Introduction

The rate of difficult laryngoscopy and intubation ranges widely from 0.5% to 24% of patients undergoing general anaesthesia among difficult studies.1–4 Patients with cervical spondylosis have a higher incidence of difficult laryngoscopy compared with patients without cervical spondylosis,5 6 and in patients with cervical spondylosis, anaesthesiologists may encounter a large percentage of unexpected difficult airways, which are associated with increased morbidity and mortality.

Airway examination is an essential component of the preoperative assessment. Preoperative assessment of a patient’s airway enables the anaesthesiologist to predict the ease of visualising the glottis and performing intubation. Predictors have variable sensitivities and specificities for identifying a difficult airway.7 Common bedside physical airway assessment tests include interincisor gap, thyromental distance and Mallampati test.8 However, these measurements have high interobserver variability, and predictive accuracy is too low to identify patients with the most difficult laryngoscopy.9–12 To increase the accuracy of preoperative evaluation, recent studies have measured anterior neck soft tissue thickness using ultrasound. Ultrasonographic measurements are rapid and convenient to perform, but ultrasonographic findings in studies of preoperative difficult airway assessment are inconsistent. This may be attributed to the operator’s skill in ultrasonographic scanning, target image recognition and measurement, patient positioning during ultrasonographic scanning and patients’ ethnic differences and operation type. Ultrasonographic predictors of anterior neck soft tissue thickness are usually performed in neck maximum extension which may be dangerous for patients with cervical spondylosis. It should be emphasised that no ultrasound study was performed in such patients with cervical spine pathology. Whether these measurements are appropriate as difficult laryngoscopy predictors requires further evaluation. To verify the accuracy of ultrasonographic measurements proposed by other studies in patients with cervical spondylosis, we used MRI to quantify anterior neck soft tissue thickness, which is more accurate than ultrasonography for evaluating soft tissues.

Materials and methods

Equipment and researchers

Neck MRI (MR750; GE Medical Systems, Milwaukee, Wisconsin, USA) examination was performed with the patient in the neutral position, and MRI indicators were measured on the lateral sagittal neck MRI film (figure 1) by an experienced radiologist blinded to group allocation and not involved in intubation and anaesthesia management. MRI data were evaluated using the radiography information system (Centricity RIS-IC CE V3.0; GE Healthcare, Little Chalfont, UK) of Peking University Third Hospital.

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Figure 1

Indicators on lateral sagittal neck MRI image. DSE, distance from skin to epiglottis; DSH, distance from skin to hyoid bone; DSS, distance from the skin to anterior to the trachea at the level of suprasternal notch; DST, distance from skin to thyroid cartilage at the level of the vocal cords; DSV, distance from skin to vocal cords.

Results

A total of 328 were initially enrolled in the study and 315 were included in the final analysis. The allocation process was presented in figure 2. Patients comprising 200 men (63.5%) and 115 women (36.5%) were included in the study. Univariate analysis demonstrated three risk factors associated with difficult laryngoscopy: male sex (p<0.01), older age (p=0.03) and body weight (p=0.02). Five MRI indicators were not significantly different between the easy and difficult laryngoscopy groups, respectively: DSH (1.73±0.54 cm vs 1.61±0.52 cm; p=0.18), DSE (3.76±0.53 cm vs 3.79±0.53 cm; p=0.72), DST (0.66±0.24 cm vs 0.59±0.23 cm; p=0.10), DSV (1.49±0.41 cm vs 1.43±0.42 cm; p=0.44) and DSS (4.53±0.72 cm vs 4.52±0.75 cm; p=0.92). The indicators are listed in table 1.

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Figure 2

Allocation process.

Adjusted by sex, age and weight, binary multivariate logistic regression analyses for the MRI indicators identified no factor that was independently associated with difficult laryngoscopy: DSH (p=0.51), DSE (p=0.12), DST (p=0.26), DSV (p=0.09) and DSS (p=0.59). ORs (95% CIs) for DSH, DSE, DST, DSV and DSS were 0.79 (0.38 to 1.62), 0.56 (0.27 to 1.16), 0.38 (0.07 to 2.04), 0.44 (0.17 to 1.13) and 1.16 (68 to 1.96), respectively. Results appear in table 2.

Discussion

To our knowledge, ours is the first study of anterior neck soft tissue thickness verified by MRI. Our results showed that the five most popular ultrasonographic indicators had no significant difference for predicting easy and difficult laryngoscopy.

Our data demonstrated that male sex, higher body weight and older age were associated with difficult laryngoscopy. A significantly greater proportion of difficult tracheal intubations has been reported in men and obese patients.15 16 An association between difficult laryngoscopy and older age has also been reported.17 Osteoarthritic changes associated with decreased thyromental distance, cervical spinal movement, interincisor distance and grade of dentition may be responsible for age-related increases in difficult laryngoscopy.4

Adding ultrasonography to airway evaluation allows for rapid visualisation of structures. With appropriate depth and probe-frequency selection, ultrasound can image any structure lying superficial to the oral, pharyngeal or tracheal air columns.18 This includes the mouth, tongue, oropharynx, hypopharynx, hyoid bone, epiglottis, larynx, vocal cords, cricothyroid membrane, cricoid cartilages and trachea.19 Ultrasonography, as a portable, non-invasive, rapid and effective examination is increasingly used for airway assessment before surgery; however, results are inconsistent.

Adhikari et al 20 reported that DSH could be used to distinguish difficult and easy laryngoscopies, and found that DSH values were higher in the difficult laryngoscopy group compared with the easy laryngoscopy group (1.69 cm, 95% CI 1.19 to 2.19 vs 1.37 cm, 95% CI 1.27 to 1.46, respectively) in a study of 51 American patients in the neutral position without a pillow. However, in a study by Reddy et al,21 DSH was not a significant predictor of difficult laryngoscopy (p=0.86), and our results were consistent with Reddy et al’s. In our study including 315 patients with cervical spondylosis in the neutral position, we found that DSH did not differ significantly between easy and difficult laryngoscopy groups (1.73±0.54 cm vs 1.61±0.52 cm, respectively; p=0.18).

Pinto et al 22 studied 74 Portuguese patients in the neutral position and found that DSE can be used effectively to predict difficult laryngoscopy; DSE was greater in the difficult laryngoscopy group compared with the easy laryngoscopy group (2.83±0.44 cm vs 2.33±0.39 cm, respectively; p<0.01). Soltani et al 23 studied 53 Iranian patients in the supine position with active maximal head tilt and chin lift and found that the correlation between the depth of the pre-epiglottic space and Cormack–Lehane grade III was weak, with a regression coefficient of 0.13 (95% CI 0.70 to 1.71; p=0. 40). In our study, DSE did not differ significantly between easy and difficult laryngoscopy groups (3.76±0.53 cm vs 3.79±0.53 cm, respectively; p=0.72).

Komatsu et al 24 studied 64 obese American patients in the neutral position without a pillow and found that DST in the difficult laryngoscopy group was shorter than that in the easy laryngoscopy group (2.0±0.3 cm vs 2.2±0.4 cm, respectively; p<0.05), but multivariate regression indicated that DST was not an independent predictor of difficult laryngoscopy (p=0.13). Therefore, the authors concluded that DST was not a good predictor and that it failed to predict difficult laryngoscopy in obese patients. In our study, we found that DST did not differ significantly between easy and difficult laryngoscopy groups (0.66±0.24 cm vs 0.59±0.23 cm, respectively; p=0.10).

Ezri et al 15 studied 50 obese Israeli patients and found that DSV and DSS in the difficult laryngoscopy group were both greater than values in the easy laryngoscopy group (2.8±0.3 cm vs 1.8±0.2 cm, respectively; p<0.01 and 3.3±0.4 cm vs 2.7±0.7 cm, respectively; p=0.01). Reddy et al 21 also found that DSV in the difficult laryngoscopy group was greater than that in the easy laryngoscopy group (0.35±0.18 cm vs 0.25±0.11 cm, respectively; p=0.01) and that DSV >0.23 cm was a potential predictor of difficult intubation with 85.7% sensitivity, 57% specificity and 61% accuracy. However, Adhikari et al 20 found no significant differences for DSV and DSS between easy and difficult laryngoscopy groups. We also found that DSV and DSS did not differ significantly between easy and difficult laryngoscopy groups (1.49±0.41 cm vs 1.43±0.42 cm, respectively; p=0.44 and 4.53±0.72 cm vs 4.52±0.75 cm, respectively; p=0.92).

Ultrasonography is convenient and useful for bedside examination, and five indicators of anterior neck soft tissue thickness at different levels measured using ultrasonography have been discussed previously. The results of the five ultrasonographic predictors are inconsistent. Considering that MRI has higher resolution than ultrasound, to obtain better soft tissue images with lower radiation hazard, we chose MRI to measure the related soft tissues. We found that none of the five indices could predict difficult laryngoscopy in patients with cervical spondylosis regardless of whether sex, age and body weight were adjusted with a binary multivariate logistic regression. The reasons for different results between our study and previous studies are as follows: (1) the five MRI indicators focused only on the local anatomy and could not reflect cervical activity and changes in head position (from neutral to extended). Therefore, these may not be meaningful indicators; however, no defined gold standard indicators exist. (2) These indices have been measured previously using ultrasonography, but measurements were not taken using standardised procedures, which have not been determined. Ultrasonographic measurements obtained by the primary investigator might have been associated with bias, and the measurement results were related to the operator’s experience. The lack of a standardised reference and methods for obtaining these ultrasonographic measurements in previous studies also indicates the potential for operator error. Patients’ necks were also positioned differently across studies, which could result in measurement error. (3) Patients’ ethnicity and operation type might also affect results. Our study participants were Asian, and previous studies included American, Portuguese or Iranian patients; studies have documented differences in neck fat tissue distribution between different ethnic groups.20 (4) Our patients had cervical spondylosis, and the presence of this condition was not mentioned for patients in previous studies. (5) Considering superficial measurements reflecting anatomical variation in an extremely anterior airway and the small sample size in previous studies, results might be skewed.

Conclusion

Our study suggests that the ultrasonographic indices (DSH, DSE, DST, DSV and DSS) used to evaluate anterior neck soft tissue thickness at different levels, measured by MRI, could not predict difficult laryngoscopy in patients with cervical spondylosis. In future studies of ultrasonographic assessment, patients’ ethnicity, position, operators’ qualifications and standardised operation procedures all must be considered. Based on high-quality and large sample size studies, the most accurate ultrasonographic indicator for difficult laryngoscopy may be determined.