Naguib et al developed 2 discriminant functions to identify patients who may have difficulty during laryngoscopy or intubation. The authors are from King Saud University College of Medicine in Riyadh, Saudi Arabia.


Patient evaluation included cephalometric analysis of lateral radiographs with the head in neutral and extended (intubation) positions.



(1) thyrosternal distance in cm

(2) Mallampati class, from 1 to 4

(3) thyromental distance in cm

(4) neck circumference in cm

(5) depth of spinous process for C2 in mm (V13)

(6) angle A (V20) in degrees from the lateral radiograph with the head extended


where (see Figures 1 and 2):

• In the extended radiograph, angle A is from the triangle ABC (between lines AB and AC).

• A the most antero-inferior point of the upper incisor tooth

• C is the antero-inferior border of the 6th cervical vertebra.

• AB is the line along the occlusal surfaces of the maxillary teeth.

• B is on the anterior aspect of C1 at a point intersecting lines AB and BC.


L1 =

= (1.1003 * (thyrosternal distance in cm)) - (2.6076 * (Mallampati class)) + (0.9684 * (thyromental distance in cm)) - (0.3966 * (neck circumference in cm)) + 4.9504


L2 =

= (1.2422 * (thyrosternal distance in cm)) - (3.368 * (Mallampati class)) + (0.966 * (thyromental distance in cm)) - (0.3192 * (depth of spine C2 in mm)) + (0.176 * (angle A in degrees)) - 10.2717



• A negative index (L1 or L2 < 0) indicates that difficulty will be encountered.

• L2 has a greater predictive ability and should be used if the results are discrepant.


NOTE: In Case 2 from the Appendix, the result reported is -0.9828. My calculation is -2.2145, both in the spreadsheet and by multiplying the numbers by hand.


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