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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 4  |  Issue : 3  |  Page : 185-190

Evaluation of upper lip bite test and thyromental height test for prediction of difficult laryngoscopy: A prospective observational study


Department of Anaesthesiology, Kokilaben Dhirubhai Ambani Hospital and Research Centre, Mumbai, Maharashtra, India

Date of Submission23-Aug-2021
Date of Acceptance05-Oct-2021
Date of Web Publication15-Nov-2021

Correspondence Address:
Dr. Aparna Date
Department of Anaesthesiology, Kokilaben Dhirubhai Ambani Hospital and Research Centre, Four Bungalows, Andheri (West), Mumbai - 400 053, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/arwy.arwy_48_21

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  Abstract 


Background and Aims: Unanticipated difficult laryngoscopy (DL) can cause life-threatening complications in the operation theatre, intensive care unit and emergency department. Various screening tests have been developed to predict DL. While modified Mallampati test (MMT) is the most commonly used one, it has poor sensitivity. Upper lip bite test (ULBT) and thyromental height test (TMHT) have been shown to be more reliable in predicting DL. The aim of this study was to compare the three tests and evaluate whether a combination of ULBT and TMHT would show better accuracy in predicting DL. Patients and Methods: A total of 109 adult patients undergoing elective surgery under general anaesthesia with endotracheal intubation were assessed using the MMT, ULBT and TMHT before surgery. The required sample size for the study was calculated using the Fisher Z test. The three tests, and a combination of ULBT with TMHT, were compared for their sensitivity, specificity, positive predictive value, negative predictive value and accuracy in predicting DL. Results: The accuracy of MMT, ULBT, TMHT and ULBT + TMHT in predicting DL was found to be 81.65%, 86.23%, 97.24% and 95.41%, respectively. MMT exhibited the lowest sensitivity and had the highest number of false negatives. TMHT exhibited the best accuracy and sensitivity, with the lowest number of false negatives. A combination of ULBT and TMHT did not improve the PPV in the sample tested. Conclusion: TMHT was found to be the best test for predicting DL, and its combination with ULBT did not improve the accuracy.

Keywords: Difficult laryngoscopy, modified Mallampati test, thyromental height test, upper lip bite test


How to cite this article:
Bhanushali A, Date A. Evaluation of upper lip bite test and thyromental height test for prediction of difficult laryngoscopy: A prospective observational study. Airway 2021;4:185-90

How to cite this URL:
Bhanushali A, Date A. Evaluation of upper lip bite test and thyromental height test for prediction of difficult laryngoscopy: A prospective observational study. Airway [serial online] 2021 [cited 2022 Jan 27];4:185-90. Available from: https://www.arwy.org/text.asp?2021/4/3/185/330492




  Introduction Top


Unanticipated difficult intubations can cause airway trauma, oesophageal injury, aspiration, need for surgical airway, brain damage and death.[1] Prior prediction of a difficult airway allows us to be prepared with the appropriate equipment, experienced personnel and a plan for an alternative airway management strategy.

Although bedside screening tests are often used alone or in combination to predict difficult laryngoscopy (DL), a perfect airway assessment tool does not exist as yet.[2] Modified Mallampati test (MMT) remains the most frequently used test for airway assessment. However, a large meta-analysis has shown that this test is not adequate as a stand-alone test for predicting DL.[3] Upper lip bite test (ULBT) evaluates the range and freedom of mandibular movement and the architecture of the teeth. These factors play a key role in facilitating laryngoscopic intubation.[4] Thyromental height test (TMHT) measures the distance between the anterior borders of the mentum and the thyroid cartilage, with the patient in supine position and the mouth closed. This is thought to be a more reliable predictor of DL than the MMT since quantitative measurements eliminate interobserver variability.[5]

The primary objective of this study was to compare the accuracy, reliability and sensitivity of these three tests in predicting DL. We also sought to assess whether a combination of the ULBT and TMHT would improve the accuracy of airway assessment.


  Patients and Methods Top


This was a prospective observational, single-centre study, conducted over 10 months. The study protocol was approved by the Institutional Scientific and Ethical Board (ISEB). Since this was an observational study, consent waiver was granted by the ISEB. The trial was registered with the Clinical Trial Registry, India (Registration Number CTRI/2017/11/010602).

Patients over the age of 18 years undergoing elective surgery under general anaesthesia with endotracheal intubation were selected for the study. Patients, who were pregnant, those undergoing emergency surgery, those exhibiting altered level of consciousness, those uncooperative or unable to follow commands and those in whom awake fibreoptic intubation was planned were excluded from the study.

The Fisher Z test was used to calculate sample size required for the study. The expected incidence of DL is reported to be approximately 11.3%.[4] Allowing for a 6% variation, at a 95% confidence level, the required sample size was calculated to be 109 patients. This was further verified using the Statistical Analysis System (SAS) 9.2 software (SAS Institute, NC, USA), which recommended a minimum sample size of 97 patients (supplementary data).

Demographic information (such as age, height, gender, weight and body mass index [BMI]), history of medical conditions and investigations were recorded. Patients were given tablet pantoprazole 40 mg and their routine medications on the morning of surgery. Patients were kept nil per oral 6 h prior to surgery. The airway was evaluated using MMT, ULBT and TMHT in the preoperative holding area and the findings noted by the principal investigator (PI) who was not involved in laryngoscopy and intubation of the cases. The above parameters were evaluated as follows:

Modified Mallampati test

The patient was asked to open the mouth wide with head in neutral position and the airway was classified as described by Mallampati et al.[6] Modified Mallampati classes III and IV were predictive of DL.

Upper lip bite test

Patients were asked to bite the upper lip with the lower teeth, and the patients were graded as Grade 1, 2 and 3 as described by Khan et al.[4] ULBT Grade 3 was a predictor of DL.

Thyromental height

The height between the anterior border of the thyroid cartilage (on the thyroid notch just between the 2 thyroid laminae) and the anterior border of the mentum (on the mental protuberance of the mandible) of the patient in supine position with their mouth closed was measured using two rulers placed vertically over the two anatomical reference points. TMHT <5 cm was predictive of DL.[5]

In our proposed approach which combines ULBT and TMHT, a laryngoscopy was predicted to be difficult if either of the tests suggested a possibility of DL.

In the operation theatre, the electrocardiogram, blood pressure, pulse oximetry and capnography monitoring were established. In addition, inspired and expired oxygen concentration along with inhalational agent monitoring was performed. Airway pressure, tidal volume and respiratory rate were also monitored. An intravenous (IV) line was secured and a balanced salt solution was started. Patients were administered glycopyrrolate (0.004 mg/kg), midazolam (0.02 mg/kg) and fentanyl (2 μg/kg) intravenously. After preoxygenation, anaesthesia was induced with IV propofol (2 mg/kg) till the loss of eyelash reflex. After confirming mask ventilation, IV atracurium (0.6 mg/kg) was administered and mask ventilation was continued till all four twitches of the train-of-four on the peripheral nerve stimulator disappeared. Before laryngoscopy, the patient's head was placed in a sniffing position using a head ring of standard size, and the operating table was placed at the level of the umbilicus of the anaesthesiologist. Laryngoscopy was done with a #3 or #4 Macintosh blade by an anaesthesiologist with a minimum experience of 2 years. The anaesthesiologist was blinded to the values of the ULBT and TMHT recorded earlier.

Laryngoscopy view was graded using the Cormack-Lehane (CL) grading without the application of any external pressure.[7] Once the grading was done, the anaesthesiologist was allowed to use external manoeuvres before performing the intubation. CL classes III and IV were considered as DL. Patients were intubated with an appropriate-sized endotracheal tube. Successful intubation was confirmed by auscultation over lung fields and waveform capnography. The CL grades recorded by the anaesthesiologist performing the intubation were recorded in the anaesthesia chart and also noted in the study pro forma.

Data entry was done in Microsoft Excel 2016 and analysed using the SAS 9.2 software. The preoperative assessment data and the laryngoscopic findings were compared to assess the predictive value of each test. Diagnostic efficacy was compared by evaluation of the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy.


  Results Top


This was a prospective observational study that aimed to evaluate the efficacy of MMT, TMHT and ULBT in predicting DL. A total of 109 patients satisfying the inclusion criteria were enrolled in the study. The patient demographics are given in [Table 1].
Table 1: Patient demographics

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Following laryngoscopy and intubation, 62 (56.9%) patients were categorised as CL Grade I, 31 (28.4%) as CL grade II, 12 (11.0%) as CL grade III and 4 (3.7%) as CL grade IV. The incidence of DL (CL grades III and IV) in our study was found to be 14.7%.

Of the 109 patients, 38 (34.9%) were MMT Class I, 65 (59.6%) were MMT Class II, 6 (5.5%) were MMT Class III and none belonged to MMT Class IV. MMT predicted only 6 patients (5.5%) to have DL. The ULBT categorised 93 patients (85.3%) as Grade 1, 11 (10.1%) as Grade 2 and 5 (4.6%) as ULBT Grade 3. This test too predicted only five patients (4.6%) to have DL. Ninety-six patients (88.1%) had a TMH > 5 cm and 13 (11.9%) had a TMH < 5 cm. Therefore, 13 (11.9%) patients were predicted to have DL based on this test. When both ULBT and TMHT were used to predict DL, 15 patients (13.8%) were predicted to have DL. The values of the tests that were studied are summarised in [Table 2] with their statistical significance summarised in [Table 3].
Table 2: Analysis of predicted and actual difficult laryngoscopies

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Table 3: Statistical evaluation of predictive tests

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It can be seen from [Table 3] that TMHT alone had a sensitivity of 81.25% and NPV of 96.87%. These values did not improve by adding the ULBT to the screening tests. On the other hand, specificity and PPV of the screening tool reduced to 97.85% and 86.66%, respectively [Table 3]. Higher false-positive rate of ULBT as compared to TMHT [Table 2] resulted in lower specificity. There was no reduction in the rate of false negative by adding ULBT to TMHT.


  Discussion Top


The All India Difficult Airway Association guidelines for difficult airway management recommend a thorough preoperative airway assessment and the use of equipment such as transnasal humidified rapid insufflation ventilatory exchange or emergency cricothyrotomy for a difficult airway scenario.[8] It is therefore essential to be able to anticipate a difficult airway and be prepared with appropriate equipment and experienced personnel. Adequate glottic exposure requires the oral, pharyngeal and laryngeal axes to be aligned. This is usually achieved by a slight flexion of the cervical spine and the extension at the atlanto-occipital joint. Failure of this alignment results in inadequate glottis exposure.[9]

Daggupati et al. have proposed a dynamic phase of laryngoscopy wherein a laryngoscope is introduced into the vallecula and the epiglottis is lifted after displacing the submandibular tissues to allow visualisation of the larynx. This phase of direct laryngoscopy is affected by temporomandibular (TM) joint movement, angulation of the epiglottis and the compliance and volume of the submandibular space.[10] MMT takes into account the size of the tongue and the oropharynx and their relationship with each other. ULBT evaluates TM joint movement, while TMHT acts as surrogate for the submandibular space. We therefore postulated that using a combination of ULBT and TMHT may improve the prediction of DL.

A Cochrane Review of screening tests used to predict DL found the incidence of CL grades 3 and 4 to be 11%.[2] The incidence of DL in this review was found to be 14.7%. The MMT is commonly used in preoperative evaluation because it is quick and easy to perform. Our study found that the PPV of MMT was 16.67% and the false negatives were 13.8%. False-negative predictions are of greatest concern to the airway operator as they can have life-threatening consequences. This suggests that relying solely on MMT to predict DL can result in a significant number of patients with unanticipated difficult airway. A meta-analysis of 55 studies has also shown that MMT is a poor predictor of DL.[3] Apart from interobserver variability, lack of patient cooperation with maximal mouth opening and inadvertent tongue protrusion or phonation during the test may result in inaccurate results.[11]

ULBT has been found to be more sensitive than the MMT in predicting DL.[2] ULBT assesses TM joint function and the prognathic ability, as well as the prominence of maxillary teeth. Khan et al. have found a PPV of 100% with a Class 3 ULBT.[4] Our study found the PPV of ULBT to be 60%, with 11.9% false negatives.

TMHT, proposed as a single test for the prediction of DL, is an objective test that measures the height between the thyroid cartilage and the mentum. It is postulated that this acts as a surrogate for anthropometric measures including mandibular protrusion, dimensions of the submandibular space and the anterior position of the larynx. A cutoff of 50 mm has been suggested for prediction of DL.[5] The original study comparing TMHT with thyromental distance (TMD) and sternomental distance (SMD) found that TMHT exhibited better sensitivity in predicting DL (82.6%) than TMD (21.73%) and SMD (13.04%). TMHT also had the lowest number of false negatives (4%).[5] Our study found the PPV of TMHT to be 100%, with 2.8% false negatives. TMHT was found to give the highest accuracy and sensitivity as well.

In an attempt to improve the accuracy of screening tests, investigators have tried to combine two or more tests.[12],[13] ULBT and TMHT assess different aspects of the airway and are objective measures. Therefore, we evaluated the accuracy of ULBT and TMHT used together in predicting DL. Patel et al. combined MMT with SMD and TMD and found that the sensitivity of prediction of DL went up from 27% for MMT alone to 100% for the combination.[13] However, SMD and TMD require full head extension which may not be possible in patients with cervical spine problems. Gender-related differences are seen in SMD but not in TMHT, although the height and weight do differ with gender.[5],[14],[15]

We found TMHT to be the most accurate test in predicting DL. However, the addition of ULBT to it did not reduce the number of false negatives; instead, it reduced the specificity and accuracy of the test. ULBT requires patient cooperation which may not be possible in unconscious or uncooperative patients.

Other studies have also found TMHT to be a single reliable predictor of DL.[16],[17] However, Yabuki et al. found that TMHT alone does not reliably predict DL in the Japanese population. They point out that most of the studies evaluating TMHT are based in the South and West Asian population and may not be equally applicable to patients of other ethnicities due to differences in cranial measurements.[15] Using backward upward rightward pressure (BURP), the incidence of DL in their study was 1%. However, their sample size was calculated anticipating a DL of 6%. A larger sample size would be needed for reliable results. Larger studies in patients of different ethnicities will need to be conducted to evaluate TMHT.

It was not the objective of this study to evaluate these screening tests in patients with high BMI. However, there were 26 patients with a BMI above 30 kg/m2 and TMHT served as an accurate predictor of DL in them. MMT predicted easy intubation in two patients with a BMI ≥35 kg/m2. However, TMHT accurately predicted DL. Further studies are required to assess whether TMHT is a more accurate predictor of DL than MMT in obese patients.

A limitation of our study was that patients with restricted mouth opening were not included in the study since they were to undergo awake fibreoptic intubation. In patients with trismus or oral growths, MMT is indispensable to predict DL. Second, BURP was not used in this study while determining the CL score. DL should ideally be reported following the use of BURP as it reduces the incidence of DL and hence the PPV of a screening test.


  Conclusion Top


TMHT was found to be a better predictor of DL than ULBT and MMT. The addition of ULBT to TMHT did not improve its accuracy in predicting DL. ULBT and TMHT were both found to be reliable predictors of easy intubation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Apfelbaum JL, Hagberg CA, Caplan RA, Blitt CD, Connis RT, Nickinovich DG, et al. Practice guidelines for management of the difficult airway: An updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiology 2013;118:251-70.  Back to cited text no. 1
    
2.
Roth D, Pace NL, Lee A, Hovhannisyan K, Warenits AM, Arrich J, et al. Airway physical examination tests for detection of difficult airway management in apparently normal adult patients. Cochrane Database Syst Rev 2018;5:CD008874.  Back to cited text no. 2
    
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Lundstrøm LH, Vester-Andersen M, Møller AM, Charuluxananan S, L'hermite J, Wetterslev J, et al. Poor prognostic value of the modified Mallampati score: A meta-analysis involving 177 088 patients. Br J Anaesth 2011;107:659-67.  Back to cited text no. 3
    
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Khan ZH, Maleki A, Makarem J, Mohammadi M, Khan RH, Zandieh A. A comparison of the upper lip bite test with hyomental/thyrosternal distances and mandible length in predicting difficulty in intubation: A prospective study. Indian J Anaesth 2011;55:43-6.  Back to cited text no. 4
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Myatra SN, Shah A, Kundra P, Patwa A, Ramkumar V, Divatia JV, et al. All India difficult airway association 2016 guidelines for the management of unanticipated difficult tracheal intubation in adults. Indian J Anaesth 2016;60:885-98.  Back to cited text no. 8
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Seo SH, Lee JG, Yu SB, Kim DS, Ryu SJ, Kim KH. Predictors of difficult intubation defined by the intubation difficulty scale (IDS): Predictive value of 7 airway assessment factors. Korean J Anesthesiol 2012;63:491-7.  Back to cited text no. 11
    
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Khan ZH, Arbabi S. Diagnostic value of the upper lip bite test in predicting difficulty in intubation with head and neck landmarks obtained from lateral neck X-ray. Indian J Anaesth 2013;57:381-6.  Back to cited text no. 12
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Patel B, Khandekar R, Diwan R, Shah A. Validation of modified Mallampati test with addition of thyromental distance and sternomental distance to predict difficult endotracheal intubation in adults. Indian J Anaesth 2014;58:171-5.  Back to cited text no. 13
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Yabuki S, Iwaoka S, Murakami M, Miura H. Reliability of the thyromental height test for prediction of difficult visualisation of the larynx: A prospective external validation. Indian J Anaesth 2019;63:270-6.  Back to cited text no. 15
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Rao KV, Dhatchinamoorthi D, Nandhakumar A, Selvarajan N, Akula HR, Thiruvenkatarajan V. Validity of thyromental height test as a predictor of difficult laryngoscopy: A prospective evaluation comparing modified Mallampati score, interincisor gap, thyromental distance, neck circumference, and neck extension. Indian J Anaesth 2018;62:603-8.  Back to cited text no. 16
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