Submandibular Sonography - Journal of Ultrasound in Medicine

ORIGINAL RESEARCH
Submandibular Sonography
Assessment of Hyomental Distances and Ratio, Tongue Size,
and Floor of the Mouth Musculature Using Portable Sonography
Jacek A. Wojtczak, MD, PhD
Received September 22, 2011, from the Department
of Anesthesiology, University of Rochester
School of Medicine and Dentistry, Rochester, New
York USA. Revision requested October 10, 2011.
Revised manuscript accepted for publication November
9, 2011.
I thank BK Medical (Peabody, MA) for the
loan of the Flex Focus ultrasound system, Catherine
Smith, NP, for help with patient recruitment,
Jerrold Lerman, MD, for correcting the manuscript
and designing Figure 3, and Michelle Richau for
help with preparing and formatting the images.
This work was presented in part at the American
Society of Anesthesiologists Annual Meeting;
October 15, 2011; Chicago, Illinois.
Address correspondence to Jacek A. Wojtczak,
MD, PhD, Department of Anesthesiology, Box
604, University of Rochester School of Medicine
and Dentistry, 601 Elmwood Ave, Rochester, NY
14642 USA.
E-mail: gryf70@yahoo.com
Abbreviations
BMI, body mass index
Objectives—The aim of this study was to evaluate the feasibility of sonography as a tool
to measure the hyomental distance ratio and tongue size in obese patients with a large
neck circumference. The hyomental distance ratio is a predictor of difficult laryngoscopy
that may result in difficult intubation.
Methods—Five obese and 7 morbidly obese adult patients with a history of either difficult
or easy intubation had a submandibular sonographic examination performed in the
supine position. The hyomental distance was measured while the head was placed in
the neutral position and then in a hyperextended position, and the ratio of two distances
(hyomental distance ratio) was calculated. Their tongue volumes were derived from
multiplication of the midsagittal cross-sectional area of the tongue by its width obtained
from transverse sonograms.
Results—The mean hyomental distance ratios ± SD in 6 patients with difficult intubation
and 6 with easy intubation were 1.02 ± 0.01 and 1.14 ± 0.02 (P < .002), respectively.
Tongue volumes did not differ statistically between the two groups: 137 ± 29
cm 3 (difficult intubation) and 168 ± 34 cm 3 (easy intubation). The mean body mass indices
were 36.3 ± 6.0 and 43.2 ± 7.2 kg/m 2 in the difficult and easy intubation groups.
The mean neck circumferences were 44.5 ± 7.4 and 46.8 ± 7.0 cm in the difficult and
easy intubation groups.
Conclusions—Sonography allows bedside measurements of the hyomental distance
ratio and tongue size in morbidly obese patients. Preoperative assessment of the hyomental
distance ratio may predict difficult laryngoscopy resulting in difficult intubation.
Key Words—airway assessment; difficult intubation; hyomental distance ratio; sonography;
tongue size
Difficult or failed tracheal intubation by direct laryngoscopy is
a major and life-threatening complication during induction
of general anesthesia. 1 Sonography has been used for preintubation
airway assessment, 2–4 but it is unclear which sonographic
parameters are useful as predictors of difficult laryngoscopy and intubation.
Visualization of the glottis during laryngoscopy depends
on several factors, including extension of the head at the occipitoatlantal
(0–C1) and atlantoaxial (C1–C2) joints. Extending the head
at the subaxial level (C2–C7) bows the cervical spine forward, which
raises the larynx and the glottis anteriorly out of the line of vision. 5,6
Recently, Takenaka et al 7 showed that the extension angle of occipitoatlantal
and atlantoaxial joints measured radiologically correlates
well with surface measurements of the hyomental distance ratio in
patients with a body mass index (BMI) of less than 30 kg/m 2 .
©2012 by the American Institute of Ultrasound in Medicine | J Ultrasound Med 2012; 31:523–528 | 0278-4297 | www.aium.org

Wojtczak—Submandibular Sonography
The hyomental distance is the distance between the
hyoid bone and the mandibular mentum. The hyomental
distance ratio is defined as the ratio of the hyomental distance
in the neutral position with that at the extreme of
head extension. The hyoid bone is attached to the styloid
process of the occiput by the stylohyoid ligament. Therefore,
the distance between the hyoid bone and the basiocciput
remains constant during extension/flexion of the
neck. 8 When the occipitoatlantal-atlantoaxial complex is
extended, the mentum moves away from the hyoid bone,
increasing the hyomental distance. 8 The movement results
from rotation of the head around the basiocciput.
In contrast, when the neck is extended at the subaxial
level without occipitoatlantal and atlantoaxial joint extension,
the mentum does not move in relation to the hyoid
bone because head rotation does not occur. Therefore, the
hyomental distance remains unchanged.
Palpation of the body of the hyoid bone may be very
difficult if not impossible in obese and morbidly obese patients
with a large neck circumference. Therefore, we evaluated
the feasibility of measuring the hyomental distance
ratio as well as volumes of the tongue and muscles of the
floor of the mouth in obese patients using submandibular
sonography.
Materials and Methods
Patient Population
With the approval of the Institutional Research Subjects
Review Board, written informed consent was obtained
from 12 patients (5 female and 7 male, ages 24–76 years)
undergoing preoperative evaluation in the anesthesia
clinic.
Anatomy of the Suprahyoid Airway
Figure 1 illustrates the suprahyoid anatomy corresponding
to the submandibular sonograms presented in this study.
The drawings are inverted and oriented to match the sonograms
that follow.
Sonographic Measurements of Hyomental Distances
and Volumes of the Tongue and Muscles of the Floor of
the Mouth
A curved low-frequency (5-MHz) transducer and a Flex
Focus 400 ultrasound system (BK Medical, Peabody, MA)
were used to visualize the tongue and shadows of the hyoid
bone and mandible (Figure 2). The midsagittal and coronal/transverse
scans were transferred from the ultrasound
machine to a laptop computer and analyzed using ImageJ,
the image-processing software developed by the National
Institutes of Health (http://rsb.info.nih.gov/ij/; accessed
August 17, 2010).
The mentum and hyoid bone appear in midsagittal
scans as hyperechoic structures with hypoechoic shadowing.
The hyomental distances in the neutral and headextended
positions were measured from the upper border
of the hyoid bone to the lower border of the mentum.
The midsagittal scans were also used to measure the
cross-sectional area of the tongue. Transverse scans obtained
in the midsection of the tongue (at the glossal end
of the genioglossus muscle) provided a measure of the
tongue width, which was measured between the most distant
points on its upper surface (Figure 2, bottom panel).
The tongue volume was derived from multiplication of the
midsagittal cross-sectional area by the tongue width. This
method most likely overestimates the volume because it
assumes that the tongue width remains the same at the
level of the mentum and the hyoid bone as it is in the midsection.
Indeed, our preliminary results 9 showed that the
mean tongue volume in 10 healthy volunteers was 111 ±
4.1 cm 3 using above formula versus 106 ± 3.9 cm 3 using
the 3-dimensional submandibular sonographic method,
which detects the true geometry of the tongue. Because
the difference is relatively small, a correction factor was not
used in this study.
The cross-sectional area of the muscles of the floor of
the mouth was the sum of the areas of geniohyoid, mylohyoid,
and diglossus muscles (Figure 2) in the transverse
scans in the midsection of the tongue. The muscle volume
of the floor of the mouth was derived from multiplication
of the cross-sectional area of the floor of the mouth muscles
by the hyomental distance in the neutral position.
For sonographic examinations, patients were positioned
with the torso and head on a firm examination table.
They were instructed to look straight up, keeping their
heads in a neutral position, to close their mouths, and to
Figure 1. Upper airway anatomy based on midsagittal (left) and transverse
(right) anatomic sections. The drawings are inverted and oriented
to match the sonograms that are presented in the following images. DG
indicates diglossus muscle; E, epiglottis; GG, genioglossus muscle; GH,
geniohyoid muscle; H, hyoid bone; M, mandible; MH, mylohyoid muscle
(“mandibular” diaphragm); and TS, tongue surface.
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keep the tongue on the mouth floor. After a first series of
scans, the patients were instructed to extend their necks
maximally while taking care to avoid lifting their shoulders
off the table. Then a second series of ultrasound scans was
performed. The neutral head position before each scan series
was verified using a goniometer to measure the angle
(90°) between the surface of the table and the patients’
mentum.
Airway Assessment and Digital Photography
A visual examination of the oropharynx was performed
with the patients sitting upright (mouth wide open, neck
flexed, not vocalizing, and tongue extended) and scored
using the Mallampati 10 classification with the Samsoon
and Young 11 modification (class I, soft palate, fauces, pillars,
and uvula visible; class II, soft palate, fauces, and uvula
visible; class III, only soft palate and base of uvula visible;
and class IV, only hard palate visible). The circumference
of the neck was measured in each patient in the upright
position at the level of the superior border of the thyroid
cartilage.
Figure 2. Positioning of the ultrasound probe and sonographic anatomy
of the suprahyoid airway in midsagittal (left) and transverse (right) scans.
The midsagittal cross-sectional area in this patient was 21 cm 2 , and the
tongue width was 5.4 cm. The calculated tongue volume (midsagittal
cross-sectional area × tongue width) was 113 cm 3 . The transverse crosssectional
area of the floor of the mouth muscles was 4.7 cm 2 , and the hyomental
distance in the neutral position was 5.2 cm. The calculated
muscle volume of the floor of the mouth (cross-sectional area of the floor
of the mouth muscles × hyomental distance in the neutral position) was
24.4 cm 3 . DG indicates diglossus muscle; GG, genioglossus muscle;
GH, geniohyoid muscle; H, hyoid bone; M, mandible; MH, mylohyoid
muscle; Pal, palate; and S, tongue surface.
Bony and cartilagenous landmarks such as the angle of
the jaw, hyoid bone, and mentum were identified by palpation
and marked with standard-diameter (19- or 8.5-
mm) paper stickers. Digital photographs of the oropharynx
and frontal/profile photographs of the head and neck in
the sitting position were obtained, followed by photographs
of the patients’ head and neck in supine neutral and
head-extended positions.
Tracheal Intubation
All patients had undergone general anesthesia with tracheal
intubation. Anesthesia records from previous anesthetics
were reviewed to evaluate the ease of intubation. The
laryngoscopic grades of Cormack and Lehane 12 that were
recorded during tracheal intubation were obtained. The
Cormack-Lehane laryngoscopic classification has 4 grades
(grade 1, glottis, including anterior and posterior commissures,
visible; grade 2, only posterior aspects of glottis visible;
grade 3, only epiglottis visible; and grade 4, epiglottis
not visible, only hypopharynx visible).
Statistical Analysis
The statistical analysis was performed with Prism software
(GraphPad Software, Inc, San Diego, CA). Data are
presented as means ± standard deviations. An unpaired
2-tailed t test was performed to determine statistical significance.
P < .05 was considered significant.
Results
The patient population in this study consisted of obese and
morbidly obese patients with a large neck circumference.
Their BMIs ranged from 30.1 to 52.3 kg/m 2 , with 7 patients
being morbidly obese (BMI >40 kg/m 2 ). The neck
circumferences of these patients ranged from 36.8 to 53.3
cm, with 9 patients having circumferences of greater than
40 cm. In spite of the increased submandibular soft tissue
mass in this group of patients, sonography enabled rapid
and efficient measurements of hyomental distances and
the hyomental distance ratio.
Table 1 lists the demographic data and suprahyoid airway
dimensions in the 12 patients. The mean hyomental
distance ratio in 6 patients who presented with a history of
difficult intubation was 1.02 ± 0.01, and the ratio in 6 patients
whose airway was easy to intubate was 1.14 ± 0.02
(P < .002). The hyomental distance ratio data are also presented
in a graphic form in Figure 3.
The mean hyomental distance in the neutral position
did not differ significantly between the two groups: 51.3 ±
5.3 mm (difficult intubation) versus 57.5 ± 4.3 mm (easy
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Wojtczak—Submandibular Sonography
Table 1. Demographic Data and Suprahyoid Airway Dimensions
A/S BMI NC MP CL In HMD n
HMD e
HMDR TV MVFM
26/F 45.2 40.6 II 1, E 60.7 69.5 1.14 132 24.7
43/M 30.7 43.2 II 2, E 55.2 64.4 1.16 166 21.0
24/M 47.5 51.3 IV 2, E 60.4 67.9 1.12 209 48.0
73/M 34.9 39.4 IV 3, D 49.3 50.2 1.01 172 29.4
76/F 30.2 36.8 IV 3, D 50.0 50.5 1.00 145 20.7
45/M 42.3 52.1 III 1, E 62.5 69.8 1.12 192 49.0
45/M 41.3 48.3 II 1, E 55.5 63.1 1.13 187 50.5
62/M 41.7 49.5 II 4, D 61.4 63.1 1.02 130 49.0
38/F 52.3 45.7 III 1, E 51.0 59.2 1.16 124 33.1
48/F 36.2 50.9 II 4, D 52.9 56.0 1.05 167 39.1
55/M 44.9 53.3 III 4, D 47.1 48.2 1.02 98 47.0
57/F 30.1 37.5 II 4, D 47.3 47.9 1.01 114 24.0
A/S indicates age (y)/sex; BMI, body mass index (kg/m 2 ); CL In, Cormack-Lehane laryngoscopic grade and ease of intubation assessment (D,
difficult; E, easy,); F, female; HMD e
and HMD n
, hyomental distance in the head-extended and neutral positions (mm); HMDR, hyomental distance
ratio; M, male; MP, Mallampati class; MVFM, muscle volume of the floor of the mouth (cm 3 ); NC, neck circumference (cm); and TV, tongue volume
(cm 3 ).
intubation), although the difference in the mean hyomental
distance in the head-extended position, 52.6 ± 5.8 mm
(difficult intubation) and 65.5 ± 4.1 mm (easy intubation),
did differ significantly (P < .01). Tongue volumes did not
differ significantly between the two groups, 137 ± 29 cm 3
(difficult intubation) and 168 ± 34 cm 3 (easy intubation).
The floor of the mouth muscle volumes were 34.8 ± 11
cm 3 in the difficult intubation group and 37.7 ± 13 cm 3 in
the easy intubation group. The mean BMIs were 36.3 ± 6
kg/m 2 in the difficult intubation group and 43.2 ± 7.2
kg/m 2 in the easy intubation group. The mean neck circumferences
were 44.5 ± 7.4 and 46.8 ± 7 cm in the difficult
and easy intubation groups, respectively.
Examples of patients in whom the airway was found to
be difficult and easy to intubate are shown in Figures 4 and
5, respectively. The top panels in both figures show digital
Figure 3. Hyomental distance ratio (HMDR) values in patients with a
history of difficult (open circles) and easy (filled triangles) intubation.
Means, standard deviations, and ranges are shown.
photographs of the patients (profile and supine with
the head in the neutral and hyperextended positions),
and the bottom panels show frontal photographs with
the mouth open and two submandibular sonograms (left,
midsagittal scan; right, transverse scan).
The patient in Figure 4 whose airway was impossible
to intubate had a hyomental distance ratio of 1.02. His photographs
show that he could extend his neck, although the
extension took place at the subaxial level of the cervical
spine without occipitoatlantal and atlantoaxial joint extension.
Therefore, his lower cervical spine is most likely bowing
forward, which lifts the larynx and the glottis forward
out of the line of view. 5,6 This state might explain why his
airway was impossible to intubate. Consistently, the airway
of the patient in Figure 5 who had a hyomental distance
ratio of 1.14, which indicates good occipitoatlantal
and atlantoaxial joint extension, was easy to intubate.
The patient in Figure 4 had unusually well-developed
muscles of the floor of the mouth. The combined volume
of the tongue and the muscles of the floor of the mouth
was 179 cm 3 in this patient versus 156 cm 3 in the patient in
Figure 5. Further questioning revealed that this patient was
a music teacher and had been playing trombone for more
than 40 years. It is likely that the continuous exercising of
the muscles of the floor of the mouth induced muscular
hypertrophy.
Discussion
Limited occipitoatlantal and atlantoaxial joint extension and
large tongue volumes can prevent adequate visualization of
the glottis and lead to an unexpected difficult tracheal intu-
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Figure 4. Images from a 62-year-old male patient (eighth patient in Table
1). Body mass index, 41.7 kg/m 2 ; hyomental distance in the neutral position,
61.4 mm; hyomental distance in the head-extended position, 63.1
mm; hyomental distance ratio, 1.02; midsagittal cross-sectional area,
30.2 cm 2 (bottom left sonogram); tongue width, 4.3 cm (bottom right
sonogram); tongue volume, 130 cm 3 ; muscle volume of the floor of the
mouth, 49 cm 3 ; Mallampati class, II; Cormack-Lehane grade, 4; failed intubation
after several attempts by 3 experienced anesthesiologists.
Video laryngoscopy (GlideScope; Verathon, Inc, Bothell, WA) was attempted
without success. The patient was awakened, and planned surgery
was canceled. DG indicates diglossus muscle; GH, geniohyoid
muscle; H, hyoid bone; M, mandible; and MH, mylohyoid muscle.
bation. 1 Bedside assessment of the occipitoatlantal and
atlantoaxial joint extension capacity using the hyomental
distance ratio may be useful in predicting difficult laryngoscopy,
as shown by Huh et al 13 in a group of Korean patients
with a mean BMI of less than 25 kg/m 2 . The main
finding of this study is that submandibular sonography allows
measurements of hyomental distance ratios and
tongue volumes in obese patients (BMI 30.1–52.3 kg/m 2 ).
Because of the increased submandibular tissue mass in
these patients, measurements of the hyomental distance
by palpation may be difficult or even impossible. Using a
hard–plastic bond ruler, Huh et al 13 evaluated the predictive
value of surface hyomental distance ratio measurements
in 213 consecutive nonobese adult patients
undergoing elective surgery and anesthesia with tracheal
intubation. They analyzed various predictors alone and in
all combinations. The hyomental distance ratio alone had
the highest predictive validity for difficult laryngoscopy
with an optimal cutoff point of 1.2. At this cutoff point, the
hyomental distance ratio had sensitivity of 88% and specificity
of 60% for predicting difficult laryngoscopy. Obese
patients in our study were either difficult or easy to intubate.
The sonographic hyomental distance ratios in the difficult
intubation group were in the 1 to 1.05 range, and
those in the easy intubation group were in the 1.12 to 1.16
range, consistent with the data of Huh et al. 13 The difference
between the mean hyomental distance ratio values in
the two groups was highly statistically significant.
Singh et al 14 presented a comprehensive sonographic
assessment of the suprahyoid and infrahyoid airways. In a
recent study by Prasad et al, 15 the same group assessed
agreement between computed tomography and sonography
for visualization of airway structures. All airway measurements
were closely related except for the hyomental
distance. They suggested that in some patients, the presence
of a “high-lying” or cephalad position of the hyoid
bone may alter the structures visualized by the suprahyoid
sonographic window. It is also possible that the position of
the head and neck was slightly different during both examinations,
although a standard headrest was used during
the scans. Because the hyomental distance ratio is a ratio of
two linear distances, its accuracy should not change if both
linear distances are affected to the same extent by altered
sonographic structure visualization.
The line of vision to the glottic opening during laryngoscopy
is likely to be affected both by the size of the
tongue and sufficient room in the mandibular space for the
tongue to be displaced into. 1,10 In this study, the tongue
volumes were not very different between the difficult and
easy intubation groups. One of the reasons for the lack of
a significant difference could have been the small study size.
It is also possible that the tongue volume can be a predictor
of difficult intubation only if expressed as a ratio of the
available mandibular volume.
Figure 5. Images from a 26-year-old female patient (first patient in Table
1). Body mass index, 45.2; hyomental distance in the neutral position,
60.7 mm; hyomental distance in the head-extended position, 69.5 mm;
hyomental distance ratio, 1.14; midsagittal cross-sectional area, 24.9 cm 2
(bottom left sonogram); tongue width, 5.3 cm (bottom right sonogram);
tongue volume, 132 cm 3 ; muscle volume of the floor of the mouth, 24.7
cm 3 ; Mallampati class, II; Cormack-Lehane grade, 1; easy intubation. DG
indicates diglossus muscle; GH, geniohyoid muscle; H, hyoid bone; M,
mandible; and MH, mylohyoid muscle.
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Wojtczak—Submandibular Sonography
In this study, 5 of 7 morbidly obese patients were easy
to intubate. Whether tracheal intubation is more difficult in
obese patients is debatable. 1 Brodsky et al 16 studied 100
consecutive patients with a BMI of greater than 40 kg/m 2
and concluded that neither absolute body weight nor BMI
is associated with intubation difficulties. Rather, they found
that a large neck circumference of 40 cm showed a 5%
probability of problematic intubation, and of a circumference
of 60 cm showed a 35% probability. This observation
was not confirmed by our study.
In summary, this study shows that the hyomental distances
in the neutral and head-extended positions, hyomental
distance ratio, and volumes of the tongue and
muscles of the floor of the mouth can be measured in
obese patients with a large neck circumference by performing
a noninvasive submandibular sonographic examination.
Preoperative assessment of the hyomental
distance ratio may predict difficult laryngoscopy resulting in
difficult intubation.
10. Mallampati SR. Clinical sign to predict difficult tracheal intubation (hypothesis).
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13. Huh J, Shin HY, Kim SH, Yoon TK, Kim DK. Diagnostic predictor of
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14. Singh M, Chin KJ, Chan VWS, Wong DT, Prasad GA, Yu E. Use of
sonography for airway assessment. J Ultrasound Med 2010; 29:79–85.
15. Prasad A, Yu E, Wong DT, Karhanis R, Gullane P, Chan VWS. Comparison
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16. Brodsky JB, Lemmens HJ, Brock-Utne JG, Vierra M, Saidman LJ. Morbid
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