Submandible Angle in Nonobese Patients with Difficult Tracheal Intubation

The study protocol was approved by our ethical committee (Graduate School of Medicine, Chiba University, Chiba, Japan), and written informed consent was obtained from each subject.

As a routine clinical practice at our institute (Department of Anesthesiology, Chiba University Hospital, Chiba, Japan), patients who presented DTI during direct laryngoscopy at induction of general anesthesia are invited for postoperative consultation to our difficult airway clinic for the purpose of informing them of the difficulty encountered during laryngoscopy and of airway reevaluation. The difficulty is determined and confirmed by experienced staff anesthesiologists when the Cormack and Lehane grade of the laryngeal view during direct laryngoscopy is 3 or 4 at application of external laryngeal pressure.5Our standard procedure for direct laryngoscopy in adults is the use of the Macintosh No. 3 blade under the sniffing position (approximately 8 cm head elevation with cushions). Between December 2001 and April 2005, 63 patients with difficult laryngoscopy were assessed in our difficult airway clinic. The modified Mallampati score,6thyromental distance, upper and lower incisor distance with maximum mouth opening, and neck movement were reassessed by investigators (N.S. and S.I.). The intubation difficulty score (IDS) developed by Adnet et al.  7was calculated based on the report by our anesthesia staff. The IDS is a sum of the following seven parameters: the number of supplementary intubation attempts, the number of supplementary operators, the number of alternative intubation techniques used, the Cormack and Lehane grade minus 1, the lifting force applied during laryngoscopy, the necessity of applied external laryngeal pressure, and the position of the vocal cords at intubation. A score of 0 represents an ideal intubation. An IDS between 1 and 5 represents slight difficulty, and an IDS greater than 5 represents moderate to major difficulty in tracheal intubation. Exclusion criteria in this study were (1) emergency surgery, (2) age younger than 20 yr, (3) obesity (body mass index greater than 30 kg/m²), (4) limited mouth opening (less than 30 mm), (5) limited neck extension, (6) use of full denture(s), (7) apparent craniofacial abnormalities, (8) patients with rheumatoid arthritis, and (9) an IDS less than 5. These exclusion criteria were determined for clarification of nonfunctional craniofacial features in these patients. To minimize racial differences, subjects were limited to the Japanese.8Among the 63 patients with DTI, 23 male and 18 female patients met the criteria.

Thirty-two control subjects (16 males and 16 females) whose trachea was easily intubated were recruited during the period between December 2001 and April 2005 and selected to match the sex, age, weight, and height of the DTI patients. To facilitate potential differences between the groups, ETI was determined as a laryngeal view (Cormack and Lehane) of 1 or 2 without external laryngeal pressure and an IDS less than 5. The exclusion criteria above mentioned except No. 9 were also applied for the selection of control subjects.

To characterize craniofacial features, frontal and lateral digital photographs (COOLPIX4500; Nikon, Tokyo, Japan) of the neck, full face, and head of each subject at standing and looking straight ahead with the mouth closed were taken. Care was taken to adjust the height of the camera to eye level, whereas the distance between the camera and the subject was not standardized. Thirty-nine anatomical landmarks on each lateral photograph and 84 anatomical landmarks on each frontal photograph were determined as shown in figure 1and used for both measurements of craniofacial dimensions and construction of an “average” face for each patient group. Lower face width and height relative to the middle face width were measured on the frontal photographs (fig. 1A). Mandible length relative to the lateral middle face width, mandible position angle, and submandible angle were measured on the lateral photographs (fig. 1B). An investigator (Y.K.) performed blind measurements of the craniofacial dimensions.

To visualize potential differences of the craniofacial features between DTI and ETI patients, an “average” craniofacial appearance for each patient group was constructed by composing digital photographs by matching anatomical landmarks defined in figure 1between the photographs (N.S.). An “average” frontal face was constructed with use of computer programs developed by Harashima et al.  (Face Tool; EEIC Engineering Department, University of Tokyo, Tokyo, Japan). A morphing computer program#was used for construction of an “average” profile. In theory, a morphing facial image on the coordinates U(x, y) is produced by composing two facial images on U1(x1, y1) and U2(x2, y2) to a composite proportion (t); U(x, y) = (U1 − U2) * t + U2, 0 ≤ t ≤ 1. An “average” face is obtained when t = 0.5, and therefore, an “average” face of two different patients consists of 50% facial property from each patient. Both color and shape of the faces were averaged in the morphing process. Repetitive production of the “average” face for pairs of patients produced an “average” face within each group of patients. Accordingly, the number of photos required for production of the group “average face” is 2ⁿin total; therefore, 16 was chosen as the maximum closest number within the total subject number of each group. Because our purpose was to clarify specific features of DTI patients, we produced an “exaggerated DTI face” by using photographs of eight DTI patients with extreme craniofacial variables determined by multiple regression analysis for each sex.

The Mann–Whitney rank sum test was used to assess differences of anthropometric and craniofacial variables between the groups. Spearman correlation analyses were performed between physical assessments for predicting difficult intubation such as the modified Mallampati score, thyromental distance, and mouth opening and craniofacial dimensions. Multiple logistic regression analysis was performed by using all craniofacial dimensions as independent variables and DTI as a positive response. A logistic regression model was produced with variables that had significant coefficients. P < 0.05 was considered to be significant. All variables were presented as median (25th–75th percentile).

Anthropometric parameters, airway assessments, and laryngeal view during direct laryngoscopy and intubation difficulty score are presented for each patient group in table 1(male) and table 2(female). Body habitus did not differ between ETI and DTI groups, whereas higher Mallampati score, shorter thyromental distance, and reduced distance between upper and lower incisors were evident in DTI patients. Tracheal intubation was more difficult in the DTI group by definition.

Results of craniofacial dimension measurements are presented in tables 1 and 2. Lower face width was significantly narrower in DTI females than in ETI females. Mandible position angle was significantly smaller in male DTI males than in ETI males. Submandible angle was significantly larger in both male and female DTI patients than in ETI patients. Figures 2 and 3present “average” faces, visualizing the results of the craniofacial dimension analyses. More receded mandible and increased submandible angle are evident in the profile composites of DTI patients (figs. 2D and 3D). The differences of facial appearance between DTI and ETI patients are more evident in the profile (figs. 2C, 2D, 3C, and 3D) than in frontal composites (figs. 2A, 2B, 3A, and 3B).

To determine the significant craniofacial variables associated with DTI, multiple logistic regression analysis was performed for each sex. We found that the submandible angle was a significant and independent variable associated with DTI among the craniofacial variables for both sexes (table 3). Based on this result, an “exaggerated DTI face” in which increased submandible angle were exaggerated was created for each sex by averaging faces from eight patients with greater submandible angle (fig. 4). Increased submandible angle was evident in both male (fig. 4A) and female (fig. 4B) “exaggerated DTI faces.”

Table 4presents results of correlation analyses between physical assessments for predicting difficult intubation and craniofacial dimensions. Mandibular position angle was indirectly associated with Mallampati score and directly associated with thyromental distance. Submandible angle was indirectly associated with thyromental distance in both sexes, whereas significant associations were observed between submandible angle and all DTI predictors in males.

This is the first study, to our knowledge, of a systematic evaluation of facial appearance by use of digital photographs of patients with varying degrees of tracheal intubation difficulty. Patients with DTI were found to have a more receded mandible and increased submandible angle than age– and body mass index–matched patients with ETI. Logistic regression analysis indicated that increased submandible angle was significantly associated with DTI. Specific craniofacial features of DTI patients such as receded mandible and increased submandible angle were visually more evident in the profile than in the frontal composites.

Our craniofacial assessment was two-dimensional and surface measurements without differentiating bony structures from soft tissue, both of which create facial appearance, whereas the direct laryngoscopy is a three-dimensional procedure in the three-dimensional structure with complicated interaction between bony and soft tissue structures. Despite the limitation, it was to our surprise that distinct differences in the plane profiles captured by a digital camera were found between DTI and ETI patients. Another limitation of this study is the absence of functional structural assessments while direct laryngoscopy is a dynamic functional procedure. Presence of limited mouth opening and neck movement may interact with increased submandible angle to further increase difficulty in tracheal intubation, a topic that necessitates further investigation, because these patients were excluded in this study. Finally, our subjects were nonobese Japanese, and results of this study may differ from those performed in whites and other races because obesity is more common in North America and Asians have a smaller and more receded mandible than whites.8Nevertheless, we believe that differences of the “average Japanese” face between patients with and without DTI provide clues to investigate mechanisms of DTI in any race, while control of the racial background is a necessity.

Although our results clearly indicate increased submandible angle as a specific feature in DTI patients, a role of increased submandible angle in the mechanisms of DTI remains unclear. Submandible angle could be increased by either excessive submandible soft tissue or foreshortened mandible. While we only included nonobese patients in this study, increased submandible angle is a common feature of morbidly obese patients due to excessive submandible soft tissue. DTI in obese patients is controversial.9–11It should be noted that the highest reported prevalence of DTI in obese patients is 16%,9suggesting that tracheal intubation is easy in most obese patients with excessive submandible soft tissue. Accordingly, excessive submandible soft tissue alone may contribute little to the DTI mechanisms.

A single DTI predictor does not satisfactorily predict DTI, and a combination of DTI predictors increases DTI predictability,12suggesting that multiple factors may be involved in the DTI mechanisms. In fact, increased submandible angle was associated with various DTI predictors such as Mallampati score, interincisor distance, and thyromental distance (table 4), suggesting that the feature is a result of multiple contributing factors to DTI mechanisms. Accordingly, it is possible that DTI in patients with increased submandible angle results from difficulty in displacement of relatively excessive oropharyngeal soft tissue, particularly the tongue base, through narrow submandible space due to a foreshortened mandible. These speculations must be tested in the future studies by assessing dynamic structural movement during direct laryngoscopy.

Although clinical significance of facial appearance for DTI prediction seems to be widely accepted by anesthesiologists, only a study performed by Wilson et al. ,13to our knowledge, systematically evaluated it. Five risk factors, including weight, head and neck movement, jaw movement, receded mandible, and buck teeth, were identified in 75% of DTI patients with a cost of falsely identifying 12% of ETI patients. In accordance with their study, DTI patients in our study had a receded mandible, and our results further indicate a potential predictive value of increased submandible angle.

Difficult tracheal intubation may occur as a result of accumulation of mild to moderate difficulty during the tracheal intubation process, and the combination of Mallampati score and thyromental distance increased the positive predictive value.12Therefore, the addition of submandible angle assessment to the conventional predictors may improve DTI prediction, although submandible angle alone may not completely predict the multifactorial DTI events. Although DTI-predictive values of the submandible angle are currently uncertain, a large-scale, prospective, randomized, multicenter study must be performed in the future. Careful assessments of the profile are advantageous for identifying receded mandible and submandible angle in each preoperative patient.

Although this is the first study presenting the “average” face of DTI patients using the morphing computer technique, an “average” face has been created for various groups of subjects, and its usefulness has been reported.3,4Certainly, the averaging process may decrease variation of color and shape when performed for a nonspecific general population. However, specific features would emerge on the “average” face when this technique is applied to a definite group of subjects. For example, likeness to the target face was better in an “average” face than in four different composites produced by eyewitnesses.4Interestingly, an “average” face of young females is more attractive than individual faces, and an “average” face produced by highly attractive females is more attractive than the “average young female face,” indicating that averaging process would clarify phenotypes of our interest.3When there are specific features associated with DTI, variations of the DTI patient faces are possibly due to colors and dimensions unrelated to DTI, which are likely to be randomly distributed. Therefore, specific features would emerge on the average faces for the definite DTI group while decreasing the variations. In fact, increased submandible angle clearly emerged on the “average” DTI patient face, and it was more prominent in the “exaggerated DTI face.”

Construction of the “average” DTI patient face may have several clinical significances. First, visualization of the specific profiles of DTI patients can correct emphatic images of DTI patient derived form wording such as “receded” or “increased” and decrease variability of DTI determinations based on facial appearance among anesthesiologists. Second, it may have an educational value for both paramedical staff as well as physicians who may encounter tracheal intubation.

Anatomical balance between the mandible size and soft tissue volume surrounding the pharyngeal airway seems to determine the airway size14,15; therefore, anatomical unbalance caused by either a small mandible or an increased amount of soft tissue is considered to result in development of obstructive sleep apnea (OSA). In fact, the small mandible is a feature of nonobese OSA patients,16and increased submandible soft tissue is reported to have value for predicting OSA.17Accordingly, the prominent facial features such as receded mandible with increased submandible angle in DTI patients coincide well with those in OSA patients. Assuming mask ventilation is more difficult in OSA patients than in non-OSA patients because of an abnormally collapsible pharyngeal airway18; the similarity of facial features suggests commonality of anatomical background and pathophysiology of DTI and difficult mask ventilation. In fact, DTI is common in OSA patients,19and difficult mask ventilation is often accompanied by DTI.20 

In conclusion, increased submandible angle characterizes craniofacial features of patients with DTI. The specific feature of difficult tracheal intubation is more evident in the profile.

The authors thank Sara Shimizu, M.D. (Head of the Department of Plastic Surgery, JFE Kawatetsu Chiba Hospital, Chiba, Japan), who greatly helped to improve this manuscript.