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Department of Anesthesia, Kaiser Permanente Medical Center, Hayward, California
To the Editor:
Even though several previous studies have questioned and challenged its relevancy as a predictor (1–5), thyromental distance (TMD) is still widely used in almost every airway study to date. In our view, TMD is a tenacious misconception that has contributed to the present confusion and frustration in understanding and predicting difficult intubation (6). For example, although El-Ganzouri et al. (7) did not discuss TMD directly in their 1996 multivariate risk index study, their data showed TMD correctly predicted only 7% of all difficult intubation cases (93% false negative). In a recent study by Brodsky et al. (8) on morbid obesity and tracheal intubation, TMD was found to be no different between easy and difficult (problematic) intubations. In one patient with failed direct laryngoscopy, the TMD was 14 cm long. A MacIntosh No. 4 blade revealed a Grade 3 view.
Taken together with previous literature (1–5) and findings presented in El-Ganzouri and Brodsky et al.’s studies (7,8), we suggest that TMD falls short as a useful predictor because measuring TMD is fundamentally an incorrect approach. Measurement of TMD originated as a quantitative assessment of "receding jaw," yet we have found no correlation between receding jaw and the mechanism of difficult laryngoscopy (9). We have previously shown that in patients with a caudally positioned larynx, the mandibulohyoid distance (MHD) is long and the tongue is largely contained in the hypopharynx, causing difficult ventilation, difficult intubation, or sleep apnea (10). Although MHD is measured vertically and TMD obliquely, patients who have a long MHD/caudal larynx/large hypopharyngeal tongue will also have a long TMD. These patients are candidates for difficult intubation, yet their long TMDs will indicate easy intubation. Such cases may account for the large false negatives associated with the use of TMD in prediction.
Another confusing concept is the "anterior larynx" often used to describe patients with difficult intubation. We believe the term "anterior larynx" is a misnomer. For instance, in a laryngoscopy Grade 3 patient, we may try with effort to lift up the tongue with the blade, yet only the epiglottis can be seen. At such time, the deeper, unreachable larynx appears situated "anterior" to the line of sight. Thus, while "anterior larynx" may describe the failure to bring the larynx into view, it obscures the real issue of a caudal larynx/large hypopharyngeal tongue that causes difficulties (2,10). Therefore, we propose "caudal larynx" to be the anatomically correct term to indicate the cause of difficult laryngoscopy.
Since the 1990 closed claims study (11), difficult airway management has become a top patient safety issue in clinical practice. After more than a decade’s effort, our specialty is now at a crossroads. Perhaps it is time for practitioners to think differently and examine new concepts and insights (2,9,10).
References
This article has been cited by other articles:
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  E. R. Worth and J. L. Benumof Haeckel's Recapitulation Theory Should Not Be Used as a Defense of the ASA Difficult Airway Algorithm * Response Anesth. Analg., April 1, 2004; 98(4): 1192 - 1192. [Full Text] [PDF]
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J. L. Benumof, H.-c. Chou, and T.-l. Wu Both a Large and Small Thyromental Distance Can Predict Difficult Intubation * Response Anesth. Analg., November 1, 2003; 97(5): 1543 - 1544. [Full Text] [PDF]
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