Introduction
The methods developed for the management of laryngeal disorders in laryngology and
phoniatrics have led to an increasing demand for more individualized, real, and accurate
measurements of glottic structures. Measurements of the glottic area and its structures
provide precious data for the assessment of the various laryngeal pathologies and
of some phonosurgeries and procedures, such as correction of glottic insufficiency
following laryngeal palsy.
Endoscopic laryngeal imaging does not enable adequate absolute measurements of the
glottic area or length because both flexible and rigid laryngeal endoscopes have a
fairly broad field depth, resulting in the expectation of large alterations in the
distance from the endoscope to the glottic area through examination with varying measurements.
There is also some distortion of the laryngeal images depending on the angle between
the instruments and the glottic area and of the endoscopy system itself. Several authors
have discussed these sources of errors and proposed standardized examination procedures
to lessen the errors and carry out reproducible examinations.[1]
[2]
[3] Moreover, accurate absolute measurements from laryngeal examination video recording
demand a known calibration distance to relate.
Computed tomography (CT) is a standard tool for the assessment of laryngeal neoplasms
and trauma.[4]
[5] Axial scans obtained during phonation at the plane of the true vocal fold (VF) can
identify soft tissue and cartilage. The length of the membranous part of the VF (from
the anterior commissure to the tip of the vocal process) is invariable if the subjects
phonate at a stable, constant pitch in a stable voice recorder.[6] Thus, CT measurement of the VF length (VFL) might be used to calibrate and estimates
of VFL from stroboscope video recording and so the glottic region during phonation
can also be estimated.
Therefore, preoperative CT assessment is essential to identify the anatomical landmarks
and dimensions of the larynx, their relationships to the surrounding structures, and
to evaluate the exact extension of the disease to aid in the proper management and
the performance of a safe and effective surgery. But, in certain cases, such as those
of neoplasms, trauma, or revision, these landmarks may be absent or distorted, increasing
the dissection difficulty and the risk of complications. Therefore, it is paramount
to identify the dimensions of the larynx preoperatively and its landmarks to guide
the surgeons.
In the literature, some attempts to measure the VFL have been made in cadavers,[7] in a recently removed larynx,[8] or based on X-rays.[9]
[10] The results obtained from cadavers could not reflect the physiological VFL in a
live person, so they do not provide helpful clinical information. Although using lateral
X-rays of the neck to estimate the VFL might be convenient and provide practical data,
it is difficult to measure the VFL precisely, and vague landmarks are used to estimate
the VFL. In lateral radiographs, the VF thickness renders the reference points obscure
and makes data interpretation hard.
Thus, the present study aimed to provide more precise measurements of the VFL and
of the subglottic area in live subjects through CT. The results may influence laryngeal
surgery, especially the endoscopic approach and phonosurgery.
Methods
The current retrospective study was conducted on 100 CTs of the larynx (200 vocal
cords) at the Otorhinolaryngology Departments of two universities between July 2021
and January 2023. The informed consent form was signed by all subjects, and the study
was approved by the institutional review board.
Patients younger than 18 years, with a history of trauma or surgery in the larynx
or neck, and those with congenital anomalies and/or malignancies were excluded.
All subjects were submitted to 64-slice CT scans (Lightspeed VCT 64, GE HealthCare).
The protocol of the 64-slice multidetector CT (MDCT) scans were as follows: 0.625 mm
of detector width, sections with a width of 1.5mm, and a 0.5-mm interval reconstruction.
130KV and 150 mA/sec with 1.5sec scan time.
The axial scans of the larynx were performed with the subject in the supine position
and breathing quietly. A high-resolution algorithm was used to enhance the appearance
of the delicate bony details.
The scans were reviewed in a routine standardized manner in order not to miss small
details. The VFL from the anterior commissure to the tip of the vocal process was
measured. The anteroposterior (AP) and transverse dimensions, along the axial planes,
were measured at the level of the cricoid cartilage ([Fig. 1]). The statistical analysis was conducted using IBM SPSS Statistics for Windows (IBM
Corp.) software, version 25.0. Values of p < 0.05 were considered statistically significant.
Fig. 1 Measurements of the length of the vocal folds (A) and of the dimensions of the subglottic area (B).
Results
The present study included 100 CT scans (200 sides) of 48 male (48%) and 52 female
subjects (52%) with a mean age of 53.4 (range:13–87) years.
The mean AP VFL was of 16.8 ± 2.6 (range: 12.5 to 22.9) mm ([Table 1]), which was significantly longer in male (mean = 18.4 ± 2.2 mm; range = 15.6–12.9 mm)
compared to female subjects (mean = 15.3 ± 1.95 mm; range = 12.5–19 mm) (t = 7.4687;
p < 0.0001) ([Table 1]).
Table 1
Computed tomography measurements of the length of the vocal cords and dimensions of
the subglottic area
|
Mean vocal cord m length (mm)
|
Mean subglottic area dimensions
|
|
Transverse (mm)
|
Anteroposterior (mm)
|
|
All subjects
|
16.8 ± 2.6
|
14.88 ± 2.18
|
17.5 ± 2.9
|
|
Males (n = 48)
|
18.4 ± 2.2
|
16.4 ± 2.46
|
18.7 ± 2.9
|
|
Females (n = 52)
|
15.3 ± 1.95
|
14.3 ± 1.8
|
16.49 ± 2.47
|
|
Student's
t
-test
|
7.4687
|
3.4466
|
4.1124
|
|
p
-value
|
< 0.0001
|
0.0008
|
< 0.0001
|
The mean AP diameter of the subglottic area was of 17.5 ± 2.9 (range = 12.7–24.9)
mm, which was longer in male (mean = 18.7 ± 2.9 mm; range = 14.2–24.9 mm) compared
to female subjects (mean = 16.49 ± 2.47 mm; range = 12.7–20.8 mm)
The mean width at the level of subglottic area was of 14.88 ± 2.18 (range: 12.3–21.1)
mm, and it was significantly wider in male (mean = 16.4 ± 2.46 mm; range: 13.2–21.1 mm)
compared to female subjects (mean = 14.3 ± 1.8 mm; range: 12.3–17.8 mm) (t = 4.8979;
p < 0.0001) ([Table 1]). Therefore, all the dimensions of the larynx were found to be significantly smaller
in female compared to male subjects.
There was strong positive correlation between the AP diameter of the subglottic area
and the VFL: the longer the AP diameter of the subglottic area, the longer the VFL
and vice versa. (r = 0.7181; p < 0.0001)
There was moderate positive correlation between transverse diameter of the subglottic
area and the VFL: the longer the transverse diameter of the subglottic area, the longer
the VFL and vice versa (r = 0.6718; p < 0.0001).
Discussion
The VFL can be used as calibration distance to assess the absolute area and length
of glottal structures and/or masses or lesions based on an examination with flexible
or rigid laryngeal endoscopes and on video recordings during phonation.
The dimension of the membranous glottis appears to be an appropriate calibration length
because its anatomical landmarks could be accurately determined by the laryngoscopic
examination and based on the CT scans.
Laryngoscopy is considered the standard tool for laryngeal examination; it is a simple
and valued option for the assessment of laryngeal lesions or the glottic gap, but
it is not suitable for the measurement of the VFL or of the dimensions of lesions.
This limits its quality for the measurement of the absolute areas.
The length of the outstretched VF can also be observed and measured distinctly through
direct laryngoscopy under general anesthesia. But, under general anesthesia, there
are dimension differences that could arise from the muscle tone loss or stretch by
the endotracheal tube insertion or possibly caused by the introduction of the laryngoscope
into the larynx. All these aspects could change the VF shape and could cause VF thinning
and widening of the base of a benign lesion.[11] This factor should be taken into consideration when performing functional VF surgeries
under general anesthesia. That is why the CT measurement appears to be more real,
reliable, and accurate.
Knowledge of the VFL is significant for professionals who rely on their voices for
work, such as teachers and singers. However, a simple and risk-free method that enables
the collection of normative reference values for the laryngeal structures has not
yet been fully developed.[12]
Improved optic instruments, such as flexible or rigid laryngoscopes with video recording
provide a superior view of the anatomy of the surface of the larynx. If stroboscopy
is used, the glottic vibrations at the free VF edges are distinctly seen during phonation,
and the degree of glottic closure can be estimated. Moreover, the disturbance of the
VF vibrations can be instantly examined during phonations.[13]
[14] Videostroboscopy is more sensitive to camera rotation, the side movement of the
laryngoscope, and patient movements, which give the VF delocation.[15]
However, the endoscopic laryngeal images obtained with these tools do not provide
an adequate absolute measurement of glottic area or length because of the fairly wide
depth of field for flexible and rigid laryngeal endoscopes, with large distance variations
between the endoscope and the VF throughout the examination, causing varied area and
length measurments. In addition, there are significant distortions in the larynx images
depending on the angle between the endoscope and the VF and on the endoscope, system
used (called barrel-shaped distortions).[2]
[13]
Various methods have been used in the past to measure VFL[7] in cadaveric larynges, but they often used a fixation or plastination process for
the specimens. Hu et al.[9] have attempted to measure the true VFL in live individuals using methods such as
photography, plain films, ultrasound, and laser. Kim et al.[10] used the X ray but could assess only the laryngeal airway, not the inner structure
of the larynx. Litman et al.[16] used magnetic resonance imaging (MRI) scans to measure the larynges of children
under sedation, which could alter the dimensions. Moreover, the unavailability of
the MRI and patient fears regarding the device make it difficult to depend on it.
And Sankar et al.[17] used a three-dimensional digital model of the human larynx derived from the published
literature and radiographs.
Computed tomography (CT) has become one of the gold-standard tools for laryngeal evaluation,
providing high-quality axial scans of true VF and of the laryngeal cartilage. Laryngeal
CT is a standard method to evaluate neoplastic and traumatic laryngeal lesions. The
axial scans obtained during phonation at the plane of the true VF could clearly identify
laryngeal soft tissue and cartilage. The length of the membranous part of the VF (from
the tip of the vocal process to the anterior commissure) is invariable when the subject
phonates at a stable, constant pitch and in a stable voice register.[10]
Thus, CT measurements of the real VFL, from the vocal process to the anterior commissure,
could be used to estimate the VFL through stroboscopic video recordings in which the
laryngeal structures and glottal area can be identified and estimated.[18]
The advantages of measuring the absolute VFL through CT are that the measurements
are easy and accurate, with the subject breathing quietly and without the introduction
of instruments in the air passage that may disturb it or cause patient discomfort,
and without need for local anesthetics or any procedures through the airways. A disadvantage
is the radiation dose, which is minimal, because high-resolution CT scans of the neck
region are associated with a relatively small dose of radiation, which makes this
type of scanning technique safe.[5]
[19]
Therefore, we chose CT to determine the different dimensions of the larynx and VF.
In the current study, the mean AP VFL was of 16.8 ± 2.6 (range: 11.8–22.9) mm, which
was significantly longer in male subjects (mean = 18.4 ± 2.2 mm; range = 10.9–19.8 mm)
compared to female subjects (mean = 15.3 ± 1.95 mm; range = 10.9–22.9 mm). The mean
width of the subglottic area was of 14.88 ± 2.18 (range = 8.9–21.1) mm, which was
significantly wider in male subjects (15.6 ± 2.13 mm; range = 9.1–21.1 mm) compared
to female subjects (mean = 14.18 ± 1.99 mm; range = 8.9–17.8 mm). There was a positive
correlation between the AP and transverse diameters of the subglotic area with the
VFL: the longer the AP and transverse diameters of the subglottic area, the longer
the VFL and vice versa.
Previous studies have measured the mean VFL in healthy male and female adults, and
the respective results were as follows: 13.8 ± 2.92 mm and 10.7 ± 1.63;[20] 22.09 ± 3.07 mm and 17.55 ± 0.92 mm;[21] 15.3 mm and 13.5 mm;[9] 16.11 ± 2.62 mm and 14.10 ± 1.54 mm; [22] and 24.9 mm and 17.5 mm.[8] In all of these studies, it was clear that male subjects has longer VFL than female
subjects, but there are wide variations in the measured VFL in different studies and
regions, reflecting the importance of normative data for each ethnic group. The mean
VFL is of 14.9, 16.0, 16.6, 18.4, 19.5, and 20.9 mm for sopranos, mezzo-sopranos,
altos, tenors, baritones, and basses, respectively.[23] These variations reflect the importance of individual VFL measurements for each
subject.
Hertegard et al.[18] repeated CT scans for several pitches in one of their male subjects, who showed
a maximum variation in the measured length of the membranous part of the glottis of
0.35 mm, that is 3% more. This male subject and a female subject were examined through
CT during phonation with normal and loud intensity in the frequencies 110 Hz and 220 Hz
respectively. For the male subject, they found no length difference, but the female
subject presented a 0.5-mm shortening of the membranous part of the glottis during
loud phonation compared to normal phonation. Therefore, the differences in FVL during
phonation are minimal and insignificant.
In addition, measurements of the dimensions of subglottic area at the cricoid cartilage
is are highly valuable and important because it is the only complete ring in the airway
representing the maximum caliber through which the endoscopy or endotracheal tube
can be passed, and can be considered as the main cartilage around which the larynx
framework is made-up. The dimensions measured in the current study could be highly
valuable for the evaluation of the vocal cord range and also for the planning of laryngeal
surgeries.
Conclusion
The present study updates the knowledge of laryngeal dimensions from a CT perspective
to improve surgeons' and radiologists' awareness for optimum and safe surgeries.
Bibliographical Record
Mohammad Waheed El-Anwar, Ali Awad, Atef Hussain, Ehsan Hendawy, Mohammad A. El Shawadfy,
Mohamed Ibrahim Abdelzeem Heggy, Mohamed Adel Mobasher. Laryngeal Dimensions: A Computed
Tomography Study. Int Arch Otorhinolaryngol 2025; 29: s00451810117.
DOI: 10.1055/s-0045-1810117
