017;80:6-15.
7Geeraerts T, Launey Y, Martin L, Pottecher J, Vigué B, Duranteau J, et al. Ultrasonography of the optic nerve sheath may be useful for detecting raised intracranial pressure after severe brain injury. Intensive Care Med 2007;33:1704-11.
8Choi SH, Min KT, Park EK, Kim MS, Jung JH, Kim H. Ultrasonography of the optic nerve sheath to assess intracranial pressure changes after ventriculo-peritoneal shunt surgery in children with hydrocephalus: A prospective observational study. Anaesthesia 2015;70:1268-73.
9Robba C, Cardim D, Tajsic T, PietersenJ, Bulman M, Donnelly J, et al. Ultrasound non-invasive measurement of intracranial pressure in neurointensive care: A prospective observational study. PLoS Med 2017;14:e1002356.
10Legrand A, Jeanjean P, Delanghe F, Peltier J, Lecat B, Dupont H. Estimation of optic nerve sheath diameter on an initial brain computed tomography scan can contribute prognostic information in traumatic brain injury patients. CritCare 2013;17:R61.
11Sekhon MS, McBeth P, Zou J, Qiao L, Kolmodin L, Henderson WR, et al. Association between optic nerve sheath diameter and mortality in patients with severe traumatic brain injury. NeurocritCare 2014;21:245-52.
12Lee SH, Jong Yun S. Diagnostic performance of optic nerve sheath diameter for predicting neurologic outcome in post-cardiac arrest patients: A systematic review and meta-analysis. Resuscitation 2019;138:59-67.
13Ohle R, McIsaac SM, Woo MY, Perry JJ. Sonography of the optic nerve sheath diameter for detection of raised intracranial pressure compared to computed tomography: A systematic review and meta-analysis. J Ultrasound Med 2015;34:1285-94.
14Agrawal A, Cheng R, Tang J, Madhok DY. Comparison of two techniques to measure optic nerve sheath diameter in patients at risk for increased intracranial pressure. Crit Care Med 2019;47:e495-501.
15Potgieter DW, KippinA, Ngu F, McKean C. Can accurate ultrasonographic measurement of the optic nerve sheath diameter (a non-invasive measure of intracranial pressure) be taught to novice operators in a single training session? Anaesth Intensive Care 2011;39:95-100.
16Bäuerle J, Lochner P, Kaps M, Nedelmann M. Intra- and interobsever reliability of sonographic assessment of the optic nerve sheath diameter in healthy adults. J Neuroimaging 2012;22:42-5.
17Moretti R, Pizzi B, Cassini F, Vivaldi N. Reliability of optic nerve ultrasound for the evaluation of patients with spontaneous intracranial hemorrhage. Neurocrit Care 2009;11:406-10.
18Finnie JW. Neuroinflammation: Beneficial and detrimental effects after traumatic brain injury. Inflammopharmacology 2013;21:309-20.
19Killer HE, Jaggi GP, Miller NR. Papilledema revisited: Is its pathophysiology really understood?Clin Exp Ophthalmol 2009;37:444-7.
20Hansen HC, Lagrèze W, Krueger O, Helmke K. Dependence of the optic nerve sheath diameter on acutely applied subarachnoidal pressure-An experimental ultrasound study. Acta Ophthalmol 2011;89:e528-32.
21Bhandari D, Udupi Bidkar P, Adinarayanan S, Narmadhalakshmi K, Srinivasan S. Measurement of changes in optic nerve sheath diameter using ultrasound and computed tomography scan before and after the ventriculoperitoneal shunt surgery in patients with hydrocephalus-A prospective observational trial. Br J Neurosurg 2019;33:125-30.
22Schott CK, Hirzallah MI, Heyman R, Lesky DN, Brant EB, Callaway CW. Ultrasound measurement of optic nerve sheath diameter pre- and post-lumbar puncture. Ultrasound J 2020;12:26.
23Wang LJ, Chen LM, Chen Y, Bao LY, Zheng NN, Wang YZ, et al. Ultrasonography assessments of optic nerve sheath diameter as a noninvasive and dynamic method of detecting changes in intracranial pressure. JAMA Ophthalmol 2018;136:250-6.
24Gao Y, Li Q, Wu C, Liu S, Zhang M. Diagnostic and prognostic value of the optic nerve sheath diameter with respect to the intracranial pressure and neurological outcome of patients following hemicraniectomy. BMC Neurol 2018;18:199.
25Bäuerle J, Niesen WD, Egger K, Buttler KJ, Reinhard M. Enlarged optic nerve sheath in aneurysmal subarachnoid hemorrhage despite normal intracranial pressure. J Neuroimaging 2016;26:194-6.
26Kim EJ, Koo BN, Choi SH, Park K, Kim MS. Ultrasonographic optic nerve sheath diameter for predicting elevated intracranial pressure during laparoscopic surgery: A systematic review and meta-analysis. Surg Endosc 2018;32:175-82.
27Maissan IM, Dirven PJ, Haitsma IK, Hoeks SE, Gommers D, Stolker RJ. Ultrasonographic measured optic nerve sheath diameter as an accurate and quick monitor for changes in intracranial pressure. J Neurosurg 2015;123:743-7.
28Bala R, Kumar R, Sharma J. A study to evaluate effect of PEEP and end-tidal carbon dioxide on optic nerve sheath diameter. Indian J Anaesth 2019;63:537-43.
29Chin JH, Seo H, Lee EH, Lee J, Hong JH, Hwang JH, et al. Sonographic optic nerve sheath diameter as a surrogate measure for intracranial pressure in anesthetized patients in the Trendelenburg position. BMC Anesthesiol 2015;15:43.
30Kim SH, Kim HJ, Jung KT. Position does not affect the optic nerve sheath diameter during laparoscopy. Korean J Anesthesiol 2015;68:358-63.