Kok Tong Lee, MD, MMed1,2
E-mail: ktong1984@gmail.com
Rhendra Hardy Mohamad Zaini, MD, MMed1,2
E-mail: rhendra@gmail.com
Wan Mohd Nazaruddin Wan Hassan, MD, MMed1,2
E-mail: nazarudin@usm.my
Chandran Nadarajan, MD, MMed2,3
E-mail: drchandran@usm.my
Yee Cheng Kueh, PhD4
E-mail: yckueh@usm.my
Lee Min Foo, MD, MMed5
E-mail: sunsun80@gmail.com
Mohamad Ibariyah Iberahim, MBBS1,2
E-mail: driba@usm.my
Soon Eu Chong, MD, MMed6
E-mail: sechong@usm.my
1Department of Anesthesiology and Intensive Care, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
2Hospital Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
3Department of Radiology, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
4Biostatistic Unit, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
5Hospital Raja Perempuan Zainab II, 15200 Kota Bharu, Kelantan, Malaysia.
6Regenerative Medicine Cluster, Advanced Medical and Dental Institute, Universiti Sains Malaysia, Bertam, 13200 Kepala Batas, Penang, Malaysia.
Correspondence:
Rhendra Hardy Mohamad Zaini
Institution: Department of Anesthesiology and Intensive Care, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, (Malaysia)
Phone: (60)19-9106665
E-mail: rhendra@gmail.com
ABSTRACT
Objective: This study was designed to evaluate the diagnostic performance of STOP-BANG score and mandibulohyoid distance (MHD) towards difficult laryngoscopy among obstructive sleep apnea patients.
Cross-Sectional Study
Methodology: Forty-one patients who had STOP-BANG scores of >3 and required tracheal intubation for general anesthesia were recruited in this cross-sectional study. MHD was measured through lateral cephalometry. After induction of anesthesia, an anesthesiologist who was blinded to patient’s profile performed a laryngoscopy and evaluated the Cormack-Lehane grading. Sensitivity, specificity, positive and negative predictive values were determined for STOP-BANG and MHD. Binary logistic regression, receiver operating characteristics (ROC) and correlation analyses were employed.
Results: Body mass index, neck circumference, and Mallampati scores were higher in the difficult laryngoscopy group. MHD was longer in difficult laryngoscopy group (25.40 ± 5.67 mm) than easy laryngoscopy group (20.17 ± 4.28 mm; p = 0.002). STOP-BANG score was higher in difficult laryngoscopy group (5.86 ± 0.96 vs. 4.30 ± 0.98; p < 0.001). The combination of MHD and STOP-BANG score improved the quality of diagnostic test in predicting the laryngoscopy status with area under ROC of 87.6%, compared to each isolated parameter (i.e., MHD = 75.7%, STOP-BANG = 85.5%). Both MHD and STOP-BANG scores were significantly positive correlated (r = 0.42, p = 0.006).
Conclusions: STOP-BANG score and MHD were useful in predicting difficult intubation. The diagnostic performance improved further when combining both parameters.
Key words: Airway assessment; Difficult airway; STOP-BANG; Lateral cephalometry; Mandibulohyoid distance; Obstructive sleep apnea
Citation: Lee KT, Zaini RHM, Wan Hassan WMN, Nadarajan C, Kueh YC, Foo LM, Iberahim MI, Chong SE. Combination of STOP-BANG score and mandibulohyoid distance in the prediction of difficult airway among obstructive sleep apnea patients. Anaesth. pain & intensive care 2019;23(2):204-210
Received – 13 October 2018, Reviewed – 22 February 2019, Accepted – 26 March 2019
INTRODUCTION
Obstructive Sleep Apnea (OSA) is a disorder in which breathing tend to be obstructed briefly during sleep and repeatedly “stop temporarily”. It has a prevalence of 9% to 38% in the general population.1 For anesthetist, the significance of OSA is the association with perioperative respiratory adverse events. One of the major concerns is the risk of difficult intubation.2 Studies has shown that OSA patients are associated with higher incidence of difficult intubation.3,4 Unanticipated difficult intubation was most frequently encountered primary airway problem, accounting for 39% of all complications of airway events5 and 11% of anesthesia related situational awareness errors contributing to mortality and morbidity.6
The identification of OSA during preoperative assessment may help to prevent adverse events.7 Singh et al8 reported that 69% of the surgical patients had OSA, and up to 60% of the moderate to severe OSA were undiagnosed preoperatively. Thus, the American Society of Anesthesiologist (ASA) has recommended a routine screening of OSA preoperatively.
There are several screening tools for OSA. The STOP-BANG questionnaire is the simplest to use and has a proven validity.9 A score of ≥3 predicts OSA,10,11 while a higher score may be associated with increased risk of difficult airway.7,10,12 To date, there is no single airway test that can reliably predict a difficult airway,13 comparing to a combination of multiple tests.14,15 Radiological methods of using mandibulohyoid distance (MHD) (Figure 1) may increase the sensitivity and specificity in this matter.16-18 This study was designed to evaluate the diagnostic performance of STOP-BANG questionnaire, MHD and the combination of both parameters in the evaluation of difficult laryngoscopy.
Figure 1: Lateral cephalometry. GO, Gonion; H, Hyoid bone; ME, menton; MHD, mandibular plane-hyoid bone distance;
METHODOLOGY
Prior to this prospective, cross-sectional trial, ethical approval was obtained from Human Research and Ethical Committee of Universiti Sains Malaysia (Reference: USM/JEPeM/ 15010010) It was registered and conducted from May 2015 to February 2016 in Hospital Universiti Sains Malaysia (ClinicalTrials.gov identifier: NCT03105388). Forty-one patients with ASA class I or II, aged between 18 to 75 years old, who were posted for surgery requiring tracheal intubation under general anesthesia, and fulfilled the STOP-BANG score of > 3 were recruited. A STOP-BANG score of ≥ 3 was chosen because it has a very high sensitivity and high negative predictive value for moderate to severe OSA. Patients who were pregnant, fasted for less than six hours, or those with gastroesophageal reflux disease were excluded. Written consent was obtained from each patient.
Data recorded include age, gender, ASA classification of physical status, height, weight, thyromental distance, neck circumference, and modified Mallampati scores (Samsoon and Young)19 were recorded. Lateral cephalometry was taken in a neutral head position. Mandibular-hyoid distance was measured.
Standard monitoring was applied before induction of anesthesia i.e. electrocardiography, non-invasive blood pressure monitoring and pulse oximeter. Anesthesia was performed by a skilled anesthesiologist who was blinded towards the cephalometric measurements as well as the STOP-BANG scores of the patient. After preoxygenation, intravenous (IV) fentanyl 1.5-2 µg/kg and IV propofol 1-2 mg/kg, were administrated in titration. After loss of consciousness, 0.9 mg/kg IV rocuronium was given. When the state of paralysis was achieved guided by train-of-four (TOF) monitoring, patient’s head was placed in sniffing position to facilitate intubation. Without pressing the thyroid cartilage, the airway was evaluated and graded following Cormack-Lehane grading system using direct laryngoscopy. The patient was subsequently intubated with appropriate-sized endotracheal tube using a video laryngoscope.
After intubation, mechanical ventilation was conducted with volume control ventilation, at tidal volume of 8 ml/kg ideal body weight, rate of 12 breaths per minute. Anesthesia was maintained with inhalational agent Sevoflurane with the minimum alveolar concentration (MAC) of 1.0. After operation, all patients were reversed using Sugammadex.
Airway management Parameters
Cormack-Lehane (CL) grading system was used as the standard outcome, comparing with MHD and STOP-BANG score. Patients with a CL grade I and II were grouped into Group A (easy laryngoscopy), while patients with CL grade III and IV were grouped into Group B (difficult laryngoscopy).
Statistical Analysis: The STATA 14 was used for data entry and analysis. All numerical variables were described using mean ± SDs and categorical variables were described using frequency and percentages. Independent sample t-test was used to compare the MHD and STOP-BANG score between Group A and B. MHD and STOP-BANG were recoded into two categories based on recommended cut-off point of 20 mm16 and 5 mm20 respectively. The association between laryngoscopy status with categorized MHD (>20 mm) and STOP-BANG (≥5) were tested using Pearson Chi-square test. The sensitivity, specificity, positive and negative predictive values for categorized MHD and STOP-BANG in predicting easy and difficult laryngoscopy was determined.
A comprehensive evaluation was done by using binary logistic regression analysis to examine the predictability of MHD and STOP-BANG on laryngoscopy status. The optimal cutoff point for sensitivity and specificity of MHD and STOP-BANG based on present data were determined through receiver operating characteristic (ROC) analysis. A list of cutoff point for diagnostic tools, sensitivity, specificity, negative and positive predictive value and accuracy were described in a table. Pearson correlation analysis was used to determine the correlation between MHD and STOP-BANG score.
RESULTS
A total of 41 patients with mean age of 49.59 ± 13.32 years old participated in the study. There were no statistically differences between two groups with respect to their age and gender (Table 1). The mean Mallampati score was 2.24 ± 0.54, the mean of neck circumference was 44.37 ± 4.71 cm.
For MHD, the mean ± SD was 22.85 ± 5.64 mm. Difficult laryngoscopy were observed in 21 (51.2%) of the 41 patients. Based on CL grading system, 31.7% of the participants were in grade I, 17.1% were grade II, 39.0% were in grade III and 12.2% were in grade IV.
MHD was found to be larger in difficult laryngoscopy group (25.40 ± 5.67 mm) as compared to easy laryngoscopy group (20.17 ± 4.28 mm; p-value = 0.002) (Table 2). STOP-BANG score was higher in difficult laryngoscopy group (5.86 ± 0.96) compared to easy laryngoscopy group (4.30 ± 0.98; p < 0.001). The categorized mandibular distance and STOP-BANG were also significantly associated with difficulty in laryngoscopy.
By using MHD alone (MHD > 20 mm), 23 out of 41 participants (56.1%) were predicted to be difficult. The sensitivity of this parameter was 76.2%, positive predictive value was 69.6%, negative predictive value was 72.2% and the accuracy was 70.7%. By using STOP-BANG score model alone (STOP-BANG > 5), 29 (70.7%) participants were predicted to be difficult and 12 (29.3%) participants were deemed easy. Based on this score, the sensitivity for predicting difficult laryngoscopy was 95.2%, positive predictive value was 69.0%, negative predictive value was 91.7%, and the accuracy was 75.6%.
Binary logistic regression showed that MHD was a significant predictor of laryngoscopy status (OR = 1.23, p = 0.006). Those with one unit (in mm) higher in MHD had 1.23 higher odd of having difficult laryngoscopy. The receiver operating characteristic (ROC) curve and its corresponding area under the curve of MHD was 75.7%, indicated that the diagnostic performance was characteristically good (Figure 2). The detailed report of sensitivity and specificity based on several cut-off points for MHD in predicting laryngoscopy status was listed in Table 3. Based on the present data in binary regression model, the optimal cut-off point for MHD was 20 mm.
Figure 2: Receiver operating characteristic curve of STOP-BANG score and mandibulohyoid distance. (n = 41)
From the binary logistic regression, STOP-BANG score was a significant predictor of laryngoscopy status (OR = 5.40, p = 0.002). Those who score one unit higher in STOP-BANG score had 5.40 higher odd of having difficult laryngoscopy. The ROC curve and its corresponding area under the curve of STOP-BANG was 85.5%, indicating a good diagnostic performance (Figure 2). Based on the present data in binary regression model, the optimal cut-off point for STOP-BANG was five (Table 3)
When combining STOP-BANG score and MHD in the binary logistic regression model, the corresponding area under the ROC increased marginally to 87.6% (Figure 2). This indicated that the quality of diagnostic test improved when combining STOP-BANG score and MHD in predicting the outcome. Besides, correlation between STOP-BANG score and MHD was statistically significant (r = 0.421 with p = 0.006), indicating that the correlation was positive.
DISCUSSION
The aim of the study was to assess the predictive model for difficult intubation in patient at risk of OSA using STOP-BANG and MHD. Many assessment criteria had been incorporated, but do not consistently produce accurate evaluation of the risk of failed intubation.21
The major finding of this study was that a combined STOP-BANG score and MHD yield a better prediction of difficult airway. It was shown that the specificity and sensitivity of each test was not perfect, but when the tests were used in combination, the specificity and sensitivity increased.
Mallampati classification which has used widely for difficult airway assessment has been reported to be of limited value by some researchers.22-24 A major pitfall in achieving a reliable score for Mallampati classification is the failure to ensure that the patient opens the mouth and protrudes the tongue maximally. This study found that modified Mallampati score has significant value in predicting difficult laryngoscopy, consistent with findings by Mahmoodpoor et al. and Patel et al. 25,26 However, modified Mallampati score was inadequate as a standalone test to reliably predict difficult laryngoscopy or tracheal intubation.27,28 The same goes to neck circumference which has been shown to have a strong correlation with STOP-BANG score.29 The greater the neck circumference, the higher the score by Cormack Lehane classification.30 This finding was comparable to our result.
The STOP-BANG score appears to promise a good diagnostic performance in predicting difficult laryngoscopy. In this study, it showed high sensitivity, high accuracy and a reasonable specificity. STOP-BANG questionnaire is easy to use and very cost effective. The severity of OSA increases linearly as the score increases from 3 to 8.20 Findings of this study were consistent with this. A STOP-BANG score of >3 has been suggested by Toshniwal et al.10 to be associated with difficult airway. However, the authors found that using a cut-off point of >5 might yields the highest accuracy, and a relatively more balanced sensitivity, specificity, positive- and negative predictive values (Table 3).
Cephalometry has provided us substantial insight into the pathophysiology of OSA, identifying the most significant craniofacial characteristics associated with this disease. The increased in MHD was found to be significantly associated with large neck circumference and receding chin. Studies have shown that an increased in MHD in OSA patient is associated with an increased risk of difficult laryngoscopy.16 A combination of STOP-BANG score and MHD had led to a marginally improvement in the area under ROC of predicting the outcome by 87.6%.
The measurement of MHD can be done electronically with radiography, and does not depend on the skill of assessor, thus preventing inter assessor variability. Results were accurate and could be easily retrieved for re-examination. In addition, a single cephalometry has the benefits of revealing other radiographic parameters such as submandibular angle31 and facial angle.2
The MHD technique may be limited by the concern of additional ionizing radiation from lateral cephalometry. However, according to the Biological Effects of Ionizing Radiation Committee VII (BEIR VII) reports, the risk of developing a fatal cancer from a single exposure from lateral cephalometry is very minimal. Clearly the benefits of a single exposure of cephalometry with the aim of assessing ease of laryngoscopy outweighed the risk, given the risk of death from difficult airway management was as high as 40%.
There are a few limitations in this study. First, the CL grading system which was used as the outcome of this study, may not be as accurate as other classification system such as ASA Intubation Difficult score or Cook’s grading system. The grading system may also depend on operator experience, patient characteristics and clinical settings. Besides, this study may also miss out the association of limited head and neck mobility and difficult laryngoscopy, which may be significant in OSA population.32 This aspect is not able to be detected using both STOP-BANG score and MHD. Besides, the risk and incidence of difficult mask ventilation among OSA patients which is also an important issue in difficult airway, was not assessed in this study. Patient who had difficult mask ventilation after muscle relaxation had been associated with difficult intubation.10
Another limitation was the exclusion of obstetric patients, due to the risk of radiation exposure during lateral cephalometry. Similarly, the study did not include extreme age groups, which are known to have higher incidence of difficult airway. Nevertheless, sleep studies were not performed for all the participants prior to laryngoscopy due to long waiting list for this test in our institution, and the concern of cost-effectiveness.
CONCLUSION
The incidence of difficult laryngoscopy in OSA patients is not negligible. The combined STOP-BANG score and mandibulohyoid distance can improved the overall accuracy of the model to predict difficult laryngoscopy. This technique provides relatively easy, reproducible and objective way of assessment among OSA patients.
Conflict of interest: None declared.
Authors’ contribution:
KTL + CN: Literature review, collected data and drafted the manuscript
RHMZ: Designed the study, discussion, edited and finalizing the manuscript
WMNWH: Concept, analyzing data and edited the manuscript
YCK: Analyzing data, discussion and edited the manuscript
LMF + MII: Collected data, drafted and proofread the manuscript
SEC: Literature review, interpretation of data, discussion and finalizing the manuscript
REFERENCES
Table 1: Comparison between easy laryngoscopy and difficult laryngoscopy among participant’s age, bmi, neck circumference, modified mallampati score, weight, height.
Note: * Chi-square (df)
Table 2: Comparison between easy laryngoscopy and difficult laryngoscopy among STOP–BANG score and mandibulohyoid distance
Note. *Chi-square test.
MHD, Mandibulohyoid distance
Table 3: Detailed report of diagnostics parameters for mandibulohyoid distance and STOP-BANG in predicting laryngoscopy status
PPV = positive predictive value, NPV = negative predictive value
E-mail: ktong1984@gmail.com
Rhendra Hardy Mohamad Zaini, MD, MMed1,2
E-mail: rhendra@gmail.com
Wan Mohd Nazaruddin Wan Hassan, MD, MMed1,2
E-mail: nazarudin@usm.my
Chandran Nadarajan, MD, MMed2,3
E-mail: drchandran@usm.my
Yee Cheng Kueh, PhD4
E-mail: yckueh@usm.my
Lee Min Foo, MD, MMed5
E-mail: sunsun80@gmail.com
Mohamad Ibariyah Iberahim, MBBS1,2
E-mail: driba@usm.my
Soon Eu Chong, MD, MMed6
E-mail: sechong@usm.my
1Department of Anesthesiology and Intensive Care, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
2Hospital Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
3Department of Radiology, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
4Biostatistic Unit, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, Malaysia.
5Hospital Raja Perempuan Zainab II, 15200 Kota Bharu, Kelantan, Malaysia.
6Regenerative Medicine Cluster, Advanced Medical and Dental Institute, Universiti Sains Malaysia, Bertam, 13200 Kepala Batas, Penang, Malaysia.
Correspondence:
Rhendra Hardy Mohamad Zaini
Institution: Department of Anesthesiology and Intensive Care, School of Medical Sciences, Universiti Sains Malaysia, 16150 Kota Bharu, Kelantan, (Malaysia)
Phone: (60)19-9106665
E-mail: rhendra@gmail.com
ABSTRACT
Objective: This study was designed to evaluate the diagnostic performance of STOP-BANG score and mandibulohyoid distance (MHD) towards difficult laryngoscopy among obstructive sleep apnea patients.
Cross-Sectional Study
Methodology: Forty-one patients who had STOP-BANG scores of >3 and required tracheal intubation for general anesthesia were recruited in this cross-sectional study. MHD was measured through lateral cephalometry. After induction of anesthesia, an anesthesiologist who was blinded to patient’s profile performed a laryngoscopy and evaluated the Cormack-Lehane grading. Sensitivity, specificity, positive and negative predictive values were determined for STOP-BANG and MHD. Binary logistic regression, receiver operating characteristics (ROC) and correlation analyses were employed.
Results: Body mass index, neck circumference, and Mallampati scores were higher in the difficult laryngoscopy group. MHD was longer in difficult laryngoscopy group (25.40 ± 5.67 mm) than easy laryngoscopy group (20.17 ± 4.28 mm; p = 0.002). STOP-BANG score was higher in difficult laryngoscopy group (5.86 ± 0.96 vs. 4.30 ± 0.98; p < 0.001). The combination of MHD and STOP-BANG score improved the quality of diagnostic test in predicting the laryngoscopy status with area under ROC of 87.6%, compared to each isolated parameter (i.e., MHD = 75.7%, STOP-BANG = 85.5%). Both MHD and STOP-BANG scores were significantly positive correlated (r = 0.42, p = 0.006).
Conclusions: STOP-BANG score and MHD were useful in predicting difficult intubation. The diagnostic performance improved further when combining both parameters.
Key words: Airway assessment; Difficult airway; STOP-BANG; Lateral cephalometry; Mandibulohyoid distance; Obstructive sleep apnea
Citation: Lee KT, Zaini RHM, Wan Hassan WMN, Nadarajan C, Kueh YC, Foo LM, Iberahim MI, Chong SE. Combination of STOP-BANG score and mandibulohyoid distance in the prediction of difficult airway among obstructive sleep apnea patients. Anaesth. pain & intensive care 2019;23(2):204-210
Received – 13 October 2018, Reviewed – 22 February 2019, Accepted – 26 March 2019
INTRODUCTION
Obstructive Sleep Apnea (OSA) is a disorder in which breathing tend to be obstructed briefly during sleep and repeatedly “stop temporarily”. It has a prevalence of 9% to 38% in the general population.1 For anesthetist, the significance of OSA is the association with perioperative respiratory adverse events. One of the major concerns is the risk of difficult intubation.2 Studies has shown that OSA patients are associated with higher incidence of difficult intubation.3,4 Unanticipated difficult intubation was most frequently encountered primary airway problem, accounting for 39% of all complications of airway events5 and 11% of anesthesia related situational awareness errors contributing to mortality and morbidity.6
The identification of OSA during preoperative assessment may help to prevent adverse events.7 Singh et al8 reported that 69% of the surgical patients had OSA, and up to 60% of the moderate to severe OSA were undiagnosed preoperatively. Thus, the American Society of Anesthesiologist (ASA) has recommended a routine screening of OSA preoperatively.
There are several screening tools for OSA. The STOP-BANG questionnaire is the simplest to use and has a proven validity.9 A score of ≥3 predicts OSA,10,11 while a higher score may be associated with increased risk of difficult airway.7,10,12 To date, there is no single airway test that can reliably predict a difficult airway,13 comparing to a combination of multiple tests.14,15 Radiological methods of using mandibulohyoid distance (MHD) (Figure 1) may increase the sensitivity and specificity in this matter.16-18 This study was designed to evaluate the diagnostic performance of STOP-BANG questionnaire, MHD and the combination of both parameters in the evaluation of difficult laryngoscopy.
Figure 1: Lateral cephalometry. GO, Gonion; H, Hyoid bone; ME, menton; MHD, mandibular plane-hyoid bone distance;
METHODOLOGY
Prior to this prospective, cross-sectional trial, ethical approval was obtained from Human Research and Ethical Committee of Universiti Sains Malaysia (Reference: USM/JEPeM/ 15010010) It was registered and conducted from May 2015 to February 2016 in Hospital Universiti Sains Malaysia (ClinicalTrials.gov identifier: NCT03105388). Forty-one patients with ASA class I or II, aged between 18 to 75 years old, who were posted for surgery requiring tracheal intubation under general anesthesia, and fulfilled the STOP-BANG score of > 3 were recruited. A STOP-BANG score of ≥ 3 was chosen because it has a very high sensitivity and high negative predictive value for moderate to severe OSA. Patients who were pregnant, fasted for less than six hours, or those with gastroesophageal reflux disease were excluded. Written consent was obtained from each patient.
Data recorded include age, gender, ASA classification of physical status, height, weight, thyromental distance, neck circumference, and modified Mallampati scores (Samsoon and Young)19 were recorded. Lateral cephalometry was taken in a neutral head position. Mandibular-hyoid distance was measured.
Standard monitoring was applied before induction of anesthesia i.e. electrocardiography, non-invasive blood pressure monitoring and pulse oximeter. Anesthesia was performed by a skilled anesthesiologist who was blinded towards the cephalometric measurements as well as the STOP-BANG scores of the patient. After preoxygenation, intravenous (IV) fentanyl 1.5-2 µg/kg and IV propofol 1-2 mg/kg, were administrated in titration. After loss of consciousness, 0.9 mg/kg IV rocuronium was given. When the state of paralysis was achieved guided by train-of-four (TOF) monitoring, patient’s head was placed in sniffing position to facilitate intubation. Without pressing the thyroid cartilage, the airway was evaluated and graded following Cormack-Lehane grading system using direct laryngoscopy. The patient was subsequently intubated with appropriate-sized endotracheal tube using a video laryngoscope.
After intubation, mechanical ventilation was conducted with volume control ventilation, at tidal volume of 8 ml/kg ideal body weight, rate of 12 breaths per minute. Anesthesia was maintained with inhalational agent Sevoflurane with the minimum alveolar concentration (MAC) of 1.0. After operation, all patients were reversed using Sugammadex.
Airway management Parameters
Cormack-Lehane (CL) grading system was used as the standard outcome, comparing with MHD and STOP-BANG score. Patients with a CL grade I and II were grouped into Group A (easy laryngoscopy), while patients with CL grade III and IV were grouped into Group B (difficult laryngoscopy).
Statistical Analysis: The STATA 14 was used for data entry and analysis. All numerical variables were described using mean ± SDs and categorical variables were described using frequency and percentages. Independent sample t-test was used to compare the MHD and STOP-BANG score between Group A and B. MHD and STOP-BANG were recoded into two categories based on recommended cut-off point of 20 mm16 and 5 mm20 respectively. The association between laryngoscopy status with categorized MHD (>20 mm) and STOP-BANG (≥5) were tested using Pearson Chi-square test. The sensitivity, specificity, positive and negative predictive values for categorized MHD and STOP-BANG in predicting easy and difficult laryngoscopy was determined.
A comprehensive evaluation was done by using binary logistic regression analysis to examine the predictability of MHD and STOP-BANG on laryngoscopy status. The optimal cutoff point for sensitivity and specificity of MHD and STOP-BANG based on present data were determined through receiver operating characteristic (ROC) analysis. A list of cutoff point for diagnostic tools, sensitivity, specificity, negative and positive predictive value and accuracy were described in a table. Pearson correlation analysis was used to determine the correlation between MHD and STOP-BANG score.
RESULTS
A total of 41 patients with mean age of 49.59 ± 13.32 years old participated in the study. There were no statistically differences between two groups with respect to their age and gender (Table 1). The mean Mallampati score was 2.24 ± 0.54, the mean of neck circumference was 44.37 ± 4.71 cm.
For MHD, the mean ± SD was 22.85 ± 5.64 mm. Difficult laryngoscopy were observed in 21 (51.2%) of the 41 patients. Based on CL grading system, 31.7% of the participants were in grade I, 17.1% were grade II, 39.0% were in grade III and 12.2% were in grade IV.
MHD was found to be larger in difficult laryngoscopy group (25.40 ± 5.67 mm) as compared to easy laryngoscopy group (20.17 ± 4.28 mm; p-value = 0.002) (Table 2). STOP-BANG score was higher in difficult laryngoscopy group (5.86 ± 0.96) compared to easy laryngoscopy group (4.30 ± 0.98; p < 0.001). The categorized mandibular distance and STOP-BANG were also significantly associated with difficulty in laryngoscopy.
By using MHD alone (MHD > 20 mm), 23 out of 41 participants (56.1%) were predicted to be difficult. The sensitivity of this parameter was 76.2%, positive predictive value was 69.6%, negative predictive value was 72.2% and the accuracy was 70.7%. By using STOP-BANG score model alone (STOP-BANG > 5), 29 (70.7%) participants were predicted to be difficult and 12 (29.3%) participants were deemed easy. Based on this score, the sensitivity for predicting difficult laryngoscopy was 95.2%, positive predictive value was 69.0%, negative predictive value was 91.7%, and the accuracy was 75.6%.
Binary logistic regression showed that MHD was a significant predictor of laryngoscopy status (OR = 1.23, p = 0.006). Those with one unit (in mm) higher in MHD had 1.23 higher odd of having difficult laryngoscopy. The receiver operating characteristic (ROC) curve and its corresponding area under the curve of MHD was 75.7%, indicated that the diagnostic performance was characteristically good (Figure 2). The detailed report of sensitivity and specificity based on several cut-off points for MHD in predicting laryngoscopy status was listed in Table 3. Based on the present data in binary regression model, the optimal cut-off point for MHD was 20 mm.
Figure 2: Receiver operating characteristic curve of STOP-BANG score and mandibulohyoid distance. (n = 41)
From the binary logistic regression, STOP-BANG score was a significant predictor of laryngoscopy status (OR = 5.40, p = 0.002). Those who score one unit higher in STOP-BANG score had 5.40 higher odd of having difficult laryngoscopy. The ROC curve and its corresponding area under the curve of STOP-BANG was 85.5%, indicating a good diagnostic performance (Figure 2). Based on the present data in binary regression model, the optimal cut-off point for STOP-BANG was five (Table 3)
When combining STOP-BANG score and MHD in the binary logistic regression model, the corresponding area under the ROC increased marginally to 87.6% (Figure 2). This indicated that the quality of diagnostic test improved when combining STOP-BANG score and MHD in predicting the outcome. Besides, correlation between STOP-BANG score and MHD was statistically significant (r = 0.421 with p = 0.006), indicating that the correlation was positive.
DISCUSSION
The aim of the study was to assess the predictive model for difficult intubation in patient at risk of OSA using STOP-BANG and MHD. Many assessment criteria had been incorporated, but do not consistently produce accurate evaluation of the risk of failed intubation.21
The major finding of this study was that a combined STOP-BANG score and MHD yield a better prediction of difficult airway. It was shown that the specificity and sensitivity of each test was not perfect, but when the tests were used in combination, the specificity and sensitivity increased.
Mallampati classification which has used widely for difficult airway assessment has been reported to be of limited value by some researchers.22-24 A major pitfall in achieving a reliable score for Mallampati classification is the failure to ensure that the patient opens the mouth and protrudes the tongue maximally. This study found that modified Mallampati score has significant value in predicting difficult laryngoscopy, consistent with findings by Mahmoodpoor et al. and Patel et al. 25,26 However, modified Mallampati score was inadequate as a standalone test to reliably predict difficult laryngoscopy or tracheal intubation.27,28 The same goes to neck circumference which has been shown to have a strong correlation with STOP-BANG score.29 The greater the neck circumference, the higher the score by Cormack Lehane classification.30 This finding was comparable to our result.
The STOP-BANG score appears to promise a good diagnostic performance in predicting difficult laryngoscopy. In this study, it showed high sensitivity, high accuracy and a reasonable specificity. STOP-BANG questionnaire is easy to use and very cost effective. The severity of OSA increases linearly as the score increases from 3 to 8.20 Findings of this study were consistent with this. A STOP-BANG score of >3 has been suggested by Toshniwal et al.10 to be associated with difficult airway. However, the authors found that using a cut-off point of >5 might yields the highest accuracy, and a relatively more balanced sensitivity, specificity, positive- and negative predictive values (Table 3).
Cephalometry has provided us substantial insight into the pathophysiology of OSA, identifying the most significant craniofacial characteristics associated with this disease. The increased in MHD was found to be significantly associated with large neck circumference and receding chin. Studies have shown that an increased in MHD in OSA patient is associated with an increased risk of difficult laryngoscopy.16 A combination of STOP-BANG score and MHD had led to a marginally improvement in the area under ROC of predicting the outcome by 87.6%.
The measurement of MHD can be done electronically with radiography, and does not depend on the skill of assessor, thus preventing inter assessor variability. Results were accurate and could be easily retrieved for re-examination. In addition, a single cephalometry has the benefits of revealing other radiographic parameters such as submandibular angle31 and facial angle.2
The MHD technique may be limited by the concern of additional ionizing radiation from lateral cephalometry. However, according to the Biological Effects of Ionizing Radiation Committee VII (BEIR VII) reports, the risk of developing a fatal cancer from a single exposure from lateral cephalometry is very minimal. Clearly the benefits of a single exposure of cephalometry with the aim of assessing ease of laryngoscopy outweighed the risk, given the risk of death from difficult airway management was as high as 40%.
There are a few limitations in this study. First, the CL grading system which was used as the outcome of this study, may not be as accurate as other classification system such as ASA Intubation Difficult score or Cook’s grading system. The grading system may also depend on operator experience, patient characteristics and clinical settings. Besides, this study may also miss out the association of limited head and neck mobility and difficult laryngoscopy, which may be significant in OSA population.32 This aspect is not able to be detected using both STOP-BANG score and MHD. Besides, the risk and incidence of difficult mask ventilation among OSA patients which is also an important issue in difficult airway, was not assessed in this study. Patient who had difficult mask ventilation after muscle relaxation had been associated with difficult intubation.10
Another limitation was the exclusion of obstetric patients, due to the risk of radiation exposure during lateral cephalometry. Similarly, the study did not include extreme age groups, which are known to have higher incidence of difficult airway. Nevertheless, sleep studies were not performed for all the participants prior to laryngoscopy due to long waiting list for this test in our institution, and the concern of cost-effectiveness.
CONCLUSION
The incidence of difficult laryngoscopy in OSA patients is not negligible. The combined STOP-BANG score and mandibulohyoid distance can improved the overall accuracy of the model to predict difficult laryngoscopy. This technique provides relatively easy, reproducible and objective way of assessment among OSA patients.
Conflict of interest: None declared.
Authors’ contribution:
KTL + CN: Literature review, collected data and drafted the manuscript
RHMZ: Designed the study, discussion, edited and finalizing the manuscript
WMNWH: Concept, analyzing data and edited the manuscript
YCK: Analyzing data, discussion and edited the manuscript
LMF + MII: Collected data, drafted and proofread the manuscript
SEC: Literature review, interpretation of data, discussion and finalizing the manuscript
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Table 1: Comparison between easy laryngoscopy and difficult laryngoscopy among participant’s age, bmi, neck circumference, modified mallampati score, weight, height.
| Variables | Easy laryngoscopy | Difficult laryngoscopy | t-value ( df = 39)
|
p-value |
| Mean (SD) | Mean (SD) | |||
| Age years) | 51.25 (12.48) | 48.00 (14.20) | 0.777 | 0.442 |
| Gender: | ||||
| Female | 11 (55.0%) | 11 (52.4%) | 0.028 (1)* | 0.867 |
| Male | 9 (45%) | 10 (47.6%) | ||
| BMI (kg/m2) | 29.36 (5.15) | 36.54 (5.10) | -4.481 | < 0.001 |
| Neck circumference (cm) | 41.60 (1.70) | 47.00 (5.16) | -4.456 | < 0.001 |
| Mallampati | 2.00 ( < 0.01) | 2.48 (0.68) | -3.132 | 0.003 |
| Weight (kg) | 74.59 (12.99) | 92.31 (14.44) | -4.125 | < 0.001 |
| Height (cm) | 159.50 (8.22) | 158.71 (7.80) | 0.314 | 0.755 |
Table 2: Comparison between easy laryngoscopy and difficult laryngoscopy among STOP–BANG score and mandibulohyoid distance
| Variables | Easy Laryngoscopy | Difficult laryngoscopy | t-value | df | p value | ||
| Mean (SD) | n (%) | Mean (SD) | n (%) | ||||
| MHD (mm) ≤ 20 mm > 20 mm |
20.17 (4.28) | 13 (65.0) 7 (35.0) |
25.40 (5.67) | 5 (23.8) 16 (76.2) |
-3.322 7.057* |
39 1* |
0.002 0.008* |
| STOPBANG score < 5 ≥ 5 |
4.30 (0.98) | 11 (55.0) 9 (45.0) |
5.86 (0.96) | 1 (4.8) 20 (95.2) |
-5.133 12.489* |
39 1* |
< 0.001 < 0.001* |
MHD, Mandibulohyoid distance
Table 3: Detailed report of diagnostics parameters for mandibulohyoid distance and STOP-BANG in predicting laryngoscopy status
| Cutoff point for diagnostic tools | Sensitivity / Specificity in % | PPV / NPV in % | Accuracy |
| Mandibulohyoid distance: | |||
| >18mm | 85.7/ 35.0 | 58.1/ 70.0 | 61.0 |
| >20mm | 76.2/ 65.0 | 69.6/ 72.2 | 70.7 |
| >22mm | 66.7/ 70.0 | 70.0/ 66.7 | 68.3 |
| >24mm | 61.9/ 85.0 | 81.3/ 68.0 | 73.2 |
| STOP-BANG: | |||
| ≥ 4 | 100.0/ 25.0 | 58.3/ 100.0 | 63.4 |
| ≥ 5 | 95.2/ 55.0 | 69.0/ 91.7 | 75.6 |
| ≥ 6 | 57.1/ 90.0 | 85.7/ 66.7 | 73.2 |
| ≥ 7 | 33.3/ 100.0 | 100.0/ 58.8 | 65.8 |