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ORIGINAL ARTICLE
Year : 2021  |  Volume : 22  |  Issue : 1  |  Page : 47-52
 

Comparison of modified Mallampati test and thyromental height test for preoperative airway assessment: A prospective observational study


1 Department of Anesthesia, T.N.M.C. and B.Y.L. Nair Hospital, Mumbai, Maharashtra, India
2 Department of Anesthesia, Pgimer, Chandigarh, India
3 Department of Anaesthesia, Critical Care and Pain, Tata Memorial Center, Homi Bhabha National Institute, Mumbai, Maharashtra, India

Date of Submission19-Jul-2020
Date of Acceptance28-Aug-2020
Date of Web Publication22-Feb-2021

Correspondence Address:
Dr. Ketan K Kataria
Department of Anaesthesia, Critical Care and Pain, Tata Memorial Center, Homi Bhabha National Institute, Mumbai, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/TheIAForum.TheIAForum_112_20

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  Abstract 


Background: Prevision of a potentially difficult airway in the preoperative period is imperative. The available tools are evaluation of mouth opening, Mallampati test; atlanto-occipital extension; hyomental, thyromental, and sternomental distances; and upper lip bite test; thyromental height test (TMHT) is a new indicator.
Aims: To compare the effectiveness of preoperative anaesthetic airway evaluation methods of TMHT and Modified mallampati test (MMT) to predict the difficulty in intubation.
Materials and Methods: A prospective observational study was aimed to compare the effectiveness of preoperative anesthetic airway evaluation methods of TMHT and modified Mallampati test (MMT) to predict the difficulty in intubation. A total of 150 subjects were included in this study. MMT and TMHT were compared, and sensitivity, specificity, predictive values, and accuracy were calculated.
Results: On comparison, we found TMHT to be more sensitive (93.33%) than MMT (77.78%); both tests have high specificity (TMHT 91.43%; MMT 81.90%). Positive predictive value was 82.35% for TMHT and 64.81% for MMT. Similarly, negative predictive value was 96.97% for TMHT and 89.58% for MMT. Accuracy was 92.00% for TMHT and 80.66% for MMT.
Conclusions: TMHT can predict difficult intubation better than MMT (high positive predictive value). However, both TMHT and MMT predict easy intubations effectively, their negative predictive values being high.


Keywords: Difficult airway, intubation, modified Mallampati test


How to cite this article:
Chhatrapati S, Bloria S, Singh N, Paul S, Luthra A, Kataria KK, Vithani S, Omar S, Narayanan Nayanar V K. Comparison of modified Mallampati test and thyromental height test for preoperative airway assessment: A prospective observational study. Indian Anaesth Forum 2021;22:47-52

How to cite this URL:
Chhatrapati S, Bloria S, Singh N, Paul S, Luthra A, Kataria KK, Vithani S, Omar S, Narayanan Nayanar V K. Comparison of modified Mallampati test and thyromental height test for preoperative airway assessment: A prospective observational study. Indian Anaesth Forum [serial online] 2021 [cited 2021 May 9];22:47-52. Available from: http://www.theiaforum.org/text.asp?2021/22/1/47/309742





  Introduction Top


Secure airway management is mainspring of efficacious anesthesia conduct and is of prime pertinence to the anesthesiologist. Unexplained difficult intubation can prove to be taxing and troublesome. Of all the claims against anesthesiologists, major percentage involved difficult or impossible intubation.[1] In anesthetized patients, failed endotracheal intubation can cause appreciable morbidity and mortality. Complications related to difficult airway range from sore throat to airway trauma, aspiration, and esophageal injury.[2],[3] Beforehand identification of patients who may prove difficult to intubate is enticing and provides a scope to prepare for foreseen difficult intubation. The incidence of failed intubation is approximately 0.05% or 1:2230 in surgical patients.[4],[5],[6],[7],[8] The incidence for difficult laryngoscopy and difficult intubation is 9.7% and 4.5%, respectively, in Indian patients with apparently normal airways.[9]

Modalities such as airway imaging and radiography have been proposed to predict the likelihood of difficult intubation. However, these are cumbersome and the cost involved limits their widespread use as screening tests.[10] Reliability of special techniques such as acoustic reflectometry is dubious.[11] Better quantitative and noninvasive measurements such as the laryngeal caliper and bubble inclinometer render the prospective for more precise measurements, but have been sparingly used in clinical practice.[12],[13] Contrarily, different bedside difficult airway predictors such as atlanto-occipital extension; Mallampati test; upper lip bite test (ULBT); thyromental, thyrosternal, hyomental, and sternomental distances; neck circumference; and mandibular length are easily performed and bear no extra cost or inconvenience to the patient. Dismally none of these tests predict difficult intubation accurately and the quest for a single predictive test that is veracious, precise, and easily applicable sticks around.

Modified Mallampati test (MMT) introduced way back, has stood the test of time, and is still consistently being used for preoperative airway assessment, despite multiple new tests coming up. However, it has a low sensitivity and has a poor prognostic value.

In 2013, Etezadi et al. proffered a new test, the thyromental height test (TMHT) – a simple bedside test involving the assessment of the height between the anterior borders of the thyroid cartilage and the mentum, with the patient lying supine with mouth closed.[14]

Multiple studies favor development of multifactorial indices and evince them to be better than a single anatomical landmark for the prediction of difficult intubation.[15],[16] Nonetheless, we need a solitary test that is simple, precise, and dependable predictor of difficult airway. Hence, we chose to study TMHT, which is contemporary test for predicting difficult intubation and has shown propitious results.[14] The purpose of this study was to compare MMT and THMT to predict difficult intubation with direct laryngoscopy views, i.e., Cormack Lehane grading obtained during tracheal intubations.


  Materials and Methods Top


After ethical committee approval, the present prospective observational study was conducted in a tertiary health-care center in India on 150 patients of either sex undergoing elective surgery under general anesthesia with endotracheal intubation. The study was conducted over 1½ years. Patients recruited were >18 years of age and of ASA physical status Class I–III. Written informed consent was taken from all participants. Exclusion criteria included patients who were uncooperative, who refused to give consent, those undergoing emergency surgery, those with restriction in mobility of mandible or neck, those with fractures of cervical spine, abnormality of the airway or any obvious neck pathology, those with history of difficult intubation, inability to sit or open the mouth, and altered level of consciousness. The necessary sample size was estimated to be 139 with a 95% confidence interval and 80% power and alpha level of 0.05, assuming the incidence of difficult intubation to be 10% based on previous studies and a pilot study.[14] Taking into consideration an attrition rate of 10%, the final sample size was rounded off to 150. A comprehensive preanesthetic checkup was done and routine preoperative investigations relevant to the surgery were sought. A detailed procedural explanation was given to all patients.

Preoperatively, the airway was assessed using both MMT and TMHT. For MMT, the patients were made to sit at the observer's eye level with mouth fully open, tongue protruded maximally, and the patient not phonating.[5] The MMT grades were categorized into:

  • Class I: Visualization of pillars, fauces, soft palate, and uvula
  • Class II: Visualization of fauces, soft palate, and uvula
  • Class III: Visualization of soft palate and base of uvula
  • Class IV: Visualization of hard palate only
  • Class 0: Ability to see any part of the epiglottis upon mouth opening and tongue protrusion


Class III and IV were considered potential difficult intubations.

Thyromental height test

The patient was placed in supine position with mouth closed. The height between anterior border of thyroid cartilage (on thyroid notch in between the two laminae of the thyroid cartilage) and anterior border of mentum (on mental protuberance of mandible) was measured using a caliper [Figure 1]. Thyromental height of <5 cm was considered as potential difficult intubation.[14]
Figure 1: Thyromental height test image

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On the day of surgery, fasting status of the patient was confirmed. In operation theater (OT), standard ASA monitors in the form of electrocardiogram, noninvasive blood pressure (NIBP) and pulse oximeter (SPO2) were attached. Difficult intubation cart was kept ready. Intravenous (i.v.) access was secured. Premedication included injection ranitidine 1 mg/kg i.v., injection ondansetron 0.08 mg/kg i.v., injection glycopyrrolate 0.005 mg/kg i.v., injection midazolam 0.03 mg/kg i.v., and injection fentanyl 2 μg/kg i.v. The patient was preoxygenated and induced with titrated doses of injection propofol 1.5–2.5 mg/kg i.v. till the loss of verbal response. Ventilation was checked and injection vecuronium 0.1 mg/kg i.v. was administered. The patient was ventilated for 3 min with oxygen, air (50:50), and sevoflurane and then for 1 min with 100% oxygen.

Laryngoscopy was performed using a Macintosh laryngoscope by a single senior anesthesiologist having at least 3 years of experience in anesthesia. Glottis was viewed and graded using the Cormack and Lehane (CL) grading.[7] No external pressure or maneuvers were applied to optimize the view. CL grading is as follows:

  • Grade 1: Full view of the glottis
  • Grade 2a: Partial view of the glottis
  • Grade 2b: Arytenoids or posterior part of the vocal cords just visible
  • Grade 3: Only epiglottis visible
  • Grade 4: Neither glottis nor epiglottis visible


Grade 3 and 4 were considered as difficult intubation. The patient was intubated with appropriate-sized endotracheal tube. Successful intubation was confirmed by auscultation of bilateral lung fields and capnography.

Number of attempts required for successful intubation, number of failed attempts, and requirement of any special maneuvers or equipment used for intubation were recorded.

Vital parameters such as pulse, NIBP, and SPO2 were continuously monitored and recorded at following time intervals: baseline, after premedication, after induction, and after intubation.

Statistical analysis:

Data analysis was done using (IBM Corp. IBM SPSS Statistics for Windows, version 17.0. Armonk, NY: IBM Corp). Data from preoperative assessment and findings from laryngoscopy were used to assess the predictive value of MMT and TMHT for difficult laryngoscopy. Representation of qualitative data was done in the form of frequency and percentage, while quantitative data were represented using mean, standard deviation, median, and interquartile range. A pilot study was done before the actual study. On the basis of clinical experience, previous studies and the pilot study conducted and estimating the incidence of difficult airway to be 10% a sample size of 139 was drawn with a 95% confidence interval, 80% power, and alpha level of 0.05. Taking into consideration an attrition rate of 10%, the final sample size was rounded off to 150.[14]


  Results Top


Out of the 150 patients studied, 80 were male and 70 were female. The mean age of patients in the study was 36.75 ± 11.13 years, while the mean weight was 55.23 ± 36.75 kg. Seventy-five patients (50%) were of ASA Class I, 67 patients (45%) belonged to ASA Class II, and 8 (5%) were of ASA Class III.

On preoperative airway evaluation, 50 (33.3%) patients belonged to MMT Class I, 46 (30.7%) were of MMT Class II, 50 (33.3%) belonged to MMT Class III, and the rest 4 (2.7%) patients were of Class IV. The incidence of difficult intubation was 36% according to MMT. Using TMHT, 99 patients (66%) had a thyromental height of more than 50 mm, while for rest 51 patients (34%), it was <50 mm. The incidence of difficult intubation was 34% according to TMHT. Of the 150 patients, 60 (40%) were of CL Class 1, 35 (23.3%) were of CL Class 2a, 10 (6.6%) were of CL Class 2b, 43 (28.7%) were of Class 3, and remaining 2 (1.3%) were of CL Class 4. The incidence of difficult intubation, i.e., CL Class 3 and 4 in this study, was 30%. The validity of MMT and TMHT to predict the incidence of difficult intubation is summarized in [Table 1], [Table 2], [Table 3], respectively. [Table 3] compares the validity of the MMT with TMHT. Using McNemar test, a statistically significant difference was found between validity of the two tests in terms of specificity, sensitivity, positive predictive value, negative predictive value, and accuracy. TMHT was found to be superior to MMT in all the validity parameters.
Table 1: Validity of modified Mallampati test

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Table 2: Validity of thyromental height test

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Table 3: Comparison of validity of modified Mallampati test and thyromental height test

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  Discussion Top


Envisioning a potentially difficult airway in the preoperative period is pivotal. Unforeseen difficult tracheal intubation can land an anesthesiologist in a calamitous situation. Modalities such as airway radiographs and imaging are used to assess difficulties with intubation, but are either unreasonable or ponderous as screening tests.[10] There are myriad techniques to determine difficult airway, namely Mallampati test; ULBT; atlanto-occipital extension; thyromental, thyrosternal, hyomental, and sternomental distances; and neck circumference. These bedside tests are more conducive and economical for the patient.

Unfortunately, no single test has been found to specifically predict difficult intubation. Hence, the quest for a single predictive test that is veracious, precise, and easily applicable persists.

MMT, which has a high specificity, is an elementary bedside test for preoperative examination of difficult airway.[17] Nonetheless, it has low sensitivity and reliability. This is because of the effect of phonation and the absence of a clear distinction between different classes of MMT, leading to higher interobserver disparity.[15]

Recently, Etezadi et al. nreported TMHT as a novel test for the prediction of difficult intubation.[14] They showed a close association between smaller thyromental height and incidence of difficult laryngoscopy. The authors reported TMHT to be a superior than the conventionally used anatomical measurements such as TMD, SMD, and HMD. The advantages of TMHT over MMT are its relative ease of applicability and a small interobserver variability. Furthermore, no active head extension is required for the measurement of TMHT, unlike TMD.

We decided to study the TMHT in Indian population. In our study, a total of 150 patients were studied. Using MMT, the sensitivity was 77.78%. This was in accordance with the study by Ezri et al. (76%), Schmitt et al. (76%), and Kaul et al. (74%). Eberhart et al. compared ULBT with MMT and found sensitivity of MMT to be 70.2%.[18],[19],[20],[21] The specificity was 81.90%, which was comparable to that reported by Mahmoodpoor et al. (81.8%) and Adamus et al. (82.4%).[22],[23] In another study by Khan et al., specificity was found to be 66.8% in contrast to higher specificity reported by Cattano et al. (91%) and Kaul et al. (95%).[20],[24],[25]

The wide discrepancy reported in specificity and sensitivity for MMT in various studies could be due to high interobserver variance. Eberhart et al. and Karkouti et al. also reported MMT to have poor reliability for evaluation of oropharyngeal view.[21],[26] Oates et al. observed maximal mouth opening and complete tongue extrusion as prerequisite in drawing out a reliable MMT class.[27]

MMT in our study had a positive predictive value of 64.81%, which was in concordance with the findings of Kaul et al., who found that MMT had a PPV of 56.36%.[20] The negative predictive value in our study was 89.58%. Similar findings were reported by Eberhart et al. and Khan et al. who found that MMT had a negative predictive value of 93.8% and 98.4%, respectively.[21],[24]

The false negatives were 7%, which was analogous to that reported by Etezadi et al., i.e., 5%,[14] with false positives being 19%, which was close to that reported by Hester et al. (20%).[28] Similarly, Khan et al. and Etezadi et al. reported false positives of 33.4% and 12.7%, respectively, which were much higher as compared to our results.[14],[24] This could be attributed to interobserver differences in techniques of laryngoscopy, MMT classification, and/or differences in characteristics of the sample. MMT in our study had an accuracy of 80.66%, which is comparable to the study performed by Etezadi et al. (81.84%) and Adamus et al. (81.9%); however, it was higher in comparison to the study by Khan et al. (67.7%).[14],[23],[24]

The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of TMHT in our study were 93.33%, 91.43%, 82.53%, 96.97%, and 92%, respectively. In the study by Etezadi et al.,[14] the sensitivity of TMHT was 82.6%, which is lower as compared to our study (93.33%). This is due to high true positives in our study, i.e., the higher number of difficult intubations in our study. In their study, the specificity was 99.98% and negative predictive value was 98.63%, which were close to that in our study. The positive predictive value was 90.47%, which is higher than our study (82.53%). This could be by virtue of high false positives in our study due to anatomical differences and measurement errors. The accuracy of TMHT came out to be 92% in our study, while it was reported to be 98.08% by Etezadi et al.[14]

In our study, for TMHT, the sensitivity, specificity, positive predictive value, negative predictive value, and the accuracy were found to be 93.33%, 91.43%, 82.53%, 96.97%, and 92%, respectively. Whereas, for MMT, the sensitivity, specificity, positive predictive value, negative predictive value, and the accuracy were 77.78%, 81.90%, 64.81%, 89.58%, and 80.66%, respectively. Hence, all the validity parameters of TMHT were superior to MMT and were statistically significant than MMT. Analogous to our study, Etezadi et al. reported all the validity parameters of TMHT to be superior to MMT, which also came out to be statistically significant.[14] A multivariate trial by Naguib et al. showed sensitivity of 81.4%. In our study, the sensitivity with the use of single parameter, i.e., TMHT, was found to be 93.33%, which is superior to their multivariate model.[29] The specificity of TMHT in our study was 91.43%, which was tantamount to the Wilson's multivariate model (92.8%).[30]

In our study, for predicting difficult intubation, MMT had a low PPV (64.81%) as compared to TMHT (82.35%), which was statistically significant. Due to low PPV, MMT will subject many patients with easy intubation to undergo difficult intubation drill. This would incur an additional cost, time, and burden on the anesthesiologist.

In a clinical practice, the major concern of the anesthesiologist is to encounter a difficult airway (i.e., false negative predictions), when it is least expected and when one is unprepared. On the other hand, false positive predictions, although disturbing, distressing, and inconvenient, have no disastrous consequences. In our study, the percentage of false negatives with TMHT was less (2%) as compared to that for MMT (7%). Both these tests have a high negative predictive value (89.6% for MMT and 96.9% for TMHT), thus confirming the fact that they are good at predicting easy intubation.

In the present study, the overall incidence of difficult intubation using CL grading was 30%, i.e., 45 out of 150 studied patients. Out of these, 35 (23%) were correctly predicted by MMT as difficult and 42 (28%) were correctly predicted by TMHT as difficult. All 45 patients of difficult intubation had CL grade III and none of them had CL grade IV. CL grade III patients had to be intubated using either external laryngeal manipulation or by the help of gum elastic bougie. All cases were intubated in the first attempt and with no failures.

Comparison of MMT with TMHT shows that TMHT has a higher sensitivity, specificity, positive predictive value, negative predictive value, and accuracy as compared to MMT. Thus, we can conclude that TMHT can predict difficult intubation better as compared to MMT. However, both these tests are good to predict easy intubation.

Limitations

Shortcomings of the study included the fact that it was a single-center study and that the study was conducted on patients scheduled for elective nonemergent surgeries.


  Conclusion Top


All the validity parameters for TMHT were found to be superior to MMT and were statistically significant as per McNemar's test. Our study suggests that TMHT is better to predict difficult intubation in comparison to MMT. Both these tests have high negative predictive value (89.6% for MMT and 96.9% for TMHT), thus affirming the fact that both these tests can be good at predicting easy intubation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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