The incidence of difficult intubation in 2 000 patients undergoing thyroid surgery – a single center expirience Incidencija otežane intubacije kod 2 000 bolesnika podvrgnutih hirurškom le č enju štitne žlezde – iskustvo jednog centra

Background/Aim. The incidence of difficult intubation (DI) is 1–10%, and DI leading to inability to intubate occurs in 0.04% of the population. The aim of this study was to evaluate the incidence of DI in thyroid surgery and to assess possible correlation of difficult tracheal intubation with sex and primary diagnosis. Methods. We studied 2 000 consecutive patients (1 705 females) scheduled for thyroid surgery who were assessed for DI prior to general anesthesia, with respect to primary disease diagnosis and sex. Patients were divided into four groups: patients with nodal goiter (group A), polynodal goiter (group B), hyperthyroidism (group C) and thyroid carcinoma (group D). Difficult intubation was predicted using the scoring system which included 13 parameters ranged from 0 to 2. Additive score > 5 was accepted as a predictor of DI. True DI was defined as impossible visualization of glottis with direct laryngoscopy (grade III and IV). Results. Difficult intubation was observed in 110/2 000 patients (5.5%). The incidence of DI was higher in males (26/295, 8.8%) then females (84/1 705, 4.9%) (p < 0.01). The incidence of DI was highest in the group B (6.2%). Extremely DI occurred in 15/2000 patients (0.75%), the most of them in the group C (1.1%). Sensitivity of used scoring system was 91.8% and specificity 86.5%. Conclusions. The incidence of DI was higest in patients with polynodal goiter but extremly DI was present mostly in patients with hyperthiroidism. Men seem to be at higher risk for DI than women. Scoring system used in this study for prediction of DI may be useful in this patient population.


Introduction
Difficult intubation (DI) can lead to catastrophic outcome due to interruption of gas exchange.Published data indicate significance of this problem, as majority of airway related events (85%) involve brain damage or death, and as many as one third of lethal outcomes attributable solely to anesthesia have been related to inability to maintain patient airway 1,2 .
Management of the difficult airway in the general surgical population has been widely investigated.However, the incidence of the DI in patient population undergoing thyroid surgery has been studied rarely and published series are relatively small 3 .Enlarged thyroid gland may cause tracheal deviation or compression, or both, leading to DI 4 .Prediction of possible event could be helpful to the attending anesthesiologists.
Various noninvasive clinical tests can be performed to predict difficult airway maintenance.Preoperative identification of patients in whom tracheal intubation may be difficult or impossible may save lives.Although Mallampati oropharyngeal classification and Wilson scoring system provide fair preoperative evaluation of possible DI, there is no precise or ideal scoring system that predicts difficult ventilation, laryngoscopy or intubation 5,6 .Despite numerous studies which included various risk factors, false positive results have been reported, but more importantly, there are false negative values of multivariate risk indexes that can mislead anesthesiologists.Increasing number of risk factors analyzed leads to higher specificity, but unfortunately, they are still not sensitive enough.
The aim of this study was to determine the incidence of DI in thyroid surgery, to assess the correlation of DI with sex and primary diagnosis, and to validate the sensitivity and specificity of the scoring system used to predict DI.

Methods
Prospective observational study was carried out during three year period at the Endocrine Surgery Center of the Institute of Endocrinology, Clinical Center of Serbia, Belgrade.
Difficult laryngoscopy and intubation were assessed using 13 qualitative and quantative predictive tests incorporated into the scoring system (Table 1).Each test was qualified with 0 to 2 points (0 -no risk, 1 -moderate risk and 2high risk present), maximum 25 points score.Score over 5 points (R > 5) was assumed to be a predictor of DI.
Difficult intubation was defined by Cormack and Lehane (CL) criteria: any intubation with grade III or IV visualization of glottis during direct laryngoscopy, with regard of Wilson's definition where help of assistant or use of supplementary technique is also taken in the consideretion of DI 6,7 .Also, as in ASA criteria of difficult airway management, number of attempts and duration of management were noted 5 .
Patients were divided into four groups according to the presence of DI.Group I -visualization of epiglottis only (CL grade III), first attempt successful intubation, with a help of the assistant.Group II -visualization of epiglottis only (CL grade III), intubated in second or third attempt, with a help of the assistant or supplementary technique (use of tube guide, spontaneous breathing, nasotracheal intubation).Group IIIno visualization of epiglottis (CL grade IV), blind intubation, few attempts, help of the assistant and other maneuvers and techniques.Group IV -epiglottis not visible, more than three attempts of intubation, duration more than 10 minutes, or impossible intubation with conventional methods and use of fiberscope (extremely difficult intubation, EDI).
Incidence of DI and correlation with sex and primary diagnosis were statistically evaluated by Fisher's, χ 2 square tests, Student t test and ANOVA.Scoring system was evaluated by calculating its sensitivity (proportion of difficult tracheal intubations correctly predicted to be difficult), specificity (proportion of intubations correctly predicted to be easy), positive predictive value (proportion of predicted DI which actually proved to be difficult), and ROC curve analysis.
In the majority of patients (84 of 110; 76.4%) with difficult tracheal intubation, visualization of the epiglottis was possible (CL grade III, i.e. groups I and II), and they were intubated in the first (19/84) or second (65/84) attempt (groups I and II).There were 26/2 000 patients (1.3%) in whom epiglottis was not visible during direct laryngoscopy; 11 of them were intubated in the third attempt (group III), and 15 proved to be extremely difficult to intubate (group IV) (Table 4).There were no statisticaly significant difference between four grups of difficult intubation in correlation with primary diagnosis (ANOVA, f = 0.869; p > 0.05).
Most of the patients with EDI had hyperthyreoidism (group C, 1.1%).In patients with polynodal goiter, the incidence of EDI was 0.9% (group B), in those with thyroid carcinoma it was 0.7% (group D), and 0.4% in patients with nodal goiter (group A).In 3/2 000 patients (0.15%; one male), conventional tracheal intubation (CI) was not possible.Two of these three patients had hyperthyreosis and one polynodose goiter.All of them were predicted to be possible DI, and were successfully intubated with fiberscope (FI) (Table 6).
With the scoring system used, 356 patients had positive predictive tests (R > 5).True DI was found in 101 of these patients (28.37%).There were 9 patients (0.45%) in whom  screening tests were negative (R < 5), but DI was present during laryngoscopy (false negative).Sensitivity of used scoring system was 91.8%, whereas specificity was 86.5% (Table 7).

Discussion
According to available literature, the incidence of DI is 1-10%, and DI leading to inability to intubate occurs in 0.04% of population 8 .Importantly, one third of deaths attributable solely to anesthesia is related to DI and failed intubation itself 1 .Reports implicate higher DI rate in obese patients (15.5%) as compared to 2.2% in non-obese patients, together with high risk of desaturation in obese with difficult airway maintenance 9 .Finally, difficult endotracheal intubation is more common in emergency rooms (3-3.5%)than in operating rooms (1.15-3.8%) 10 .
In addition to lack of standardization of preoperative screening tests for DI, there are no reconciled opinions con-cerning definition of DI.Wilson et al. 6 define DI as inability of insertion of endotracheal tube in the first attempt, without any assistance (external pressure on cricoid or thyroid cartilage or any maneuver) or supplementary technique (placing the tube guide).Thus, regardless of fair visualization of the glottis, intubation would be regarded as difficult even if tube was properly inserted in the first attempt, but with help of the assistant or use of tube guide.Classification of Cormack and Lehane, defined in four grades: I -glottis and vocal cords visible, no difficulties; II -partial visualization of glottis (only posterior commissure or arytenoids visible); III -only epiglottis visible; IV -none of foregoing visible, with grades III and IV regarded difficult, is still widely accepted 7 .American Society of Anesthesiologists Task Force Report on the management of difficult airway formulated comprehended practice guidelines and recommendations, where difficult airway is defined as the clinical situation in which a conventionally trained anesthesiologist experiences difficulty with mask ventilation, tracheal intubation, or both 5 .Description of DI included more than three attempts of proper insertion of the tracheal tube with conventional laryngoscopy or required more time than 10 minutes.
In a prospective study of 10.507 patients, where CL criteria of DI were accepted, the authors found that 5.1% patients had grade III visualization of the glottis, and 1% had grade IV 2 .All patients with grade IV were successfully intubated with fiberscope.
Another prospective study of 778 patients showed 9.3% incidence of difficult laryngoscopy (CL III and IV) and intubation without help of the assistant 6 .Decrease of incidence to 5.9% was obtained if external pressure on cricoid cartilage was applied 6 .Savva 11 published data using the same criteria, and revealed 1.14% incidence of DI.
Smith et al. 12 published that the incidence of DI was 1% in general population, and 2-3% in obstetric patients.According to recent study, incidence of DI is 2-8% in North America and 1-4% in UK 13 .In this study, in one of 20 cases of DI, it was impossible to see any part of vocal cords using conventional laryngoscopy (CL grade III and IV).The inci- dence of DI was 1% in general population, and intubation was impossible in 1:2 000 patients.
Our study was focused on population of patients scheduled for thyroid surgery where we defined DI using CL criteria: any intubation with grade III or IV visualization of glottis during direct laryngoscopy, with regard of Wilson's definition where help of assistant or use of supplementary technique was also taken in the consideretion of DI 6,7 .Also, as in ASA criteria of difficult airway management, number of attempts and duration of management were noted 5 .The incidence of DI was 5.5%, and that is comparable with the incidence in general population, as well as with incidence found in the study of Bouaggad et al. 14 .
Higher incidence of difficult intubation was found in two studies dealing with the population of patients scheduled for thyroid surgery.Amathieu et al. 15 published that the incidence of DI in population of patients with goiter was 11.1%, while Adnet et al 16 founded the incidence of DI to be 8%.Amathieu et al. 15 had the highest incidence of DI although they did not have any explanation for that, since they did not found that the presence of palpable goiter as well as one seen on the echography influenced the presence of DI.
In our previous study that included 200 patients with thyroid disorders, frequent anatomical deformities of the airway, and the definition of DI based on Wilson's criteria only, the incidence of DI was as high as 17.5% 17 .Obviously, different institutions and different clinical criteria involved in the definition of the DI could lead to very different results and calculations of its incidence.
We found that male patients scheduled for thyroidectomy had higher incidence of DI and EDI as compared to female patients.Male sex was also found to be a risk factor for DI in the study of Bouaggad 14 .
Data from our study showed that, regarding primary diagnosis, the highest incidence of DI was in the group of patients undergoing thyroid surgery for polynodal goiter and the lowest in the group of patients with nodal goiter.This may occur due to the fact that nodal goiters are smaller and have less pronounced impact on the larynx and trachea position and dislocation.Interestingly, EDI was most frequently present in patients with hyperthyroidism.The incidence of DI was highest in patients with big polynodal goiters, following by thyroid carcinoma, and at the end by small nodal goiters.However, differences in the incidence of DI between the groups did not reach statistical significance.There are very few information in the literature about the incidence of DI in the groups of patients with the different pathology of the thyroid gland, and most of them did not show any association among those two 15 .Malignancy was the only factor that influenced presence of DI 14 .Although previously published data suggested that large goiter was not associated with greater difficulty with intubation, our data show that this claim is not fully correct 14 .In fact, big goiters sometimes do not dislocate larynx or trachea, but rather wrap them diffusely, leaving central position of neck structures unaffected.However, they do cause a dislocation that leads to DI more frequently than other pathological changes of the thyroid gland.
The incidence of EDI or impossible intubation in our study population was a bit higher than the incidence reported in general population (0.12% vs 0.04%) 18 .
Some published studies revealed different specificity of the scoring systems used to predict DI in general population.El-Ganzouri et al. 2 found specificity of their scoring system 84%, Wilson et al. 6 88%, and Arne J et al. 19 96%.However, sensitivity of these scoring systems was rather low, 70%, 75% and 93%, respectively 2,6,17 .For the current study, we have used our scoring system that revealed sensitivity of 91.8% and specificity of 86.5% to predict DI.False negative results were present in only 9 of 2 000 patients (0.45%).ROC curve analysis showed that our scoring system was good diagnostic test with largest number of true positive tests and the least number of false positive results in all confidential intervals.It appears that high sensitivity and specificity of our scoring system proved to be useful in this patient population.
It should be noticed that there are very few studies published on this topic, especially in recent years, most of them were observational and included rather small number of patients.Our study included large number of patients with the different pathology, so we believe that some of our conclusions could be relevant about the incidence of the DI in this patient population and that suggested scoring system could be usefull to everyday practitionars.

Conclusion
The incidence of DI was higest in patients with polynodal goiter but extremly DI was present mostly in patients with hyperthiroidism.Men seem to be at higher risk for DI than women.Our scoring system for prediction of DI may be useful in this patient population.