Accepted Manuscript Placement of a cervical collar increases the optic nerve sheath diameter in healthy adults

Casey M. Woster, Michael D. Zwank, Joseph R. Pasquarella, Sandi S. Wewerka, Jeffrey P. Anderson, Jessica T. Greupner, Sakib Motalib PII: DOI: Reference:

S0735-6757(17)30700-3 doi: 10.1016/j.ajem.2017.08.051 YAJEM 56936

To appear in: Received date: Revised date: Accepted date:

14 April 2017 23 August 2017 23 August 2017

Please cite this article as: Casey M. Woster, Michael D. Zwank, Joseph R. Pasquarella, Sandi S. Wewerka, Jeffrey P. Anderson, Jessica T. Greupner, Sakib Motalib , Placement of a cervical collar increases the optic nerve sheath diameter in healthy adults, (2017), doi: 10.1016/j.ajem.2017.08.051

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ACCEPTED MANUSCRIPT TITLE: Placement of a Cervical Collar Increases the Optic Nerve Sheath Diameter in Healthy Adults RUNNING HEAD: Cervical Collar Affects Optic Nerve Sheath Diameter

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Casey M. Woster, MD, RDMS – Corresponding Author Department of Emergency Medicine Regions Hospital 640 Jackson Street MS11102F Saint Paul, MN 55101 (office) 651-254-2055 (fax) 651-254-5607 [email protected]

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AUTHORS & AFFILIATIONS:

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Michael D. Zwank, MD, RDMS (1)

Sandi S. Wewerka, MPH (1)

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Joseph R. Pasquarella, MS (1)

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Jeffrey P. Anderson, ScD, MPH (1) Jessica T. Greupner, MD (1)

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Sakib Motalib, MD (2)

1. Regions Hospital Department of Emergency Medicine

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2. Georgetown University Hospital / Washington Hospital Center MEETING PRESENTATIONS:

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2016 SAEM Great Plains Regional Meeting, Iowa City, IA, September 24 2017 AIUM Annual Convention, Orlando, FL, March 28 FUNDING:

This work was supported by the Resident IME funding from HealthPartners Institute. Abstract Introduction Blunt head trauma is a common cause of increased intracranial pressure (ICP). Ultrasound measurement of the optic nerve sheath diameter (OSND) is an accurate and non-invasive way to detect increased ICP. Blunt trauma patients are often immobilized in a rigid cervical spine collar. Our

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objective was to describe the changes in ONSD following the placement of a c-collar and determine if any changes were time-dependent. Methods We performed a prospective cohort study measuring the ONSD of healthy volunteers before and after placement of a c-collar. Two physicians obtained the measurements. Each eye was scanned twice using a standardized technique. This was done before c-collar placement, 5 minutes after placement and 20 minutes after placement. A mean of both eyes was calculated and analyzed using descriptive statistics. An intraclass correlation coefficient (ICC) was used to assess inter-rater reliability. Results Twenty study participants with a mean age of 37.1 years old were enrolled. The mean baseline ONSD was 3.77 mm (95% CI 3.48-4.07). The mean ONSD five minutes after the c-collar was placed was 4.47 (95% CI 4.17-4.78). The mean ONSD at 20 minutes after C-collar placement was 4.53 (95% CI 4.13-4.92). These changes were statistically significant (p=0.003 and <0.001). Reliability was relatively strong overall (ICC = 0.74; 95% CI: 0.65, 0.81). Conclusion The placement of a cervical collar increased the ONSD at 5 minutes and this change remained increased at 20 minutes. Future study should assess whether similar results are found in patients with blunt head trauma.

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Optic Nerve Sheath Diameter, Cervical Collar, Trauma

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1. INTRODUCTION

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Head injury is a significant cause of morbidity and mortality in trauma patients. Increases in intracranial pressure (ICP) can lead to decreased cerebral perfusion and resultant tissue injury. It is important to be able to detect increasing ICP in a timely manner so treatment can be initiated to avoid additional brain injury. Measuring the optic nerve sheath diameter (ONSD) has been advocated as a non-invasive and rapid way to assess for increased ICP.1-12 Traditional methods of ICP monitoring such as external ventricular devices are invasive, timeconsuming and can only be inserted by trained providers usually only at trauma centers.

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The optic nerve is surrounded by a protective sheath and is located posterior to the globe. It is easily assessed with ultrasound. This is typically done with a linear probe, measuring the diameter of the optic nerve sheath 3 mm posterior to the globe where the contrast is greatest. The upper limit of normal for adults is 5 mm, with a diameter greater than this being considered abnormal.1 Numerous previous studies have shown that an increased ONSD measured by bedside ultrasound correlates with increased ICP2-12, with one study demonstrating a correlation between ONSD >5mm and ICP >20 cm H2O.6 A recent systematic review and meta-analysis done by Ohle, et al. found an increased ONSD to be 95.6% sensitive and 92.3% specific for elevated ICP, with high positive and negative predictive values.13

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Trauma patients with head injuries frequently have the cervical spine at least temporarily immobilized by placement of a cervical collar (c-collar). It has been shown that the placement of a c-collar increases ICP.14-19 Venous outflow obstruction in the neck caused by the c-collar is a proposed mechanism for this increase in ICP20. The increase in ICP has been shown in cadavers to occur simultaneously with application of a c-collar.21 It is possible that the increased ONSD seen in trauma patients may be related to the c-collar. If the application of a c-collar is found to increase the ONSD, this may complicate the use of ONSD measurement as a non-invasive estimation of ICP in trauma patients. To our knowledge, no study has examined the possible effect of c-collar application on ONSD. We sought to measure ONSD before and after placement of a cervical collar. Our study objectives were to describe changes in optic nerve sheath measurement following the placement of a c-collar and to determine if these changes are dependent on the length of time immobilized by a c-collar. 2. METHODS

This was a prospective, observation cohort study that was conducted at a tertiary-care hospital. Healthy adult hospital employees greater than 18 years of

ACCEPTED MANUSCRIPT age were included. Individuals with a previous significant eye injury or malignancy and those with a history of a traumatic brain injury within the past 12 months were excluded. The study was approved by the institutional review board. Study participants were recruited via email, as well as at educational conferences and departmental meetings. Each participant received a gift card in return for participation in the study.

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2.2. DATA COLLECTION Participants were placed in the supine position and instructed to close their eyes. After applying a generous amount of sterile ultrasound gel to each closed eye, a 7.5 MHz linear probe with ophthalmic setting (Sonosite Edge, FUJIFILM) was utilized to obtain the images. Two physicians experienced with ultrasound obtained the measurements. Each eye was scanned individually in both sagittal and transverse planes. Measurements of the ONSD were taken at the accepted standard location 3 mm posterior to the retina (Figure 1). The measurements for each eye were recorded twice and then a mean binocular ONSD was calculated. This served as a baseline for the subsequent measurements.

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Following this baseline measurement, a rigid c-collar was fitted for each individual participant according to appropriate neck size and applied in standard fashion. ONSD measurements of each eye were then measured twice and the binocular mean was again calculated similar to the baseline measurements. This was done at time points 5 minutes and 20 minutes after c-collar application. The participants remained supine throughout the study period.

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2.3. ANALYSIS Descriptive statistics including means, standard deviations, ranges, frequency counts, and proportions were calculated to summarize patient characteristics and ONSD measures. Statistical comparisons of ONSD by time point were conducted using Wilcoxon-Mann-Whitney tests. Additionally, we constructed linear regression models to evaluate patient correlates of ONSD. Results from these models are reported as β estimates with 95% confidence intervals (CIs). To assess concordance of the two raters, we calculated intraclass correlation coefficients (ICCs) for the full complement of measures, and by time point. For interpretation of ICCs, we adhered to the guidelines put forth by Cicchetti. 22 All analyses were conducted using SAS (v9.4) or R (v3.3.0). ICC estimates and 95% CIs were obtained using the ICC9 macro, written and made publicly available by Hertzmark and Spiegelman.23 A significance level of α=0.05 was assumed throughout, and any reported p-values are two-sided. 3. RESULTS

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Twenty study participants with a mean age of 37.1 years were enrolled (Table 1). There were 15 females and 5 males. BMI ranged from 19.0 to 45.8 kg/m2. The distribution of 240 pooled ONSD measures (two eyes, two raters) by three time points is shown in Figure 2. At baseline, mean ONSD was 3.77 mm (95% CI: 3.60, 3.94). At 5 minutes after placement of the c-collar, mean ONSD was 4.47 mm (95% CI: 4.29, 4.65), and at 20 minutes, mean ONSD was 4.53 mm (95% CI: 4.33, 4.73). ONSD at 5 minutes and at 20 minutes were significantly increased compared to the ONSD at baseline (p values< 0.001). ONSD did not significantly change from 5 minutes to 20 minutes (p=0.580).

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Of the available patient measures, only BMI was significantly associated with increased ONSD at baseline. Specifically, the ONSD among individuals with a BMI greater than 30 kg/m2 was on average 0.55 mm larger (95% CI: 0.20, 0.90; p = 0.003). However, this difference was not present after the c-collar had been placed (Figure 3).

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Inter-rater reliability between the two sonographers was relatively strong overall (ICC = 0.74; 95% CI: 0.65, 0.81). The distribution of differences in rater measures by time point is shown in Figure 4. Concordance for ONSD at baseline was moderate (ICC = 0.69; 95% CI: 0.51, 0.82). Agreement was strongest at 20 minutes (ICC = 0.79; 95% CI: 0.65, 0.88), and weakest at 5 minutes (ICC = 0.59; 95% CI: 0.38, 0.77) following c-collar placement.

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4. DISCUSSION

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In this study, the placement of a cervical collar increased the ONSD at 5 minutes and the ONSD remained increased at 20 minutes after collar placement. It is likely the ONSD increased quickly but may not continue to increase with time, as there was no significant difference between the ONSD when measured at 5 minutes and at 20 minutes. Based on previous literature, we do not suspect that having the patient remain supine during the entire study played a role in the observed increase in ONSD, and this position most appropriately simulated the care of a head-injured trauma patient.24

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The measurement of the ONSD with ultrasound can be challenging, as obtaining a contiguous view of the ONSD may be limited by patient factors or by provider experience. As evidenced by our inter-rater reliability, ONSD measurements can vary between providers. As the optic nerve is not exactly perpendicular when attaching to the retina, edge artifact can be generated based on the angle of incidence of the ultrasound beam or the angle the optic nerve is entering the posterior eye. This minor variance was addressed by the sonographer using the lens or the iris as standard reference points, but could not fully account for the anatomic variance on how the optic nerve entered the eye. As mentioned previously, there was no significant difference between the ONSD at 5 minutes as compared to 20 minutes. For this reason, it is unlikely that any difference in

ACCEPTED MANUSCRIPT the amount of time taken to obtain each measurement by the two sonographers played a role in the observed inter-rater variation.

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Although c-collar application led to a significant increase in the ONSD, the mean remained below the accepted upper limit of 5 mm. The current results as well as another recent publication question the validity of this accepted standard. In a study by Goeres, the mean ONSD in 120 healthy volunteers to be 3.70 mm in men and 3.60 mm in women.25 It is possible that a lower cutoff may be more accurate. More research is needed to determine how much of an increase in the ONSD should be expected after the placement of a c-collar. Once established, this anticipated increase could then be used to more accurately detect an increased ICP in an immobilized trauma patient.

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4.1. LIMITATIONS There are several limitations to our study. We were unable to blind the sonographers to the presence of a c-collar and thus bias may have been present in measuring the ONSD before and after c-collar placement. In addition, study participants were healthy volunteers which may limit the external validity of our results when applied to trauma patients. Two experienced sonographers performed all ultrasounds and the results may not be generalizable to less experienced providers. A prospective trial involving trauma patients with head injuries is needed to explore the utility of ONSD measurements in this population.

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4.2. CONCLUSION In a cohort of healthy volunteers, placement of a rigid c-collar significantly increased the size of the optic nerve sheath diameter. This increase in size should be considered when using the ONSD as a means of detecting elevated ICP in trauma patients. A future study involving trauma patients will be undertaken to demonstrate if this effect is similar in patients with traumatic injuries.

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Figure 4

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ACCEPTED MANUSCRIPT 13. Ohle 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 Jun;34(7):1285-94. 14. Craig GR, Nielsen MS. Rigid cervical collars and intracranial pressure. Intensive Care Med. 1991;17(8):504-5.

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20. Stone, MB, Tubridy CM, Curran R. The effect of rigid cervical collars on internal jugular vein dimensions. Acad Emerg Med. 2010 Jan;17(1):100-2.

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21. Lyon M, Ganapathy P, Burbacher T, et al. Time correlation of optic nerve sheath diameter to increasing intracranial pressure in a cadaveric model. Annals of Emergency Medicine. 2011 OCt;58(4):S273.

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ACCEPTED MANUSCRIPT Table 1. Baseline Characteristics of the Study Population VARIABLE Total

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100%

Mean ± SD

37.1 ± 10.6

[Min, Max]

[27, 62]

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BMI

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Female

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Gender

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Age

Mean ± SD

28.1 ± 7.8

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[Min, Max]

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C-Collar Size Small

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Regular

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Neckless

[19.0, 45.8]

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25%

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15%