Difficult exposure to the larynx, although not a common situation in laryngology, can cause problems for the surgeon, especially if he or she is unprepared, and for the patient at risk of laryngeal injury and a failed procedure. It is important to predict this situation to inform patients about DLE risks prior to surgery and to allow the surgeon to prepare by adding various tools, such as curved instruments and endoscopes, to his or her default setting . Therefore, we conducted this prospective study to establish reproducible measures that can predict DLE.
The term DLE is still not well defined in the literature. Roh et al.12 proposed a classification for laryngeal vision as follows: grade 1: full view of the vocal folds; grade 2A: partial view of the vocal folds, but the anterior commissure is not seen; grade 2B: partial view of the vocal folds (less than half); grade 3: only the arytenoids are visible; and grade 4: the entire glottis and arytenoids are hidden. They defined grades 3 and 4 as DLO. piazza et al.13 defined DLE as visualization of the anterior commissure by a small diameter laryngoscope in the sniff position and external compression or the impossibility of visualization of the anterior commissure and limitation of exposure to the posterior third of the vocal cord. Hekiert et al.14 suggested a visual analog score (VAS) from 1 to 10, rated by the laryngologist, with 1 being the least difficult and 10 the most difficult. While this definition varies according to the authors, the majority agree that exposure limited to the posterior third or less of the vocal cord defines DLE9,12,13,15† We applied the protocol proposed by Hsiung, which consists of evaluating exposure to the larynx after the patient is placed in the sniff position, external back pressure, and changing to a smaller laryngoscope if exposure is not complete.9† Therefore, in this study, DLE was defined as exposure of the larynx limited to the posterior third of the vocal cord after the above efforts.9†
Factors leading to difficult intubation are closely related to factors leading to difficult exposure during suspension laryngoscopy. Therefore, patients who are difficult to intubate are usually considered to be likely DLE candidates8,12,15† Likewise, a high Mallampati score can predict high Cormack-Lehane scores and consequently DLE8.9.15† This was confirmed in this study because we found a strong correlation between high Mallampati and Cormack scores and DLE.
Because these scores depend on anatomical factors such as obesity, restricted mouth opening, macroglossia and retrognathia, we chose to evaluate these factors to discover which are independent predictors for DLE. Therefore, we measured the mouth opening to evaluate restricted mouth opening, as well as the HMD, DTMD, VTMD, HTMD and SMD to evaluate retrognatia and the neck length, neck circumference, weight and BMI and thereby assess the impact of obesity and, especially an excess of tissue in the cervical region when exposed to the larynx. We used all these measurements not only in the neutral position, but also in the position of the head with full extension to optimize other parameters, as we believe that cervical extension capacity plays an important role in predicting DLE. Our results showed that restricted mouth opening (< 49 mm), DTMD < 67 mm in the neutral position, DTMD < 82 mm and SMD < 157 mm at full head extension were statistically related to DLE in the univariate analysis. In the multivariate study, only the DTMD in the neutral position appeared to be correlated with the DLE (p: 0.049) when controlling the HTMD at full extension.
In fact, the results in the literature review of the impact of these physical measurements are variable, and their predictability has been questioned.
In their prospective study of 93 patients undergoing suspension laryngoscopy with 22 cases of DLE, Pinar et al. found that mouth opening was not correlated with DLE8† In another prospective study involving 73 patients with 13 cases of DLO, this correlation was not found12† Some authors suggest that a fully opening mouth is a good predictor for difficult intubation, but not for DLE12.16†
In a multivariate study, Roh et al. found that DTMD < 55 mm and HTMD < 40 mm in a neutral position are correlated with DLE and concluded that retrognathia predicts DLE12† For Pinar et al., this correlation was found with measurement of the HMD and SMD in the full extension position, which are related to the degree of retrognathia and the length of the neck.8† In other studies, however, those measurements were not good predictors for DLE9.15†
According to our results, DTMD appears to be correlated with DLE in controlling the HTMD. This means that for patients without retrognathia, the risk of DLE is higher in patients with a shorter neck. In fact, the HTMD is more specific for the evaluation of retrognathia. Therefore, when the HTMD is the same, the DTMD will essentially depend on the length of the neck.
The statistical analysis of the radiographic parameters showed that the effective maxilla length and atlanto-occipital distance were independent predictors for DLE (p: 0.015 and 0.001, respectively). In fact, the effective maxillary length was significantly greater in the DLE group. The increase in maxillary length reflects overgrowth of the maxilla and results in protruding upper teeth17,18often cited as a cause of difficult direct laryngoscopy18† We believe that this measurement reflects not only an increase in the length of the upper jaw, but also an increase in the mass of the contents. Therefore, an increase in maxillary length can increase the length, thickness, and total surface area of the soft palate and increase the tongue area.
The atlanto-occipital distance was significantly lower in the DLE group than in the control group, with medians of 4 mm and 6.5 mm, respectively, for both groups. Based on the defined cut-off values, a threshold value of 6 mm was established. This distance has largely been used to predict and measure difficult intubation, especially in neutral positions17, and it reflects cervical extension capacity. Limited extension has been correlated with problems with laryngeal exposure. This finding was confirmed in this study, in which the atlanto-occipital distance was significantly shorter in the DLE group than in the control group. different authors17.19 reported a large variation in the atlanto-occipital distance between DLE patients and non-DLE patients, and this parameter was considered an important predictor of difficult laryngoscopy. Nichol et al.19 showed that a decrease in the atlanto-occipital opening or even contact of the posterior tubercle of the atlas with the occipital protuberance could be seen on both neutral and head-extended lateral cervical radiographs and could reduce exposure of the larynx.
Kikkawa et al.10 found that the atlanto-occipital distance, measured on radiographic film performed with the head extended, was lower in the DLE group, with a median of 0.068 ± 0.15 versus 0.114 ± 0.22 in the control group, although the difference between the groups was not significant.
piazza et al.13 analyzed the degree of neck flexion extension by asking the patient to straighten the neck and then measuring the arc from this position to that of full flexion of the neck on the chest. They found that a degree of flexion extension of less than 90° was significantly correlated with DLO. Paul et al.15 measured the atlanto-occipital extension degree and found it to be a good predictor of DLE if less than 19.5° (p: 0.001).
Other factors, including the presence of mandibular tori, a history of radiation therapy, etc., can lead to DLE.
The main limitation of the present study was that the most reliable parameters for difficult laryngeal exposure were based on radiographic measurements, meaning that lateral cervical radiographs are ordered for each patient who will undergo phonosurgery. This examination, while not time consuming or invasive, represents an additional source of radiation exposure for the patient. However, we believe that X-ray examination can effectively predict the risk of difficult exposure to the larynx; therefore, the surgeon can prepare for this event and avoid laryngeal trauma, incomplete resection or even abortion of the procedure by using different modalities such as 30° and 70° endoscopes and angled surgical instruments specially designed for this situation2as well as flexible fibroscopy with two surgeons, as described by Kuang-Chih and Chih-Shin20† Therefore, we believe that allowing the surgeon to perform phonosurgery in the best conditions far outweighs the drawbacks of ordering a radiographic X-ray. We are also well aware that not all hospitals worldwide are equipped with a standard X-ray machine, even though this is considered basic radiological equipment. In these cases, we recommend that this imaging at least be reserved for patients who have clinical predictive factors for possible difficult laryngeal exposure, especially in cases of an anterior lesion (of the anterior commissure and/or the anterior 1/3 of the vocal cords). In these cases, imaging can be performed at the nearest facility equipped with this machine.