Sonographic measurement of the epiglottis in normal Chinese adults

Hong Kong Journal of Emergency Medicine Sonographic measurement of the epiglottis in normal Chinese adults CW Chau, HH Chan, CP Wong, TW Wong, CC Lau Objectives: (1) To assess the normal range of thickness of the epiglottis by means of ultrasound measurement. (2) To evaluate inter-observer agreement in measuring the thickness of the epiglottis of normal individuals by ultrasound. (3) To assess the association between biological factors and the thickness of the epiglottis. Methods: Fifty adult volunteers working at a local accident and emergency department were recruited. The thickness of the epiglottis was measured by means of ultrasound examination, which was performed twice by two emergency physicians at different time. The study subjects' age, sex, height and body weight were recorded. Results: The mean thickness of the epiglottis was 0.236 cm and the standard deviation was 0.020. Male subjects had thicker epiglottis. Interobserver agreement of the two emergency physicians who performed the ultrasound scan was very good. Multiple regression models showed that sex and height were useful predictors of the thickness of the epiglottis. Conclusions: Bedside ultrasound assessment of the epiglottis is an easy, rapid and reliable method to evaluate its thickness. Further studies are needed to evaluate the thickness of the epiglottis in patients with epiglottitis before it can be put into clinical use. (Hong Kong j. emerg.med. 2010;17:429-434) 50 0.236 0.020 Keywords: Adult, epiglottis, observer variation, ultrasonography Correspondence to: Chau Cheuk Wai, MBBS, MRCS, DPD Pamela Youde Nethersole Eastern Hospital, Accident and Emergency Department, 3 Lok Man Road, Chai Wan, Hong Kong Email: hunterjphoto@yahoo.com.hk Chan Hok Hang, MBChB, FHKCEM, FHKAM(Emergency Medicine) Wong Chi Pang, MBBS, FHKCEM, FHKAM(Emergency Medicine) Wong Tai Wai, MBBS, FRCSEd, FHKAM(Emergency Medicine) Lau Chor Chiu, MBBS, MRCP(UK), FHKAM(Emergency Medicine) Introduction Ultrasonography has become more popular and important in emergency practice in recent decades. Many new ultrasound techniques have been incorporated into daily clinical practice in the emergency department (ED). 1 Focussed sonographic assessment at the bedside can give more information about the patient's illness on top of physical

430 Hong Kong j. emerg. med. Vol. 17(5) Nov 2010 examination. Chest, abdominal, vascular and cardiac ultrasound examinations are now common practice in the ED. However, sonographic measurement of the epiglottis is rarely performed in our daily clinical practice. In a local study, Lam et al 2 stated that the occurrence of acute epiglottitis in adults has been increasingly recognised in recent years. The paper also showed that the local prevalence rate was estimated to be 1.8/ 100,000, higher than its paediatric counterpart in many countries. However, adult epiglottitis is a diagnosis easily missed despite its potentially fatal outcome. The lack of specific symptoms in adult has resulted in 12.6% of misdiagnosis at the initial ED visits. 2 An ideal emergency department diagnostic test of acute epiglottitis should be simple, rapid, noninvasive, highly accurate and easily performed at the bedside by emergency physicians (EP). Flexible endoscopy is a good test but it is invasive and requires more skill. Lateral neck X-ray is the key investigation for suspected epiglottitis at the moment. However, its overall sensitivity in detecting epiglottitis is only about 89.4%. 2 Ultrasonography of the epiglottis may prove particularly useful in those unstable patients who should not be sent to the radiology department. Ultrasonographic measurement of the epiglottis in healthy adults has been studied in western populations. 3 However, the size of the epiglottis as measured by ultrasonography has not been studied among Chinese. The aims of this study were to measure the thickness of the epiglottis in normal Chinese adults, study the biological factors affecting the thickness of the epiglottis as well as to evaluate inter-observer agreement of the measurements. Methods This study was a prospective, observational study. The study was conducted in the ED of Pamela Youde Nethersole Eastern Hospital in Hong Kong from 1st December 2009 to 31st March 2010. The participants included 50 staff in the ED. Exclusion criteria were known laryngeal disease, recent acute upper airway infection, and known operation on the larynx. Verbal consent was obtained from all participants. Ultrasound scan was performed using the 12 MHz linear probe of the Esaote MyLab 30 machine. Subjects were imaged in either lying or upright seated position with the neck neutral or mildly extended. The epiglottis was located above the thyroid cartilage and was scanned in the short axis in all the subjects. Scanning was performed by two EPs (CW and HH) who were trained in emergency ultrasound. Scanning of the same subject was performed at different times by each EP who was blinded to the previous measurement results. The standard view of the epiglottis (Figure 1) can be found by simply placing the linear transducer probe at the level just above the thyroid cartilage in the transverse plane. This view of the epiglottis in ultrasound will show up a "face-appearance". The two hypoechoic "eyes" are the sternohyoid and the omohyoid muscles, and the "mouth" is the cross section of the epiglottis. There is no "nose" in this "face". The "faces" would sometimes be "smiley faces" (Figure 1a) or "sad faces" (Figure 1b) depending on the shape of the epiglottis. The epiglottis is shown as a hypoechoic structure and behind it, the hyperechoic dense artefacts are due to air. The thickness of the epiglottis was measured at the middle of the hypoechoic structure (Figure 1). The biological parameters of the participants including sex, age, height and body weight were recorded. Statistical analysis The central tendency and variation of the measurements of the two investigators, as well as those of the baseline characteristics of the subjects, were compared using means and standard deviations. Inter-observer agreement was analysed by plotting the difference against the mean of the measurements of the two EPs (EP1 and EP2). This kind of plotting was proposed by Bland and Altman. 4

Chau et al./sonographic measurement of the epiglottis 431 (a) (b) Figure 1. (a) Ultrasound scan showing the "face appearance" standard view. The two stars represent the "eyes" made up by the hyoid muscles. The "mouth" representing the epiglottis is indicated by the arrow. The thickness of the epiglottis is measured by the two crosses over the "mouth". This scan is showing a "smiley face". (b) This epiglottic ultrasound is showing a "sad face". The effect of age, sex, height, body weight and body mass index (BMI) on the thickness of the epiglottis was determined by multiple linear regression models, which were constructed by stepwise backward approach. The model with the least value of Akaike Information Criteria (AIC) was chosen as the best model. 5 All p values were two-sided and the significance level was chosen as 0.05. All statistical analyses were performed with the use of statistical software R version 2.10.1. Results The epiglottis thicknesses of the 50 subjects were measured by the two EPs. The baseline characteristics of the subjects are shown in Table 1. The measurements of EP1 and EP2 were compared in Table 2. The means of the measurements of the two investigators can be used as an estimate of the thickness of the epiglottis of the normal population. The average thickness of epiglottis was 0.236 cm and the standard deviation was 0.020. The mean and median of EP2's measurement were slightly smaller than those of EP1, so were the lower and upper quartiles. There was an outlier in EP2's measurement, which corresponded to a subject whose epiglottis thickness was 0.31 cm. The same subject got the same measurement by EP1, but its value was not shown as an outlier because of the relatively higher median and upper quartiles of EP1's data. The plotting of the differences against the means of measurements of the two emergency physicians is shown in Figure 2. We can see that: (1) almost all data Table 1. Baseline characteristics of the study subjects Mean Standard deviation Age (year) 35.4 7.9 Sex (male %) 50% Height (metre) 1.656 0.087 Body weight (kg) 60.01 11.95 Body mass index (kg/m 2 ) 27.74 3.03 Table 2. Comparison of emergency physicians' (EP1 and EP2) measurement of epiglottis thickness Mean Standard deviation EP1 EP2 Mean (cm) 0.24 0.23 Median (cm) 0.24 0.23 Standard deviation (cm) 0.028 0.021 Range (cm) 0.20-0.31 0.20-0.31 EP1=emergency physician 1, EP2=emergency physician 2

432 Hong Kong j. emerg. med. Vol. 17(5) Nov 2010 EP1=emergency physician 1, EP2=emergency physician 2 Figure 2. Difference-vs-mean plot. points are located between the two lines of the mean± 2 times SD, and (2) there is no pattern or trend observed on the distribution of the data points. Therefore, we can conclude that the two observers agree with each other very well. Table 3 depicts the various multiple linear regression models constructed. The model with sex and height as the explanatory variables has the smallest value of AIC (-398.56), hence it is the best model. Summary statistics of the best fitted model was shown in Table 4. Since the adjusted R 2 of the selected model is 0.2306, the two explanatory variables (sex and height) can only account for about 23% of the variation of the thickness of epiglottis of the subjects. Moreover, the coefficient estimates showed that, on average, male subjects had thicker epiglottis; and that the taller the subject, the thinner was the epiglottis. However, the p value of the height variable is larger than 0.05, therefore it is not a statistically significant explanatory variable in this model. Thus, the regression equation of the best fitting model is: Thickness of epiglottis (cm) = 0.3413 + 0.0268 Sex - 0.0718 Height (m) Table 3. Stepwise backward approach in choosing the best multiple linear regression model Model Explanatory variables included AIC 1 Age, Sex, Body weight, Height, and BMI -393.84 2 Age, Sex, Body weight, and Height -395.84 3 Age, Sex, and Height -397.81 4 Sex and Height -398.56 5 Sex -398.00 6 Height -398.53 AIC=Akaike Information Criteria; BMI=body mass index Table 4. Summary statistics of the best fitting model Term Coefficient estimate Standard error p value Intercept 0.3413 0.0728 <0.001 Sex Male = 1, Female = 0 0.0268 0.0079 0.001 Height - 0.0718 0.0457 0.12 Adjusted R 2 =0.2306

Chau et al./sonographic measurement of the epiglottis 433 Discussion Werner et al 3 did a sonographic assessment of the epiglottis in Cleveland which showed a mean epiglottic thickness of 0.239±0.015 cm with man's epiglottis thicker than woman s. In our study with the Chinese population, the epiglottic size was measured to be 0.236±0.020 cm and we also showed that male epiglottis was thicker than that of female. This result suggested that epiglottic thickness was quite similar in different ethnic groups. Further studies are needed to evaluate the size in case of epiglottitis. Ultrasound examination has a learning curve and is operator dependent. We are probably the first group to report on inter-observer agreement in measuring the thickness of the epiglottis. Our study showed that there was no significant difference of epiglottis mean thickness as measured by two observers. In this study, both emergency physicians had experience in ultrasound of the epiglottis. It is unsure whether this good agreement can be achieved by EPs with less experience in ultrasound. We believe that beginners could perform the examination comfortably after some guidance and practice as the examination of the epiglottis is rather easy. The standard view of the epiglottis can be found by simply placing the linear transducer probe at the level just above the thyroid cartilage. With a little tilting of the probe or asking the patient to extend his neck slightly, the standard view can usually be located. The standard view will mostly represent the base of the epiglottis where the thickness of the epiglottis would be measured by both EP1 and EP2. However, the higher median as well as the upper quartiles measured by EP1 could be due to scanning at slightly different planes of the epiglottis. Margins of the epiglottis were blurred in some volunteers and this might account for the differences in measurements by the two investigators. Despite possible variations in scanning planes, we could see that the results of the two observers actually agreed with each other by the Bland-Altman plot. The clinical relevance of measuring the epiglottis is of course in the diagnosis of acute epiglottitis. A case report on ultrasound measurement of epiglottis in acute epiglottitis showed the epiglottis thickness to be 5.1 mm, 6 which is nearly double the normal size as reported by Werner and our study. We have also performed ultrasound measurements in a few acute epiglottitis cases in which diagnosis was confirmed by an experienced ENT specialist with a fibreoptic nasopharyngolaryngoscopy. In two of the cases, the epiglottic thicknesses were 0.5 and 0.65 cm by ultrasound measurement (Figure 3). Thus, it seems that a thickness of the epiglottis of around 2 times the normal value should lead to suspicion of acute epiglottitis. Of course, further studies are needed to define the best cut-off value. Figure 3. Two ultrasound scans taken in two patients with confirmed diagnosis of acute epiglottitis. The thicknesses of the epiglottis were 0.65 cm and 0.50 cm which were nearly double of the normal value (0.236 cm).

434 Hong Kong j. emerg. med. Vol. 17(5) Nov 2010 If bedside ultrasound could accurately measure the thickness of the epiglottis, it could definitely help in making the diagnosis of acute epiglottitis especially in unstable patients. If a positive correlation of the epiglottis thickness and severity of acute epiglottitis is established, bedside ultrasound can also provide an easy and quick way for repeated assessment of the epiglottis status. Our study was limited by a relatively small sample size of 50. All our subjects were relatively young adults (24 to 51 years of age) and thus the mean thickness value might not be generalisable to older adults. The ultrasound technique adopted in this study only measured the thickness but not the entire length of the epiglottis. It was due to the impedance of the ultrasound wave by air in the pharynx. Our interobserver agreement was good between the two EPs who were experienced in emergency ultrasonography. Agreement between other less experienced operators could not be guaranteed. The other limitation would be the position of the subject during scanning. As the position of the epiglottis may be slightly different in the lying or sitting position, the results of the scanning may be slightly affected. However, this positional variation may be more significant at the tip of the epiglottis but not at the base of the epiglottis, where the epiglottis is "secured" by the soft tissue in the front. Since we are measuring the base of the epiglottis, the variation may be insignificant. 7 Conclusions Bedside ultrasound assessment of the epiglottis is an easy, rapid and reliable method to evaluate its thickness. Further studies are needed to evaluate the thickness of the epiglottis in patients with epiglottitis before it can be put into clinical use. References 1. Thompson P. Evolving role of ultrasound in the emergency department. Emerg Med Australas 2008;20 (5):375-8. 2. Lam PK, Choi YF, Wong TW, Lau CC. Adult acute epiglottitis: predictors for airway intervention and intensive care unit admission. Hong Kong J Emerg Med 2009;16(4):198-207. 3. Werner SL, Jones RA, Emerman CL. Sonographic assessment of the epiglottis. Acad Emerg Med 2004;11 (12):1358-60. 4. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1(8476):307-10. 5. Akaike H. A new look at the statistical model identification. IEEE Transactions on Automatic Control 1974;19(6):716-23. 6. Bektas F, Soyuncu S, Yigit O, Turhan M. Sonographic diagnosis of epiglottal enlargement. Emerg Med J 2010; 27(3):224-5. 7. Sutthiprapaporn P, Tanimoto K, Ohtsuka M, Nagasaki T, Iida Y, Katsumata A. Positional changes of oropharyngeal structures due to gravity in the upright and supine positions. Dentomaxillofac Radiol 2008; 37(3):130-5.