All patients undergoing preoperative evaluation are assessed for anatomic features that might predict difficulty in performing endotracheal intubation under general anesthesia. Typically, at least two examinations are used: the “Mallampati Test” (MP)¹,² is performed and the thyromental distance (TMD)³ is measured. The MP test involves an examination of oropharyngeal structures that are visible when the seated patient maximally opens the mouth and extends the tongue without phonation. The TMD measures the space between the superior tip of the thyroid cartilage and the inside of the tip of the mandible. Both tests perform only modestly, with sensitivity of 30% to 60%, specificity of 60% to 80%, and positive predictive value of just 5% to 20%.⁴ Even so, the combination of MP and TMD performed better than any other bedside screening test in a meta-analysis of 35 trials studying over 50,000 subjects.⁴ In practice, anesthesiologists likely weigh other subjective factors in anticipating a difficult airway, including habitus, facial appearance, and perhaps other poorly understood hunches.

Use of a bedside examination to predict difficult intubation is considered the standard of care in modern anesthesiology practice. It has been incorporated into the difficult airway algorithm of not only the American Society of Anesthesiologists (ASA)⁵ and those of several other countries⁶ but also most recently into the World Health Organization Surgical Safety Checklist.⁷ Unfortunately, all easily performed examination systems in clinical practice perform only modestly, with sensitivities of 20% to 62%, specificities of 82% to 97%, and very low positive predictive values, generally less than 30%, unless very liberal definitions of difficulty are used.⁸ There are likely several reasons for this poor performance, including the relative rarity of difficult intubation,⁸ the multifactorial etiology and varying definition of difficult intubation, interobserver variability in test results,⁹,¹⁰ failure to validate potential systems in patients independent of those used to derive the test,⁸ and the inadequacy of the tests themselves. Conversely, experienced anesthetists almost certainly use cues other than those derived from formal bedside tests to formulate their clinical impression of the ease of intubating any given patient. There may be several anatomic factors that enter into such a judgment.¹¹ The development of a tool that is able to capture this gestalt of the experienced anesthesiologist remains an important, incompletely solved problem.

Suzuki et al¹² used digital photographs of subjects’ faces to calculate five ratios and angles from measurements derived from placement of anatomic markers on the photographs. They found one, the “submandibular angle,” to be correlated with difficult tracheal intubation. Similarly, Naguib et al¹³ measured 22 indices from plain radiographs and 8 from three-dimensional computed tomography scans of the head in patients who were easy or difficult to intubate. They constructed a model containing three bedside tests (MP, TMD, and thyrosternal distance) and two radiographic features that accurately separated the easy and difficult cohorts with an AUC of the receiver operating characteristic (ROC) curve of 0.97. Both of these previous investigations, however, used a priori assumptions of which anatomic features might relate to difficult laryngoscopy and intubation. Both also required actual measurement of anatomic features.

In contrast, we have proposed a photographic technique that models the entire physiognomy of the face with no such assumptions and no direct measurements.¹⁴ Computer software is used to reconstruct a three-dimensional model of the patient’s head from three photographs, as shown in Fig. 10-1. The relative sizes of the patient’s facial features can be measured from this model. Using photographs of patients whose ease or difficulty is already known, a statistical decision-making model can be derived that can distinguish those patients who are easy to intubate from those who are difficult. This statistical
model does not contain any a priori assumptions about the facial features that may prognosticate difficult intubation. The statistical model should, without preconditioning, model the gestalt of the anesthesiologist once sufficient example cases are provided to it.

In our initial investigation, 80 Caucasian male patients were recruited postoperatively. These patients were defined as easy to intubate if their anesthetic record described a single attempt with a Macintosh 3 blade resulting in a grade 1 laryngoscopic view (full exposure of the vocal cords).¹,¹⁵ Difficult intubation was defined by at least one of the following: more than one attempt by an operator with at least 1 year of anesthesia experience, grade 3 or 4 laryngoscopic view on a 4-point scale,¹⁵