The TMHT was found to be a more accurate predictor of difficult laryngoscopy than other single anatomical measures. MMT is one of the most widely reported methods used for prediction of difficult laryngoscopy. Although when used alone this method has a poor predictive value, it may be valuable as part of a multivariate model for prediction of a difficult laryngoscopy.18–20 Schmitt et al.21 made a modification in the TMD test: “ratio of patient’s height to thyromental distance” (RHTMD). The sensitivity of the TMD and RHTMD test was the same, while the RHTMD test showed a little better specificity.21 Al Ramadhani et al.16 found that the SMD test by itself may not be an adequate sole predictor of subsequent difficult laryngoscopy. They suggested that the SMD test is better when used as part of a series of airway assessment tests.16
The consequence of an FN prediction (i.e., laryngoscopy and intubation would be easy) may prove to be catastrophic. Thus, decreasing FN predictions (increased sensitivity) is more important than falsely predicting difficulty (false positive) for patients in whom laryngoscopy and intubation are accomplished easily.15,22 The results of the current study support previous studies regarding poor sensitivity and positive predictive values of MMT, TMD, and SMD tests. Also, specificity and negative predictive values for the 3 above-mentioned tests in the current study are comparable with that of the previous studies.16,18–24 The typical advantage of the TMHT is high sensitivity and positive predictive values in comparison with the 3 other methods of airway assessment. Naguib et al.11 reported the sensitivity of 3 multivariate clinical models (Wilson, Arné, and Naguib) as 40.2%, 54.6 %, and 81.4%, respectively. The sensitivity of the TMHT, 82.6% (CI, 74%–88%), is approximately equal to Naguib et al.’s multivariate clinical model. Also, the specificity value of the TMHT test, 99.31% (CI, 96%–99.98%), is comparable with that of the above-mentioned multivariate clinical tests.
In an analytical observational study that focused exclusively on TMD, Qudaisat and Al-Ghanem24 found that the TMD method is a surrogate for inadequate head extension rather than dimensions of submandibular space. The calculated sensitivity of TMD (21.7%) in the current study is close to that of Qudaisat and Al-Ghanem’s study (19%). The TMHT on its own is not dependent on active head extension. However, it must be noted that patients lying supine inevitably would place 98% of them in a relative head extension.25 Both SMD and TMD tests must be measured in full head extension; thus, they are dependent on the patient’s cooperation, adequate cervical spine mobility, and having no contraindication for full head extension.
A more caudal or anterior larynx is associated with difficult laryngoscopy,26 and it can be expected to correlate with a shorter thyromental height. To compensate for this, backward, upward, and rightward pressure can be used to improve the laryngoscopic view.5,27–29 This posterior displacement increases the TMD, effectively increasing the thyromental height. One advantage of the TMHT is the use of an inexpensive, easily applicable instrument for measurement of an objective quantity (distance between anterior surfaces of the mentum and thyroid cartilage). It must be mentioned that the anatomical difference and measurement errors may affect the test results. For example, the assurance of a proper horizontal alignment may require the use of a bubble device such as those on carpentry levels. However, this study found a small interobserver variability for TMHT measurement (this was performed on a small subgroup of patients using only 2 observers). In contrast, large interobserver variability is a major problem with the MMT.4
Our study has some limitations. First, the study population was limited to patients scheduled for elective, nonemergent surgeries including orthopedic (trauma, spine), thoracic, abdominal, and vascular (central, peripheral). Thus, the results are only applicable to this group of patients. Second, determination of the best cutoff point for use of the TMHT, as a difficult laryngoscopy predictor, and its analysis, as a measure of prediction, have both been performed on the same population; this may explain the good statistical results calculated for the TMHT in comparison with the other tests. Third, we calculated the ideal cutoff value for the TMHT from the data obtained in the study, whereas the cutoff value for the other tests was obtained from the literature. Although this statistical approach may lead to a weaker or a better result for the competing parameters, it should be remembered that the primary end point of this study was the TMHT, not an evaluation of existing methods. In addition, the validity indexes calculated for the competing tests were in concordance with many other studies.
In summary, the TMHT appears promising as a single anatomical measure to predict the risk of difficult laryngoscopy, but validation will require further studies in more diverse patient populations.
Name: Farhad Etezadi, MD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Farhad Etezadi has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Aylar Ahangari.
Contribution: This author helped conduct the study.
Attestation: Aylar Ahangari has seen the original study data and approved the final manuscript.
Name: Hajar Shokri.
Contribution: This author helped conduct the study.
Attestation: Hajar Shokri has seen the original study data and approved the final manuscript.
Name: Atabak Najafi, MD.
Contribution: This author helped design the study.
Attestation: Atabak Najafi has seen the original study data and approved the final manuscript.
Name: Mohammad Reza Khajavi, MD.
Contribution: This author helped conduct the study and analyze the data.
Attestation: Mohammad Reza Khajavi has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Mahtab Daghigh, MA.
Contribution: This author helped write the manuscript.
Attestation: Mahtab Daghigh has seen the original study data and approved the final manuscript.
Name: Reza Shariat Moharari, MD.
Contribution: This author helped conduct the study and write the manuscript.
Attestation: Reza Shariat Moharari has seen the original study data and approved the final manuscript.
This manuscript was handled by: Sorin J. Brull, MD, FCARCSI (Hon).
The authors thank the Research Promotion Center of Sina Hospital for technical assistance and Dr. Mehrdad Pahlevani for his friendly cooperation.
* n= (number, sample size), p= prevalence, q= (1- prevalence)
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