Discussion
Over an 8-month period of 150 airway procedures, we were able to obtain views by SL adequate for the procedure (grade S1-3) in all but one case (in which failure had been predicted preoperatively and planned flexible bronchoscopy back-up was successful) (8). This high success rate, together with our low complication rates, compares well with series from other centers specializing in laryngeal surgeries (Table 4), and is likely multifactorial in nature. We are the UK referral service for tertiary airway surgery - as such we have a high turnover of cases, including many patients undergoing regular repeated procedures under our care (albeit fewer in this time period due to COVID-19), and we have an experienced surgical, anesthetic and nursing team who regularly work together (1). Other factors which we feel contribute to this high success rate include correct patient positioning (no shoulder roll and head forward position), routine use of supraglottic jet ventilation, infrequent endotracheal intubation and our predilection for the Dedo-Pilling laryngoscope (although it must be stated that use of the Lindholm laryngoscope was not correlated with increased complications) (Figure 1).
Many other documented ‘laryngoscores’, whilst useful in research contexts, are limited in their potential for widespread clinical adoption by the need for multiple time-costly measurements of specific head and neck movements (2-7). We suggest, given that we found patient-demonstrated gross limitations to neck or chin movement to correlate with the surgical view, that the absence of problems with patients pre-operatively demonstrating the appropriate surgical position (i.e. a forward head posture with or without neck extension) is generally sufficiently predictive of procedural success without the need to measure (3, 4, 7). A ‘Mini-Laryngoscore‘ predicting glottic exposure was recently proposed by Incandela et al based solely on thyromental distance, interincisor gap and upper jaw dentition (3). However, interincisor distance and upper jaw dentition did not correlate significantly with our grade of view in this series. The anesthetic literature generally holds that BMI alone is not usually a strong independent predictor of difficult intubation unless extremely high (9) - although BMI and neck and head movement limitations were all significantly correlated with higher grade view in our series, intercorrelation of these variables could represent a confounding effect (10).
Comparing anesthetic and surgical views can provide useful management-changing information in the pre- to intra-procedure transition, such as predicting the likelihood of requiring a senior surgeon to perform the procedure, or to guide appropriate ventilatory choices. We therefore adapted the widely-used MCLS anesthetic grading system for our purposes to make the surgical score more relevant. The anesthetic goal at laryngoscopy is somewhat different as the goal is topicalization rather than to try and achieve the best view possible for intubation; in cases where intubation is considered as first-line, a video laryngoscope would usually be employed as a first choice in these patients. The use of the straight-bladed Dedo, in contrast to the curved anesthetic Mackintosh blade, may further explain the improved view of the surgical team. If faced with a potentially difficult airway, the anesthesia regimen would usually include higher doses of induction agents and muscle relaxant to improve the view for the first attempt.
In this data collection period, we had no incidences of major complications such as severe cardiovascular instability, esophageal perforation or permanent tongue sequelae, incurring only mild temporary complications (Table 3). Reassuringly, most large reported series also report an absence of severe complications (1, 8, 11-19) and the literature on these subjects is limited to isolated case reports (20). The main complication experienced by our patients, in line with other studies (18), was that of temporary sore throat (66% of patients). It can be difficult to unpick complications associated with SL itself versus the procedures that it enables (for example, ablative surgery is likely to cause throat pain in its own right irrespective of SL). Our results showed those patients with wider mouth opening had a significantly lower correlation with sore throat, although age, which also positively correlated with an increased interincisor gap, did not have a significant association with sore throat. This leads us to suggest that interincisor gap may be a useful independent clinical indicator for sore throat, although it must be acknowledged that the Spearman coefficient indicates a very modest effect only (21).
In this time period, 6.7% of patients experienced tongue symptoms, all of which were followed up and seen to resolve by the end of the second postoperative week. This is a relatively low incidence compared to other reported series (8, 12, 13, 15, 17, 19, 22, 23). There was no significant correlation between tongue symptoms and SL duration, however as might be expected, there was a moderate significant positive correlation with macroglossia (r=0.452, p=1.611x10-8). Anecdotally, one patient reported that ‘they always get tongue numbness, and it always resolves’. Given this patient was young with a low BMI, no gross limitations to movement, an average interincisor gap and no macroglossia, this leads us to hypothesize that some patients may be more at risk of developing lingual nerve compression, perhaps due to internal jaw anatomy as others have suggested (24). As with most other reported studies, we saw non-significant associations with gender, although others have suggested a female preponderance (11, 19). Detailed further studies to investigate this could form the basis for future cohort research. In this data collection period, we had no serious dental complications, although there were two cases of temporary exacerbation of existing dental/temporomandibular joint pain. Our team are extremely careful not to lever the laryngoscope on the teeth, but additional care is taken in patients who self-report painful teeth, given this is a potential sign of underlying instability requiring additional vigilance and care (12).
Reassuringly, there was no significant correlation between any investigated complication rate and more difficult views or longer procedural duration, both of which would imply greater cumulative oropharyngeal tissue compression (11, 25). However, other series report associations of longer procedural duration with increased complications (8, 17, 19). Our airway unit treats predominantly benign pathologies and is separate from the Head & Neck unit which manages malignant cases. Our cases therefore tend to have shorter procedural times, and the absence of adjuvant radiotherapy renders our patients’ oropharyngeal tissues less stiff, and dentition more stable, than that seen by the Head & Neck team. This makes comparison with other reported series difficult (1, 8, 12, 15) as most report a mix of benign and malignant cases and have much longer average procedural durations. Our findings of common and uncommon procedural complications therefore pertain specifically to benign laryngology and are not generalizable to mixed or Head & Neck patient populations. In addition, rarer risks (i.e., those with a <0.7% incidence) may not have occurred in our sample size of 150 cases and the consent process should still contain mention of more serious or long-lasting risks reported in the literature, such as tongue weakness.
The major limitation of our work is that this is an unblinded snapshot study, albeit one representative of our practice. Anesthesia regimes also varied throughout the study by anesthetic consultant, and the administered muscle relaxant dose was often incomplete on the electronic documentation system - this factor was therefore subsequently excluded from multivariate analysis. The relationships between relaxant dosage and timing between administration and visualization, and between relaxant type or dose and complication rates, are also important avenues for further study.