Thyromental Height-As a Predictor of Difficult Laryngoscopy

The fundamental responsibility of the anaesthesiologist and one of the most important steps in anaesthesia practice is the intubation and maintenance of the airway. The integral part of preanaesthetic evaluation to recognize a potentially difficult airway is the airway assessment. There are multitude of bedside screening tests which are helpful to predict a difficult airway but the accuracy is doubtful. Thus, pointing out a single reliable predictor of difficult intubation is important. Accordingly, this study aims to evaluate the practicality of thyromental height test alone as a sole predictor of difficult laryngoscopy in our present population. Ethical clearance was obtained and after taking an informed consent, a randomised prospective observational study was conducted on 315 adult patients who were posted for elective surgical procedures under general anaesthesia with endotracheal intubation. On the day before the surgery, airway was assessed and Thyromental height (TMHT) was measured. Laryngoscopy was performed intra-operatively and Cormack Lehane’s grading was noted. The evaluation of the accuracy of thyromental height in predicting difficult laryngoscopy was done by comparing the preoperative assessment data and laryngoscopy findings. In our study, the mean thyromental height observed was 5.4cm. Thyromental height at cut off of 50mm had a high negative predictive value of 94.1% and high sensitivity of 72.5%, but with low specificity of 64.2% (P value 0.000). When the cut off was Original Research Article Gothandaramalingam and Vittobaraju; JPRI, 33(48A): 223-231, 2021; Article no.JPRI.75012 224 emended to 48mm, sensitivity of the test decreased to 56.2% and specificity increased to 79.8% (P value 0.002).The conducted study demonstrates the usefulness of thyromental height. It substantiates the good sensitivity of thyromental height for predicting difficult intubation. But, the validation will require further studies in more diverse patient population.


INTRODUCTION
The fundamental responsibility of the anaesthesiologist and one of the most important steps in anaesthesia practice is the intubation and maintenance of the airway. The most remarkable cause of morbidity and mortality in anaesthesia practice is the failure in managing the airway [1]. All most all of the airway tragedies have occurred only when difficult airway was not anticipated [2]. Faultless prediction of difficult airway, guides the preparation for its management. Serious complications which might result in failed oxygenation has been associated with difficult intubation. Incidence of difficult laryngoscopy is reported in the range of 1.5-20% [3]. According to the American society of anaesthesiologists, brain damage or death has occurred in 85% of airway related events, and anaesthesia due to inability to maintain patent airway hold responsible for 1/3rd of the deaths [4]. Thus in unidentified cases, difficult intubation can be challenging for the anaesthesiologists. The second most frequent proclaimed damaging event is the difficult intubation leading to anaesthesia malpractice claims [5]. There is difficult or impossible mask ventilation in a patient with a difficult airway. This is one of the most critical emergencies faced in anaesthesia practice [6]. The integral part of pre-anaesthetic evaluation to recognize a potentially difficult airway is the airway assessment. Clinical examination and application of simple clinical tests aid in predicting most cases with a difficult airway [7]. Pre-operative prediction of patients with difficult airways can help anaesthesiologist in planning effective way of managing tracheal intubation by implementing special gadgets like fiberoptic bronchoscope, video laryngoscope etc [8,9,10,11]. There is a multitude of bedside screening tests that are helpful to predict a difficult airway but the accuracy is doubtful [12,13,14]. In order to assess difficult laryngoscopy, a range of clinical techniques have been used some of which are not sensitive or specific to anticipate difficult intubation [15,16]. Simple bed side physical examination using a single test or combination of different tests to predict difficult intubation has been tried by innumerable investigators [17,18,19]. The performance of these tests varies substantially between various studies. [20] While single measures are slightly outperformed by multifactorial indices, their accuracy is doubtful in predicting difficult laryngoscopy [18]. Thus, pointing out a single reliable predictor of difficult intubation is important. Thus a test with high sensitivity, specificity, and positive predictive value is essential. A study was done in Iran's population and it has been found out that the thyromental height test (TMHT) alone is an accurate predictor of difficult laryngoscopy [3]. Accordingly, this study aims to evaluate the practicality of the thyromental height test alone as a sole predictor of difficult laryngoscopy in our present population.

METHODOLOGY
After obtaining ethical clearance and taking informed consent, a study was conducted at Saveetha medical college hospital between March 2021 and May 2021 on 400 adult patients who were posted for elective surgical procedures under general anaesthesia with endotracheal intubation. Only the patients fulfilling inclusion criteria were enrolled in this study. It is a prospective observational study. All the patients aged between 18 and 70 years of either sex were included in this study. Patients with obvious airway malformations, requiring awake intubation, history of radiotherapy for head and neck pathology, need for rapid sequence intubation under cricoid pressure, cervical spine pathology were excluded.
On the day before surgery, a thorough preanaesthetic check-up was done. All the demographic data like age, sex, height, body weight, body mass index were recorded. History of medical illnesses, medication, surgical procedures, drug allergy and general physical examination of the patient was considered for patient evaluation. The blood pressure and pulse rate of the patient were also noted. Systemic examination was done and American Society of Anaesthesiologist (ASA) grading was determined. Routine airway assessment was done.
The thyromental height test (TMHT) was performed with the patient lying in the supine position and with his/her mouth closed, the thyromental height that is, the height between the anterior border of the thyroid cartilage and a tangential line drawn from the anterior border of mentum was obtained in the pre-operative room. The height is measured with a depth gauge.
On the day of surgery, the patient was shifted to the operation theatre and an intravenous line was secured. ECG, non-invasive blood pressure, SpO2 monitors were connected and basal vitals were recorded. Inj. Midazolam (0.15mg/kg), and Fentanyl (2mcg/kg) IV were given after preoxygenation for 3 minutes. The induction agent used was Propofol (2mg/kg) I.V. Atracurium (0.5mg/kg) was used for facilitating muscle relaxation. The patients were mask ventilated with 100% O2 for 3 min.
At the end of 3 minutes, with the patient's head in sniffing position, laryngoscopy was performed by a conventionally trained anaesthesiologist using a Macintosh 3 or 4 blade. If no laryngeal view has been achieved, external, backward, upward and right laryngeal pressure (BURP) was applied and the view obtained was assigned a grading (I to IV) according to Cormack-Lehane's (C-L) criteria. The same anaesthesiologist carried out all of these attempts. The anaesthesiologist was unaware of the airway assessments. After confirming bilateral air equity, appropriately sized cuffed oral endotracheal tube was used to intubate the patients.
The accuracy of the thyromental height test in predicting difficult laryngoscopy was evaluated using the preoperative assessment data and laryngoscopy findings. The accuracy of the thyromental height test, the sensitivity, specificity, positive and negative predictive values were calculated.
The study carried out on the thyromental height test for prediction of difficult laryngoscopy has shown that the sensitivity of the thyromental height test when compared with the Cormack-Lehane grading to be 82.6%. Thus in reference to the above findings, with a precision of +/-1%, power of study at 80% and alpha error of 5% the required sample size for the study was estimated to be 400.
Microsoft Excel was used to record the data collected from the study population. All the quantitative variables such as age, thyromental height, weight, etc., were described by employing descriptive statistics. Percentages with a 95% confidence interval were employed to describe the qualitative variables. Various indicators such as sensitivity, specificity, accuracy, positive predictive value, negative predictive value, likelihood ratio and odds ratio were calculated to test the efficacy between the different cut-off points for the thyromental height. The significance of difference for qualitative variables was tested using Fisher exact tests. A 'p' value of less than 0.05 was taken to denote a significant difference. All statistical analyses were performed using SPSS version 18.0(SPSS Chicago).

DISCUSSION
Unexpected difficult airway has always been a major concern for anaesthesiologists [21]. Serious complications particularly failed oxygenation has occurred due to difficult intubation [22,23]. As per ASA closed claim audit, the significance of difficult or failed tracheal intubation is well acknowledged as a major cause of morbidity and mortality in anaesthetic practice [24].
Numerous anatomical characteristics and pathological conditions like Pierre Robin syndrome and Ludwig's angina have been suggested to be useful in assessing predicted difficult intubation by altering or distorting the regional anatomy of the airway [25,26,27,28].
Unexpected difficult intubation is a risk to the patient's life and is a challenge to the anaesthesiologist [29]. In the absence of pathological conditions, radiographic methods cannot be used routinely for the prediction of difficult intubation as it is time-consuming [30,31].
The necessity to anticipate potentially difficult tracheal intubation with an accurate marker, even before laryngoscopy, has received more significance but the success is limited [32]. Preoperative prediction of difficult airway helps not only in managing the intubation time better, but also decreases morbidity related to airway management [33,34,35].
It is seen that weight, jaw movement, head and neck movement, buck teeth, receding mandible, Modified Mallampati classification, sternomental distance, thyromental distance, mouth opening and Wilson risk score are commonly used, but are not foolproof to anticipate difficult intubation [36,37,38].
Thus, there is a necessity for a test, which is (a) easy and quick to perform, (b) highly sensitive (so that most of the difficult airway cases can be identified), (c) highly specific (so that false positive rate will be low when the test is used routinely), (d) have the least false positive and false negative values [39,40]. A single standard method is still not available that meets the criteria or a consensus regarding the reliability of the ideally preferred tests [41,42]. Lately, the thyromental height test (TMHT) has been put forward as one of the highly sensitive and specific bed side tests to predict difficult airways [3].
Etezadi et al. [3] substantiated the use of the thyromental height test. They proposed that, there is a close relation between small thyromental height and the occurrence of difficult laryngoscopy. In their study, an optimal cut-off value of 50mm was chosen. They observed that the test had high sensitivity, high specificity, high PPV, high NPV and high accuracy (82.66%, 99.31%, 90.47%, 98.63% and 98.08%) respectively at 50mm cut off. We also took a cutoff value of 50mm at which sensitivity, specificity, PPV, NPV and accuracy are (72.5%, 64.2%, 22.8%, 94.1% and 65.2%) respectively. In our study, 162 patients had TMH < 50mm and 238 patients had > 50mm. 12.6% of patients had difficult laryngoscopy (C-L grade 3 and 4) and 87.4% of patients had easy laryngoscopy(C-L grade 1 and 2). The sensitivity and NPV results of our study are comparable to the study conducted by Etezadi et al, but specificity, PPV and accuracy are remarkably lower.
An optimal cut-off value of 48 mm was chosen in our study. At this cut-off, the test had a sensitivity, specificity, PPV, NPV, and accuracy of 56.2%, 79.8%, 27.5%, 93.0%, and 77.0% respectively. At 48mm, specificity, NPV and accuracy results of our study are comparable to Etezadi et al study. But, sensitivity and PPV are comparatively less. We found that 97 patients had TMH <48mm and 303 patients had TMH >48mm. In the total 400 patients studied, true positives, false positives, true negatives and false negatives detected by the test are 27, 71, 281, and 21 respectively.
In contradiction to Etezadi et al study, we couldn't substantiate the same efficiency of the test at 50mm or 48mm, however the test has high sensitivity, high NPV at 50mm, high specificity, high NPV at 48mm and less PPV at 50mm and 48mm.
Our study is almost identical to the study done by Nilesh et al. [43] in sensitivity and NPV. But specificity, PPV, and accuracy are lower at 50mm cut off. Nilesh et al studied the efficacy of the thyromental height test in patients undergoing coronary bypass graft surgery. In their study, they had taken a cut-off value equal to 50mm. On 345 patients, 9.3% had difficult laryngoscopy and 90.7% patients had easy laryngoscopy. They found that 33 patients had TMH <50mm and 312 patients had >50mm. True positive, false positive, true negative, and false negative results detected by the test were found to be 24,9,304, and 8, respectively. It showed sensitivity of 75%, specificity of 97%, PPV of 73%, NPV of 97% and accuracy of 95%. In our study, at cut off value of 48mm, the results obtained are all similar to Nilesh et al study at 50mm. But, PPV is lesser in our study.
We are able to verify the efficiency of TMHT at 50mm, with respect to sensitivity and NPV, but the specificity of the test is better at 48mm which is similar to Nilesh et al study. But in contradiction to their study, PPV is poor in our study at 50mm and 48mm.
Selvi et al. [44]  They found the best compromise between sensitivity and specificity when they revised the cut off value of TMHT to 43.5mm. Sensitivity (64.86%), specificity (78.02%), PPV (20.87%) and NPV (96.13%) at this cut off are also comparable to our revised cutoff of 48mm. In their study among 451 patients, the test detected 24 true positives, 91 false positives, 323 true negatives, 13 false negatives at 43.5mm cut off. In comparison to Etezadi et al study, they couldn't verify the same efficiency of TMHT at either 50mm or 43.5mm. However, it could be verified with our study at both 50mm and 48mm cut off values.
As there are several anatomical variations of the airway with respect to age, sex and body habitus, there occur varying results at different cut-off points compared to earlier studies. The efficacy of TMHT to predict difficult laryngoscopy also relies on the patient's race and variations in airway anatomy. It is a must to mention that the results might be affected by anatomical differences and measurement errors.
Sensitivity is better at 50mm cut-off but specificity and accuracy are better at 48mm on comparison of the efficacy of TMHT at 50mm and 48mm. As we are concerned with difficult laryngoscopy more than easy laryngoscopy, the test which detects more true positives and fewer false negatives than more true negatives has more clinical applicability. In accordance with our study, a TMH value less than 50 mm can be used as an early warning system to alert the clinician about the probability of difficult intubation due to high sensitivity of 72.5%. This will point out most patients in whom intubation would be difficult in reality. As the NPV value is 94.1%, it can be predicted that the patients with TMH values greater than 50 mm will have easy intubation.
No morbidity, mortality or complication was recorded among the study population short after the end of general anaesthesia.

CONCLUSION
Airway screening tests should be simple, convenient and practical. Despite the fact that their validity is variable and their ability to predict unexpected difficult intubations remains restricted, they are a crucial tool for anaesthesiologists. This study demonstrates the practicality of TMHT in predicting difficult intubation which has good sensitivity and negative predictive value. But validation of this study would require further studies in the more diverse patient populations with regard to age, sex and race which may result in more revealing results. Further studies with a larger sample size and different methodology concerning age, sex and race dependent variables are required to evaluate this test.

ETHICAL APPROVAL AND CONSENT
After obtaining ethical clearance and taking informed consent, a study was conducted at Saveetha medical college hospital between March 2021 and May 2021 on 400 adult patients who were posted for elective surgical procedures under general anaesthesia with endotracheal intubation