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Should We Reevaluate the Variables for Predicting the Difficult Airway in Anesthesiology?

Türkan, Selma MD*,; Ateş, Yeşim MD*,; Cuhruk, Handan MD*,; Tekdemir, İbrahim MD

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doi: 10.1097/00000539-200205000-00055
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Inadequate ventilation, esophageal intubation, and difficult tracheal intubation are the most common mechanisms of respiratory-related adverse outcomes in the clinical practice of anesthesiology (1). Therefore, detecting patients at risk of difficult endotracheal intubation is important. Preoperative evaluation of the airway can be accomplished by various measurements of the anatomic landmarks or noninvasive clinical tests performed during physical examination. However, two reports have questioned the previously accepted variables of airway evaluation; some reports have even questioned whether true prediction can ever be possible (2,3).

Measurements of mandibular space and neck mobility are some of the criteria included in the multifactorial analysis of predicting a difficult airway (4–6); however, normal ranges of these variables in the adult population have not been clearly defined. Data about the range of these measurements in different age groups or in female and male populations are also scarce.

The aim of this study was to evaluate anatomical measurements often used to predict a difficult airway in the normal adult population, thereby revealing any age or sex-related differences in these variables. The study was performed on elective surgical patients during preoperative evaluation and in cadavers at the anatomical laboratory. Thyromental, hyomental, and sternomental distances were measured. Degree of neck extension and Mallampati scores were also recorded.

Material and Methods

Local ethics committee approval was obtained for the study. The committee decided that patient consent was not required because the measurements performed were noninvasive and had no cost. Consecutive male and female white adult patients admitted to the outpatient preoperative anesthetic clinic were enrolled onto the study. The age limit was 20–70 yr, and the following groups were assessed based on age: Group 1 (20–30 yr), Group 2 (31–49 yr), and Group 3 (50–70 yr). None of the study patients had had bone, joint, or soft tissue disorder or previous operations involving the thyroid gland or the soft tissues of the neck. Cadaver measurements were performed on 12 cadavers in the anatomy laboratory.

Digital micrometer (digital caliper) was used to perform cadaver measurements, and direct bone contact was provided. The cadaver study was designed as a pilot study to ascertain the anatomical landmarks. Reference points on bones and cartilage were protuberantia mentalis on mandibula, prominentia laryngea located in front of incisura thyroidea superior, corpus of the hyoid bone, and incisura jugularis on the sternum.

Variables recorded for the study were age (years), sex (men/women), weight (kg), height (cm), thyromental distance (cm), hyomental distance (cm), and sternomental distance (cm). Reference points for morphometric measurements were prominentia laryngea of the thyroid cartilage and symphysis of mandibula for thyromental distance, body of the hyoid bone and symphysis of mandibula for hyomental distance, and incisura jugularis of the sternal bone and symphysis of mandibula for sternomental distance. All measurements were performed with the patient’s head in midline-neutral position, neck fully extended, and the patient lying supine. A hard-plastic bond ruler was used to measure the distances, and each measurement was performed three times by two investigators (ST and YA) blinded to each other.

Extension of the neck was evaluated by a goniometer while the patient was in the sitting position. The angle of the jaw and the upper incisor teeth were the reference points. The patient was then asked to fully extend the neck while the measuring side of the goniometer was moved parallel to the upper incisors, and the maximum extension was recorded.

The scoring system described by Mallampati et al. (7,8) was used to evaluate the airway of the study patients. The patient was asked to sit upright with the head in neutral position. Then, while opening the mouth as wide as possible, maximum tongue protrusion was requested. Pharyngeal structures were then observed, and classification was performed according to the structures seen as follows: Class 1, soft palate, fauces, uvula, pillars; Class 2, soft palate, fauces, uvula; Class 3, soft palate, base of uvula; Class 4, soft palate not visible at all.

Statistical analysis was performed using the GB-Stat version 5.0 software program (Dynamic Microsystems, Inc, Silver Spring, MD). Student’s t-tests were used to compare demographic data. Analysis of variance for completely randomized groups was used to compare the measured variables. Post hoc Tukey’s Honestly Significant Differences test was used where statistical significance was obtained. The Cronbach α model was used to evaluate the interassessor reliability of the data. Fisher’s exact test was used to evaluate the distribution of patients. P < 0.05 represented significance, and data were shown as mean ± sd.


A total of 334 patients (215 men and 119 women aged 20–70 yr) were enrolled onto the study. Patients in Group 2 and 3 weighed significantly more (P < 0.01), and their heights were significantly less (P < 0.01), than the patients in Group 1 (Table 1). Groups 2 and 3 were similar in mean weight and height.

Table 1:
Demographic Data of the Patients in the Study Groups

α values obtained from the reliability analysis of the measurements performed by the two investigators were more than 0.8 in all comparisons. Morphometric measurements of the patients and neck extension values are shown in Table 2. Thyromental and sternomental distances were longer in Group 1 compared with Groups 2 and 3 (P < 0.01). Groups 2 and 3 were similar in thyromental and sternomental distance measurements. When male and female patients in the study groups were compared by mean of thyromental and sternomental distance measurements, values in Group 1 in both sexes were longer than in the other two groups (P < 0.01). The mean degree of neck extension was also greater in Group 1 compared with Groups 2 and 3 (P < 0.05). Groups 2 and 3 were similar in neck extension values.

Table 2:
Morphometric Measurements of the Patients in the Study Groups for Upper Airway Evaluation

Statistical analysis of the morphometric measurements obtained from male (n = 215) and female patients (n = 119), regardless of the age group, was also performed. Mean values of the hyomental distances were 5.9 ± 0.9 cm in male patients versus 5.6 ± 0.8 cm in female patients (P < 0.01). Mean thyromental distances were 9.9 ± 1.4 cm in male patients versus 9.7 ± 1.6 cm in female patients (P < 0.05). Mean sternomental distances were 17.8 ± 2.4 cm in male patients versus 16.3 ± 2.3 cm in female patients (P < 0.01). Neck extension values were 58 ± 11 degrees in male and 59 ± 9 degrees in female patients (P > 0.05).

Mallampati scores of the male patients (n = 215) were 43% in Class 1, 40% in Class 2, 14% in Class 3, and 3% in Class 4. Female patients (n = 119) were also evaluated for Mallampati scores; the results were 62% in Class 1, 24% in Class 2, 11% in Class 3, and 3% in Class 4. Morphometric measurements of the patients evaluated as Mallampati Class 3 and 4 are shown in Table 3. Distribution of the patients evaluated as Mallampati Class 3 and 4 in each age group was analyzed, and the number of patients in different groups evaluated as Mallampati Class 3 and 4 were similar.

Table 3:
Morphometric Measurements of the Patients Evaluated as Mallampati Class 3 and 4 in the Study Groups

Cadaver measurements were performed on 12 cadavers (8 men and 4 women). Mean age of the cadavers was 57 ± 5 yr, mean weight was 76 ± 8 kg, and mean height was 169 ± 7 cm. Mean values of the anatomical measurements were a hyomental distance of 4.6 ± 0.5 cm, a thyromental distance of 6.5 ± 0.7 cm, and a sternomental distance of 15.3 ± 0.8 cm.


In this study, the most often accepted morphometric measurements used to predict difficult intubation were performed on patients having no abnormalities of the neck structures. The results of this study indicate that preoperative evaluation of the morphometric measurements may be performed in an age and sex-specific manner. Predicting if patients are at risk of difficult endotracheal intubation is important. However, certain criteria for recognition of these patients have been defined mostly by measurements performed on patients that have already been diagnosed as difficult to ventilate or intubate (9,10). The effects of age and sex on these measurements have not been thoroughly investigated previously.

Some authors have suggested that one of the measurements may be superior to others for predicting difficult intubation. Ramadhani et al. (11) have shown that sternomental distance had an increased sensitivity and specificity for predicting subsequent difficult laryngoscopy. However, the patient group in their study was limited to women of childbearing age only. Even though Ramadhani et al. (11) concluded that the sternomental distance was not affected by age, obstetric patients might not have reflected a broad age group. In our study, sternomental distance measurements were affected both by age and by sex. Younger patients (20–30 years) and male patients in global assessment had longer mean sternomental distance measurements (P < 0.01).

Chou and Wu (10) found that mandibulohyoid distance was a determining factor in predicting difficult intubation. The measurement of the mandibulohyoid distance was performed on cervical spine radiographic films with patients in neutral position. However, it has been concluded by Mc Intyre (5), and later by Randell (12), that radiological measurements have not been successful to reveal sensitive criteria for prediction of difficult intubation, and radiographic studies were at their best regarded as valuable in understanding problems encountered during laryngoscopy. In this study, hyomental distance was the only morphometric measurement that was similar in all age groups. Although the effect of sex was significant, the difference obtained was minor (5.6 ± 0.8 cm in female patients versus 5.9 ± 0.9 in male patients).

Among all the morphometric measurements, thyromental distance has been questioned the most for its value in predicting difficult intubation (2,12). It has been reported that thyromental distance was neither sensitive nor specific enough to be used as the only predictor of a difficult laryngoscopy (12–14). In this study, thyromental distance was also both age and sex dependent (P < 0.01). The difference in measurement of thyromental distance related to the age group seemed larger than the sex-related difference.

Neck extension has also been considered as one of the variables in predicting difficult intubation. Tse et al. (15) studied a relatively large patient group (n = 471); however, they evaluated only three variables, which were Mallampati classification, neck extension, and thyromental distance. They concluded that these tests, alone or in combination, were not reliable in predicting difficult intubation. Unfortunately, the effect of age or sex on any of the variables mentioned above has not been evaluated. In this study, the degree of mean neck extension was larger in the younger age group (20–30 years) compared with the relatively elderly groups (31–70 years) (P < 0.05). Sex did not have an effect on the mean degree of neck extension.

Morgan and Mikhail (4) have reviewed the criteria for predicting difficult intubation. Neck extension <35 degrees, hyomental distance <7 cm, and sternomental distance <12.5 cm were stated to be related to difficult intubation. In this study, mean neck extension values and mean sternomental distance values in different age groups and sex were greater than the mentioned variables. However, the mean hyomental distances in all the study groups and the cadavers were less than the stated limit of 7 cm for difficult intubation. Janssens and Hartstein (6) and Janssens and Lamy (16) recently developed a new scoring system for predicting difficult intubation in which a thyromental distance <6 cm is related to airway difficulty. They have stated that studies based on the airway difficulty score were in progress (6). Although the value of thyromental distance has been questioned as a variable in defining difficult intubation, its limit remains far less than the mean values obtained in this study. Therefore, a thyromental distance <6 cm may actually represent a difficulty.

In conclusion, several studies have addressed different variables alone or in combination to determine an ideal evaluation method of predetermining difficult intubation. The results of this study may help the clinician to assess adult patients before surgery in terms of age and sex-specific morphometric airway measurements. Adequate data of normal values may be used to identify patients that are outside the range and therefore may be challenging. Hyomental distance is the only variable that seems to be unrelated to age and sex. It therefore may be useful for evaluating patients of different ages and sex for airway measurements. Scoring systems, including multiple variables such as the intubation difficulty scale (17) and the airway difficulty score (6), seem to require more investigation and perhaps inclusion of more definite variables until an ideal method of evaluation has been achieved.


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