Difficult visualization of the larynx (DVL) is a major cause of difficult intubation in many patients.1 Therefore, preoperative identification of those patients at risk for difficult laryngoscopy is important in adopting safer alternative strategies for the induction of anesthesia and intubation. However, whether true prediction is possible and which variables should be used for evaluation remain subjects of debate.
The hyomental distance (HMD) has been used to estimate the mandibular space, but the HMD alone was shown to have only a modest degree of diagnostic accuracy.2 Recently, Takenaka et al.3 defined the ratio of the HMD in the neutral position and at the extreme of head extension as the hyomental distance ratio (HMDR) and demonstrated that it was a good predictor of a reduced occipitoatlantoaxial (OAA) complex extension capacity in patients with rheumatoid arthritis. In a cinefluoroscopic study,4 the OAA extension angle required to expose the glottis during direct laryngoscopy was found to be at least 12°. On the other hand, Hastings and Wood5 measured the OAA extension angles with an external angle finder affixed to the head of normal subjects and demonstrated its value as 23°. In the study of Urakami et al.,6 the OAA extension capacity could not exceed 23° in approximately two thirds of the 20 normal subjects. Therefore, we believe that there are some cases in which the angle for an optimal laryngoscopic view cannot exceed the extension capacity, even in apparently normal patients. In such cases, optimal visualization of the glottis would require maximal head extension during laryngoscopic intubation. Thus, an assessment of the extension capacity of the OAA complex is an important component of preoperative tests for predicting DVL. However, no study has quantified its diagnostic validity for predicting DVL. The purpose of the present study was to evaluate the usefulness of the HMDR for accurately predicting DVL in apparently normal patients, by examining the following preoperative airway predictors, alone and in combination: the modified Mallampati test, HMD in the neutral position, HMD and thyromental distance (TMD) at the extreme of head extension, and HMDR.
The study protocol was approved by the hospital ethics committee, and written informed consent was obtained from all of the participating patients. We studied 213 consecutive adult patients scheduled to undergo general anesthesia requiring tracheal intubation for elective surgery. Exclusion criteria included a gross anatomical abnormality, recent surgery of the head and neck, upper airway disease (e.g., maxillofacial fracture or tumors), loose teeth, or those requiring a rapid sequence or awake intubation.
All testing was performed by a single investigator who was well trained in our planned test but not involved in intubating the trachea. As one of the comparative airway evaluation tests, we initially performed the modified Mallampati test.7 Subsequently, we kept the patients in the supine position, with the head on a firm operating table. The patients were instructed to look straight ahead, keep the head in the neutral position, close the mouth and not swallow. A hard-plastic bond ruler was pressed on the skin surface just above the hyoid bone, and the distance from the tip to the anterior-most part of the mentum was measured and defined as the HMD in the neutral position (Fig. 1). The patients were then instructed to extend the head maximally, taking care that the shoulders were not lifted while extending the head. The HMD was measured again in this position, and this variable was defined as the HMD at the extreme of head extension. Using the same method in this position, the straight distance from the anterior-most part of the mentum to the thyroid notch was measured and defined as the TMD at the extreme of head extension (Fig. 1). The HMDR was calculated as the ratio of the HMD at the extreme of head extension to that in the neutral position.
After all of the airway evaluations were completed, standard monitors were applied, and anesthesia was induced with fentanyl 1 μg/kg, thiopental 5–7 mg/kg, and vecuronium 0.1 mg/kg to facilitate tracheal intubation. Laryngoscopy was performed after the loss of the fourth twitch in the train-of-four in response to ulnar nerve stimulation. All laryngoscopies were performed with the patient placed in the sniffing position8 (head placed on a 6-cm firm pad with a gel ring) using a #4 Macintosh blade. A single experienced anesthesiologist, blinded to the results of the airway assessments, performed all of the direct laryngoscopies and classified the laryngoscopic view according to the modified Cormack and Lehane grade (C-L grade).9 Easy visualization of the larynx was defined as a Grade 1 or 2 view, and DVL was a Grade 3 or 4 view on direct laryngoscopy.
The following statistical analyses were performed with MedCal 7.3 for Windows (MedCal software, Mariakerke, Belgium). First, a univariate analysis was performed to assess the association of each demographic parameter and airway predictor with DVL. An unpaired t-test was used for continuous variables, and the χ2 test or Fisher’s exact test, as appropriate, was used for noncontinuous variables. Second, the receiver operating characteristic (ROC) curves were constructed to explore the trade-offs between the sensitivity and specificity of each test. The ROC area under the curve (AUC), which ranges from 0.5 to 1.0, equals the probability of correctly predicting DVL.10 Therefore, the optimal cutoff points of each test were determined at the maximum of the AUC for the corresponding ROC curve. For the modified Mallampati test, Grade 3 or 4 was predefined as a predictor of DVL.11 Third, using these cutoff points, true and false positives, true and false negatives, and the sensitivity, specificity, positive predictive value, and negative predictive value of each test were calculated. Then, all possible combinations of the modified Mallampati test and single predictors that were shown to be relevant to DVL in the univariate analysis were formulated, and the diagnostic validity profiles were calculated and compared among the combinations. Lastly, the diagnostic accuracy of the HMDR versus that of the other proven single predictors was assessed by calculating the AUC for each ROC curve. The AUC is a performance indicator equivalent to the nonparametric concordance measure, Somers D, and the difference between two ROC areas is half the difference between the corresponding Somers D values.12 The AUC values were compared using the nonparametric method of Delong et al.,13 which is based on the Mann–Whitney U statistic. In all cases, statistical significance was defined as P < 0.05.
The larynx was difficult to visualize in 26 (12.2%) of the 213 patients (Table 1). No failed tracheal intubations occurred. In the univariate analysis, significant differences were observed in the HMD and TMD at the extreme of head extension and in the HMDR between the DVL and easy visualization of the larynx patients (Table 1).
The diagnostic validity profiles from the single tests for the derived test criteria are shown in Table 2. The HMDR, with a sensitivity of 88%, was the most sensitive of the single tests. Combinations of the three tests that were relevant to DVL in the univariate analyses and the modified Mallampati test resulted in increased specificity at the expense of decreasing the sensitivity. Although the HMDR and the HMD at the extreme of head extension was the combination with the best result, it showed a poorer diagnostic validity profile than the HMDR alone (Table 3).
The ROC curves of the three single predictors that were relevant to DVL in the univariate analyses are shown in Figure 2. The AUC values for the HMD and TMD at the extreme of head extension were 0.642 and 0.659, indicating a poor degree of accuracy. In comparison, the AUC for the HMDR was significantly greater, at 0.782 (Table 4; Fig. 2).
The incidence of DVL in this study was 12.2%, which is consistent with a meta-analysis of nine studies that included 14,438 patients and a DVL incidence of 6%–27%.14 The wide variations in the incidence of DVL may be related to factors such as age11 and ethnic differences among patients15,16 or types of laryngoscope blade used.17 Based on a ROC curve analysis of the variables statistically relevant to DVL, the HMDR using an optimal cutoff point of 1.2 had the highest diagnostic accuracy for predicting DVL, with an AUC of 0.782 (95% confidence interval, 0.720–0.835). According to published guidelines,18 the HMDR was the only single test with good diagnostic value (AUC of 0.75–0.92). The HMDR alone had a greater diagnostic validity profile than that of the modified Mallampati test combined with any of the variables statistically relevant to DVL.
The major advantage of the HMDR is its high sensitivity (88%), which minimizes false-negative predictions. However, the HMDR has relative low specificity (60%) and positive predictive value (23%). The high false-positive predictions based on this test may subject many patients to unnecessary procedures. The ideal test for DVL prediction should have 100% sensitivity and 100% specificity; however, sensitivity and specificity are inversely proportional to each other. We believe that minimizing false-negative predictions with the HMDR is preferable to minimizing false-positive predictions, because the higher numbers of false-negative predictions that would be associated with minimizing false-positive predictions may lead to the potentially serious scenario of failed intubation.
The HMDR was previously suggested as a new possible predictor of DVL,3 and we confirmed its utility in the present study. Radiological studies4,19 revealed that the HMD increased during extension of the head at the OAA complex and remained so during extension of the head in the subaxial regions. This means that the hyoid bone moves parallel to the cervical spine during movement of the head and neck. As a result, the HMDR alone was highly correlated with the OAA complex extension capacity despite a concurrent degree of subaxial extension.3 In addition, the HMDR is easy and quick to perform at bedside without any special devices and was found to be more accurate than direct measurement of the OAA complex extension angle using a goggles-mounted goniometer.3,6
During laryngoscopy, creating a nearly straight line from the mouth to the glottic opening depends entirely on the degree of extension of the OAA complex.6,20 The angle required to expose the glottis during direct laryngoscopy was previously reported to be at least 12°,4 and the corresponding HMDR was calculated as 1.25.3 Therefore, we prepared a plain or optical stylet, laryngeal mask airway, or fiberoptic scope in such cases and planned to adopt these alternative strategies if the first intubation trial were to fail. As a result, our calculated cutoff point of 1.2 is similar to that of the original version of the HMDR. As both cutoff points are derived from a similar ethnic group (i.e., East Asians), there is a clear need for more data on the HMDR in different ethnic groups.
Instead of the sitting position used in the original version of the study describing the HMDR, we kept the patients in the supine position during the measurement for several reasons. First, it is reasonable to evaluate the airway in the position in which laryngoscopic intubation will actually be performed. Because the hyoid bone is movable, the possibility of changes in its dimension and position because of the effect of gravity should be taken into consideration. Recently, Sutthiprapaporn et al.21 demonstrated that the body of the hyoid bone moves caudally 6.7 ± 4.4 mm in response to a change in the postural position from supine to sitting upright. Considering the significant degree of intersubject variability in the vertical hyoid bone movement, anatomic variables related to the hyoid bone are more predictive when measuring in the supine position, rather than in the sitting position. Second, our method may be more useful in the intensive care unit or general ward, where clinicians encounter patients lying in bed in need of intubation. These patients are usually connected to many monitoring instruments, drains, and IV lines; thus, several medical personnel and a great deal of time may be required to keep the patients in a sitting position for the tests. Third, our method may be applicable to patients who cannot sit up in bed or control their head and neck because of altered levels of consciousness. Although we did not measure the HMD on manual maximal extension of the head, this maneuver could replace voluntary maximal extension by the patients.
There are some potential limitations to our study design. First, intersubject variability was possible because the end point for extending the head maximally depended on the voluntary participation of each subject. We tried to clearly explain each maneuver to the patients and demonstrated it when necessary; thus, we believe that intersubject variability was of minor importance in this study. Second, intrarater variability was possible, because a single investigator performed all of the measurements at once in a test. Finally, although DVL is a major determinant of difficult intubation, it is not synonymous with difficult intubation. In this study, we defined the modified C-L Grade 3 or 4 as an indicator of DVL. In many clinical situations, however, the application of external laryngeal pressure facilitates a laryngoscopic view and intubation can be performed without difficulty in these patients. In addition, direct laryngoscopy is not the only way to secure and maintain an airway, although it is the most common means of facilitating intubation.
In conclusion, we demonstrated that the HMDR is a clinically reliable predictor of DVL and that a value of <1.2 can be used as a test threshold. Although the HMDR test alone had greater diagnostic accuracy than any combination of the tests in this study, it had relatively low specificity and a high number of false-positive results. Therefore, we recommend seeking an optimal combination of tests that includes the HMDR and other predictors and performing the tests in combination, rather than using it alone.
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