Predictive signs of difficult intubation
Pierre Diemunsch, Thierry Pottecher
Studies carried out in the United Kingdom in the 1980s established that each year, 3
pregnant women die due to impossible airway control. On a global scale, difficult or
impossible intubation kills an estimated 600 people a year1. Difficulties related to certain
extreme situations, such as battlefields and lack of specific training, are indisputable factors in
impossible airway control2.
The acute awareness of practitioners (workshops in all major congresses), the development
of fibroscopy, the introduction of algorithms and a generalization in the use of the laryngeal
mask have contributed to a decrease in complications of sudden impossible ventilation as
observed in the US over the past decade3.
Screening for high-risk situations using simple clinical signs can avoid complications but is
not sufficient on its own, because the identification of a problem does not guarantee its
resolution. Nonetheless, it is crucial to recognize patients for whom mask ventilation or
intubation is potentially difficult, in order to take preventative measures and to be prepared to
apply first-line treatment, which will be optimized by avoiding the stress of a surprise
situation.
Difficult intubation has often been associated with difficult laryngoscopy.
Laryngoscopy provides a more or less complete view of the glottis, making intubation
relatively easy. Cormack and Lehane4 have defined 4 grades of laryngoscopic view, according
to the structures that can be visualized (table 1). Intubation is easy for grade I and slightly
more difficult for grade II, but this is generally improved by external manipulation of the
larynx. Grade III corresponds to severe intubation difficulties, and grade IV usually
corresponds to an impossible intubation.
This relationship is not categorical. It is excessive to completely associate the conditions of
difficult laryngoscopy and difficult intubation. A study carried out by Wilson (table 3)
reveals considerable variations in the reported incidence of difficult laryngoscopies, defined
as corresponding to a Cormack grade greater than II. These differences are attributed to
variations between operators and to whether external laryngeal pressure is applied or not.
Moreover, these figures have very little to do with those reported for difficult intubations.
Grade
Structures identified by direct laryngoscopy
Visualization of the entire laryngeal aperture
I
Visualization limited to the posterior portion of the laryngeal aperture, incomplete
II
visualization of the cords
Visualization limited to the epiglottis, no visualization of the laryngeal aperture
III
Visualization limited to the soft palate, no visualization of the epiglottis
IV
TABLE 1
The 4 grades of Cormack and Lehane. These grades, defined for obstetric patients, are
associated with increasing orotracheal intubation difficulties. Their correlation with the 4
Mallampati classes as modified by Samsoon and Young is neither very sensitive nor very
specific.
169
Predictive signs of difficult intubation
Based on a study of 500 patients, Cook5 proposed a 3-grade classification of laryngoscopic
views. This classification is better correlated to intubation difficulties than the classical grades
of Cormack and Lehane.
Quality of the
laryngoscopy
Easy
Restricted
Difficult
Criteria of Cook
Laryngeal inlet visible
Visibility of posterior glottic structures (posterior
commissure or arytenoids)
OR of epiglottis that can be lifted
No laryngeal structures visible
OR epiglottis visible but cannot be lifted
Grade of
Cormack and
Lehane
I
II and III
III and IV
Impossible intubation
Before declaring that a maneuver is difficult or impossible, it is essential to take into account
the circumstances involved in performing it by using a definition of optimal attempt.
Definition and quantification of the difficulty are 2 other elements that should be
standardized to interpret and compare clinical study data.
Criteria of definition of optimal attempt
Orotracheal intubation
Mask ventilation
1- two-person additive/synergistic effort for
1- reasonably experienced laryngoscopist
optimal jaw thrust and mask seal
2- no significant muscle tone
2- large oropharyngeal airway
3- optimal sniff position
4- optimal external laryngeal pressure
5- change length of blade once
3- large bilateral nasopharyngeal airways
6- change type of blade once
TABLE 2
Criteria of definition of optimal attempt at intubation and mask ventilation. When these
criteria are fulfilled, failure involves the diagnosis of impossibility and requires an alternative
solution.
The advantage of the sniff position was recently contested6. As compared to a simple
extension of the head, this position only improves laryngoscopy in obese patients or when the
mobility of the neck is limited.
170
Predictive signs of difficult intubation
Incidence of various degrees of difficult intubation (16 clinical studies)
Final result:
Means required
Cormack
Incidenc
intubation
and Lehane
e (%)
Possible
Multiple attempts and/or blades
II or III
1 to 18
Possible
Multiple attempts and/or blades and/or laryngoscopists III
1 to 4
Impossible
III or IV
0.05 to
0.35
iiiv*, afterTranstracheal approach, jet ventilation, life-saving
0.0001 to
effects or
measures
0.02
death
Incidence of difficult intubation (Savva, 1994)
Intubation on bougie after 2 unsuccessful attempts
>II
4.9
Incidence of difficult laryngoscopies (Wilson, 1988)
Without laryngeal compression; n=431
>II
9.3
With laryngeal compression; n=202
>II
5.9
With laryngeal compression; n=778; prospective study
>II
1.6
TABLE 3
Incidences of various degrees of difficult intubation and difficult laryngoscopies.
*iiiv = impossible intubation and impossible ventilation (from Reisner, 1999; Savva, 1994;
Wilson, 1988).
In 1997, Adnet7 proposed a quantitative score for difficult intubation, aiming to standardize
evaluations and to permit comparisons between various predictive tests and management
techniques for difficult intubation.
Parameters
Number of attempts over 1
Number of operators over 1
Number of alternative techniques
Cormack and Lehane grade – 1 (grade 1 = 0)
Normal traction strength (0) or abnormal (1)
Laryngeal pressure: no (0) or yes (1) except for
Sellick
Vocal cords in abduction (0) or adduction (1)
Degree of difficulty (sum of parameters)
Total = intubation difficulty score
0 = easy, ideal
0 <total <=5 = slight difficulty
5 <total = moderate to major difficulty
∞ corresponds to impossible intubation
Distribution of difficulty scores
hospitalized patients; n=289 (Adnet et al., 1997)
score =0
53% (time: 6 to 50s; mean:18s)
score >5
6.3%
Distribution of difficulty scores
pre-hospitalized patients; n=311 (Adnet et al., 1997)
score =0
28.2%
score >5
16.1 % with 1 impossible (>17)
171
Predictive signs of difficult intubation
Predictive signs of intubation difficulty
We lack predictive criteria that are simple, rapid, affordable, reliable, sensitive and specific,
and that have good positive and negative predictive values.
1 – Anatomical criteria
Checking for elements that indicate potential intubation difficulties is an essential part of the
anesthesia workup. In emergencies, this assessment can be limited to checking for prostheses
and to the Mallampati classification and Wilson score (tables 4 and 5). Most assessments
proposed include common points or variable assessments of the same criteria (neck extension
and sternomental distance, for example).
1. Mallampati classification8
This is established when the patient is awake, either sitting or standing. The patient is asked
to open the mouth as wide as possible, and to stick out the tongue as far as possible, without
phonation (table 4). The classification was initially limited to the first 3 classes, and was
completed by the addition of class IV represented by a limited view of the hard palate.
The correlation with the Cormack and Lehane grades is not very reliable for Mallampati
classes II and III, because patients with these intermediary airway classifications have a
relatively uniform distribution of the 4 grades of laryngoscopic view. However, there is a good
correlation between the observance of a class I and a grade I laryngoscopy. Likewise, a class
IV is generally associated with a grade III or IV.
The mediocre performance of the Mallampati classification has been attributed to errors in
methodology, such as examining the patient in supine position, having the patient say “ah”
(phonation that falsely improves the view), or allowing the patient to arch his or her tongue
(falsely obscuring the view). Variations between observers are an additional source of false
positives and false negatives for the Mallampati classification, which is not sufficient on its
own to predict difficult laryngoscopy nor, a fortiori, difficult intubation. The insufficiency of
the Mallampati classification has been specifically shown for obese patients9. These data
indicate that this classification should no longer be considered individually capable of
predicting, with precision, what the laryngoscopic view will be10.
Combining the assessment of the mouth opening improves the specificity without altering
the sensitivity of the Mallampati classification as a predictor of difficult intubation11.
Class
I
II
III
IV
Visible structures
(patient upright, maximal opening of mouth and protrusion of tongue)
Uvula, fauces, soft palate, hard palate
Fauces, soft palate, hard palate
Soft palate, hard palate
Hard palate alone
TABLE 4
Mallampati classification modified by Samsoon and Young (addition of class IV) and
outcome of test.
172
Predictive signs of difficult intubation
Figure: Mallampati classification
Mallampati class predictive of a DI
>II
True positives (%)
55
False positives (%)
5
Ezri has suggested adding a class 0 to the 4 others. Class 0 is defined as the visualization of
the epiglottis with an open mouth and protruding tongue12. The incidence of class 0 is 1.18% in
the author’s study. It is consistently associated with a Cormack and Lehane grade I
laryngoscopic view.
2. Wilson score
Wilson’s study (1988)13 was an important development in the attempt to deductively identify
patients for whom intubation will be difficult. It should be emphasized that this study tests the
predictability of difficult laryngoscopy and not difficult intubation.
Criterion
Weight (kg)
Head and neck
mobility (degrees)
Mandibular mobility
Retrognathism
Prominence of upper
incisors
0
<90
>90
Points
1
90-110
90
2
>110
<90
MO* >5 cm or
subluxation† >0
None
None
MO* <5 cm and
subluxation† =0
Moderate
Moderate
MO* <5 cm and
subluxation† <0
Severe
Severe
TABLE 5
Wilson score. *MO: mouth opening; †subluxation: possibility of advancing the mandibular
incisors in front of the maxillary incisors (>0); or just to their level (=0); or impossibility of
advancing the mandible in relation to the maxilla (<0). A score of 2 or more is predictive of a
difficult (intubation) laryngoscopy.
173
Predictive signs of difficult intubation
Performances of the score depending on the chosen threshold (prospective study).
Choice of threshold value of Wilson score
>=4
>=3
>=2
>=1
True positives (%)
42
50
75
92
False positives (%)
0.8
4.6
12.1
26.6
No factor is all-important. Several simple indicators are assessed by the mobility of the head
and neck (cervical flexion and atlantooccipital joint extension) as well as by mandibular
mobility (distance between upper and lower incisors and temporomandibular joint mobility).
Other classical items, such as a short, thick neck, are sufficiently represented by the weight of
the patient, which is easier to assess quantitatively. The elements describing the relative place
of the tongue in the mouth and how it interferes with the view of the laryngeal aperture, are
found together in the subjective assessment of the prominence of the upper incisors and of the
retrognathism. It is easy and advisable to complete these criteria with the Mallampati
classification.
When a threshold value of 2 (Wilson score) is chosen to predict a difficult laryngoscopy,
75% of difficult cases were correctly detected (true positives) and 12.1% of easy cases were
incorrectly detected as being difficult (false positives). The author calculates that for an annual
activity of 10 000 patients, of whom 1.5% (150) have a difficult laryngoscopy, 9 true positives
are detected per month for every 3 patients who are overlooked during screening (total =12).
Out of the 9850 easy patients, 12.1% unduly prompt the implementation of specific measures
that are useless and sometimes extensive. The false positives (1192=9850*12.1%) are the
cause of 99 false alerts per month.
These data illustrate the relatively weak power of the tests and the absolute need to train all
anesthesiologists in difficult intubation techniques. This skill, combined with a sufficient
availability of fibroscopes, enables practitioners to deal with sudden difficulties and makes
anticipatory measures commonplace. This outweighs the logistical disadvantages of taking
numerous false positives into consideration.
3. El-Ganzouri score14
Established according to the same principle as the Wilson score, it includes similar criteria,
in addition to the thyromental distance, Mallampati classification and history of DI. A value of
4 or more has a better predictive value than the Mallampati classification. It is a predictive
score for difficult laryngoscopies, established from a study of 10 507 patients of whom 5.1%
are grade III and 1% are grade IV according to Cormack and Lehane.
Criterion
Weight (kg)
Head and neck mobility
(degrees)
Mouth opening
Subluxation >0
Thyromental distance
Mallampati class
History of DI
174
0
<90
>90
Points
1
90-110
90±10
2
>110
<80
>=4 cm
possible
>6.5 cm
I
no
<4 cm
not possible
6-6.5 cm
II
possible
<6 cm
III
established
Predictive signs of difficult intubation
4. The three criteria of Bellhouse
In 1988, Bellhouse15 published the results of a comparison between radiographic studies of
19 patients for whom intubation had been difficult and those of 14 patients for whom
intubation had been easy. He used these results to identify the following criteria that together
predict intubation difficulty:
- restriction of atlantooccipital joint extension (less than 35 degrees);
- reduced mandibular space;
- enlarged tongue (versus pharyngeal) size.
The atlantooccipital joint extension is measured by moving the plane of the occlusal surface
of the upper teeth. When the patient sits facing forward with his or her head erect, this plane is
horizontal. When a normal patient extends the atlantooccipital joint as much as possible, he or
she can produce a 35° angle between the horizontal and extended planes.
The mandibular space is assessed by the thyromental distance, which should be at least 5 cm
(or 3 fingerbreadths) long. This distance evaluates the horizontal length of the mandible (>9
cm). A short thyromental distance can be combined with a retrognathism and a relative
macroglossia. It generally coincides with a high Mallampati classification because the tongue
that is more globular obscures the view of the pharynx. When the mandibular space is large,
the tongue can be easily compressed into this space and the larynx, which lies relatively
posterior, can be more easily visualized with direct laryngoscopy. Forward protrusion of the
mandible contributes positively to a large mandibular space.
5. Sternomental distance
The comparison of 4 predictive tests on the same population (n=350)16 confirms that a
Mallampati classification >II is not reliable on its own. Measurement of sternomental distance
proved to be more sensitive and more specific, with a threshold value of 12.5 (head fully
extended on the neck and the mouth closed). The thyromental distance (less or equal to 6.5
cm) was less useful, as was the inability to bring the mandibular teeth anterior to the maxillary
teeth (subluxation), refuting the results reported by Wilson.
Test (n=350)
(Savva, 1994)
Mallampati >II
Thyromental distance <=6.5 cm
Sternomental distance <=12.5 cm
Subluxation 0 or <0
Sensitivity (%) Specificity (%)
64.7
64.7
82.4
29.4
66.1
81.4
88.6
85.0
Positive predictive
value (%)
8.9
15.1
26.9
9.1
6. Synthesis
Benumof (1999-2001) grouped together 11 main elements of a physical examination and the
criteria that must be met in order to indicate that intubation will not be difficult (table 6). This
evaluation uses the most relevant elements of the main tests or scores proposed. It is carried
out easily and quickly, and requires no specific equipment.
Complementary elements are obtained by questioning the patient and studying previous
anesthesia reports, keeping in mind that intubation difficulty can vary in the same patient from
one procedure to another, and even only a few hours apart17.
Obesity and mammary hypertrophy usually play important roles.
A criteria that is pathologic to the point of establishing the diagnosis of an impossible
intubation on its own is exceptionally rare. Usually, the probability of a DI is backed up by
several, converging elements. The reliability of the assessment increases with the number of
criteria that are considered. Wong confirms this conclusion in his series of 411 women. The
study indicates that pregnancy does not increase the risk of DI (prevalence of 1.99% versus
1.55% for non-pregnant women)18.
175
Predictive signs of difficult intubation
11 elements of the examination
Length of the upper incisors
Involuntary anterior overriding of the
maxillary teeth on the mandibular teeth
(retrognathism)
Voluntary protrusion of the mandibular
teeth anterior to the maxillary teeth
Criteria in favor of an easy intubation
Short incisors – qualitative evaluation
No overriding of the maxillary teeth on the
mandibular teeth
Inter-incisor distance (mouth opening)
Mallampati classification (sitting position)
Configuration of the palate
Thyromental distance (mandibular space)
Mandibular space compliance
Length of neck
Thickness of neck
Range of motion of head and neck
Anterior protrusion of the mandibular teeth
relative to the maxillary teeth (subluxation of
the TMJ)
Over 3 cm
I or II
Should not appear very narrow or highly arched
5 cm or 3 fingerbreadths
Qualitative palpation of normal
resilience/softness
Not a short neck – qualitative evaluation
Not a thick neck – qualitative evaluation
Neck flexed 35° on chest, and head extended
80° on the neck (ie sniffing position)
TABLE 6
Main elements of the examination to detect difficult intubation. The 11 items are presented in
logical order, superiorly to inferiorly (teeth followed by mouth and then neck); no element is
sufficient on its own. (TMJ: temporomandibular joint). From Benumof, 1999.
For Karkouti19, the ideal combination includes 3 airway tests: mouth opening, chin
protrusion and atlantooccipital extension. This preference is based on a multivariable analysis
of predictive criteria, in an observational study of 461 patients of whom 38 had a DI. The
conclusions reached by Bernumof are based on common sense and on the author’s expertise.
7. Paraclinical examinations for systematic detection of DI
Among the paraclinical evaluations, indirect laryngoscopy seems to be the easiest to
perform (sitting position, tongue held out by operator, angled mirror) and the easiest to
interpret. A view that is equivalent to Cormack and Lehane grades III and IV is predictive of a
direct laryngoscopy revealing the same grades and of difficult intubation. The positive
predictive value, sensitivity and specificity of this test are better than those of the Mallampati
classification and of the Wilson score20. This examination may not be possible to perform in
certain patients, including 15% who have a strong gag reflex, and others who cannot sit up or
who refuse it.
Test (n=6148, DI: 1.3%)
(Yamamoto, 1997)
Wilson score >2
Mallampati classification >2
Indirect laryngoscopy; grade >II
Sensitivity (%)
Specificity (%)
55.4
67.9
69.2
86.1
52.5
98.4
Positive predictive
value (%)
5.9
2.2
31.0
The combination of clinical and radiological criteria proposed by Naguib is interesting from
a retrospective point of view, but cannot be systematically applied as a detection tool21.
176
Predictive signs of difficult intubation
2 – High-risk groups
Intubation is generally considered more difficult in pregnant women and in otolaryngology
(ENT)22 and traumatology patients. Contradictory data have been reported, however, notably
in obstetrics17.
Moreover, certain pathologies are particularly predisposing. Among the most common of
these is diabetes. To check if a diabetic patient is at risk, ask him or her to bring the hands
together as if praying. If the fifth fingers of the hands cannot be flat against one another, there
is probably ligament thickening of the finger joints (and in the TMJ and the cervical spine),
and difficult intubation should be anticipated. Another test that has recently been proposed is a
palm print study of the patient’s dominant hand. A grade above 0 is considered a more
sensitive predictor of difficult laryngoscopy than the Mallampati classification, the
thyromental distance and the degree of neck extension23; (in this study, the praying test was not
compared to the palm print test).
Palm print
Grade 0 – View of all phalangeal surfaces
Grade 1 – Phalangeal surfaces of fourth or fifth fingers missing from print
Grade 2 – Phalangeal surfaces of the second to fifth fingers missing from print
Grade 3 – Print of fingertips only.
Acromegaly is also considered a risk factor. Difficult intubation occurs in about 10% of
patients with this disease24.
Obesity by itself, including morbid obesity (BMI>35 Kg/m2) was not always considered a
factor in difficult laryngoscopy25. The combination of obesity and lack of teeth, however, is
strongly predisposing. A more recent series conversely suggested that difficult tracheal
intubation is more common in obese than in lean patients, with a difficult intubation rate of
15.5% in obese patients (BMI>35 Kg/m2) compared to 2.2% in lean patients (BMI<30 Kg/m2)
respectively. Desaturation is common and fast occurring with difficult intubation in obese
patients26.
In general, problems linked to congenital disease, rheumatic conditions, local pathologies
and previous history of trauma are easily identified during the physical exam or by
questioning the patient. In otolaryngology, surgeons who are prepared to perform a direct
tracheal approach in case of impossible intubation are immediately available to assist the
anesthesiologist.
Lingual papillomatosis and angioedemas can also be formidable pitfalls.
Examples of obvious or less obvious situations that predispose to difficult intubation:
- congenital facial and upper airway deformities;
- maxillofacial and airway trauma (current or previous);
- airway tumors and abscesses;
- immobile cervical spine;
- fibrosis of the face and neck from burns or radiation exposure;
- obstructive sleep apnea syndrome (OSAS)27;
- history of neurosurgical procedures, with or without division of the temporal muscle, that
can lead to pseudoankylosis of the mandible28.
- tongue piercing29.
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Predictive signs of difficult intubation
3 – Clinical index
Clinical multifactorial indexes have been described to predict difficult intubation. Defining
difficult intubation as the failed attempt of 2 anesthesiologists to use basic direct laryngoscopy,
Arné30 has developed a clinical index that obtains predictive scores with a sensitivity and
specificity of 94% and 96% in general surgery, 90% and 93% in ENT non-cancer surgery, and
92% and 66% in ENT cancer surgery. The defined index was validated in a prospective study
(n=1090) after being established in an initial study (n=1200). The overall incidence of difficult
intubation for all patients is 4.2%. No impossible intubation was reported.
The factors taken into consideration are:
1. Weight, age and height
2. History of DI (if patient was informed)
3. Predisposing pathologies such as facial deformities, acromegaly, rheumatic conditions o
the neck, ENT tumors, diabetes
4. Symptoms of respiratory tract diseases such as dyspnea, dysphonia, dysphagia and sleep
apnea in particular
5. Mandibular mobility as in the Wilson score, but with a 3.5 cm mouth opening
6. Mobility of the head and the neck as in the Wilson score
7. Prominence of the upper incisors as in the Wilson score
8. Aspect of the neck: short and thick or not
9. Thyromental distance, > or <6.5 cm
10. Mallampati classification
The statistical analysis based on 1200 observations was used to assign point values to each of
these factors in proportion to regression coefficients representing the relative weight of each
predictive intubation difficulty factor, which was validated in the second prospective study of
1090 patients.
Criteria
Simplified value
Past history of DI
10
Predisposing pathologies
5
Respiratory symptoms
3
MO >5 cm or subluxation >0
0
3.5 cm<MO<5 cm and subluxation =0
3
MO <3.5 cm and subluxation <0
13
Thyromental distance < 6.5 cm
4
Mobility of head and neck >100°
0
Mobility of head and neck 80-100°
2
Mobility of head and neck <80°
5
Mallampati classification 1
0
Mallampati classification 2
2
Mallampati classification 3
6
Mallampati classification 4
8
Maximum total
48
With 11 as the threshold value
for this index, the test gave the
following results:
Sensitivity: 93%
Specificity: 93%
PPV: 34%
NPV: 99%
General population
Validation study: n=1090
Difficult intubation: 3.8%
Opinions concerning the usefulness of this type of index are sometimes very negative31.
Nevertheless, they seem to play a justifiable role in the evaluation of situations that are neither
obviously easy nor obviously difficult.
178
Predictive signs of difficult intubation
Conclusions
Predictive tests of difficult intubation are numerous. None are perfect. The reproducibility of
the tests from one observer to another is inconsistent. A certain confusion exists between
difficult laryngoscopy and difficult intubation. A certain amount of false negatives will persist,
no matter what method of detection is used.
Prevention is the best cure. However, foreseeing, when possible, does not guarantee
prevention32. There is a perceptible association in the literature between foreseeing difficulty
and preventing death due to impossible intubation. Since our final goal is the latter, we should
direct our efforts towards the management of DI as much as towards detecting it.
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