Developmental Changes of Laryngeal Dimensions in Unparalyzed, Sedated Children

The Research Subjects’ Review Board of the University of Rochester (Rochester, New York) approved this study, and written informed consent was obtained from all parents of children in the study. Children who were old enough to understand that they were participating in a research study gave their assent. Children aged 0–14 yr who presented for an elective magnetic resonance imaging scan with deep sedation were eligible to participate. Consecutive children were enrolled within the limits of investigator-related and clinical situations. Exclusion criteria included obvious anatomical deformities of the head or neck, or any other condition the investigators felt would cause abnormal laryngeal anatomy. Children who received airway management with either a laryngeal mask airway or endotracheal tube were also excluded.

The study was performed in children who were deeply sedated with 200 μg · kg⁻¹· min⁻¹propofol. Most children did not previously have an indwelling intravenous catheter and therefore initially received a mask anesthetic with sevoflurane and nitrous oxide, both of which were discontinued after intravenous catheter placement. Spontaneous ventilation was maintained throughout the procedure. A nasal cannula provided oxygen, usually at 2 l/min. Monitors included continuous electrocardiography, pulse oximetry, capnography (via  the nasal cannula), and intermittent automated blood pressure determinations (Medrad, Inc., Indianola, PA). A best attempt was made to position the child's head in the neutral position, usually with a folded sheet beneath the neck or shoulders. Occasionally, a nasal or oral airway device was used to assist with maintenance of upper airway patency if excessive head or neck movement occurred with each breath. No attempt was made to obtain images at a certain point in the respiratory cycle.

Magnetic resonance imaging was performed on a 1.5-T unit with a quadrature head coil (GE Medical Systems, Milwaukee, WI). T1 spin-echo images were obtained with 3-mm slices at 0.5-mm intervals.

Axial scans were obtained with the oropharynx as the most cephalad limit and the trachea below the cricoid cartilage as the most caudad limit. A reference sagittal image was used to help identify the proper level of the vocal cords. The cricoid cartilage was identified as the most caudad part of the larynx that was continuous posteriorly. In each child, transverse and anterior–posterior (AP) dimensions of the larynx were measured at a minimum of two levels—the vocal cords and cricoid ring. Most children had at least one additional level measured between the vocal cords and cricoid cartilage, depending on their size. Two authors measured each child's larynx independently. When there was a discrepancy between the two, the final value was decided upon by consensus or with assistance from a third author. A radiologist author was involved with measurement of every child.

Results were obtained by plotting the dependent variable (age) against the independent variables—laryngeal dimensions, and the ratio of laryngeal dimensions at different levels (Origin, Microcal Software, Northhampton, MA). The sub–vocal cord level was defined as the magnetic resonance imaging slice immediately caudad to the vocal cords but above the cricoid ring.

Linear regression was used to determine the relationship between dependent and independent variables. Kruskal-Wallis and Mann–Whitney tests were used to compare continuous nonparametric data (difference between laryngeal dimensions at different levels). A P  value of 0.05 or less was used to indicate statistical significance.

One hundred nine children initially enrolled in the study. Ten were excluded because of various reasons that included movement artifact (4), scanner malfunction (1), abnormal laryngeal anatomy (2), and corrupted optical disks that prevented retrieval of the scans (3). Therefore, 99 children are presented for analysis and are described in table 1. In the initial data analysis, males and females were analyzed separately. However, since their results were essentially the same, we report the aggregate results.

The relationship between the transverse and AP dimensions at all levels of the larynx, including the cricoid ring, did not change during development. Transverse dimensions were narrower than AP dimensions at all levels of the larynx above the cricoid ring and in most children at the cricoid ring (fig. 1). Transverse and AP dimensions increased linearly with age at all levels of the larynx (fig. 2, P < 0.001 for all dimensions).

In all children studied, the narrowest portion of the larynx was the transverse dimension at the level of the vocal cords. Transverse dimensions increased linearly in a caudad direction through the larynx (P < 0.001, fig. 3, top), while AP dimensions did not change relative to laryngeal level (fig. 3, bottom). Thus, during spontaneous breathing in unparalyzed, sedated children, the larynx is conical shaped in the transverse dimension (with the apex of the cone at the level of the vocal cords) and is cylindrical shaped in the AP dimension. These relationships are demonstrated in a representative scan (fig. 4).

The major finding of this study is that in sedated, unparalyzed children, the shape of the larynx is conical in the transverse dimension (with the apex of the cone at the level of the vocal cords) and cylindrical in the AP dimension, and does not change throughout development. This is in contrast to that of the atonic larynx, in which cadaver specimens have demonstrated that the larynx is conical with the apex at the caudally positioned cricoid ring during early childhood. 1–3During development, the transverse dimension of the atonic larynx assumes a more cylindrical shape, although the age at which this occurs is not precisely known. 4 

In publications that used measurements from cadaver specimens, the portions of the larynx cephalad to the cricoid ring could be distended since these areas do not contain full circumferential cartilage. The patients included in the present study were deeply sedated and not paralyzed. Active contraction of the laryngeal muscles influenced the dimension of the larynx above the cricoid ring, especially at the level of the vocal cords, which continue to move during spontaneous respiration. 5Indeed, lightening of consciousness in anesthetized adults is associated with increasing tracheal cuff pressures, 6and lack of paralysis in anesthetized children is associated with an audible leak at an increased airway pressure. 7This is further borne out by the increased variability of the transverse dimensions at the level of the vocal cords compared with other levels of the larynx (fig. 2, top left). Thus, although our measurements describe the glottic opening to be the narrowest portion of the larynx in spontaneously breathing, unparalyzed children, the rigid cricoid ring is functionally the narrowest portion of the larynx.

A limitation of these findings is that we did not attempt to image the larynx in any particular phase of respiration. Since vocal cord dimensions likely change in spontaneously breathing, sedated children, the dimensions that we measured should be considered to be an average measurement during movement of the vocal cords during spontaneous respiration. Therefore, it is possible that in some children, the vocal cords were “captured” in a more adducted state than usual, as in expiration. If this occurred in a significant number of our subjects, the shape of the larynx in the transverse direction would be more cylindrical than conical. An alternative explanation for our findings is the possible influence of propofol on vocal cord position. In dogs, propofol depresses vocal cord abductor (posterior cricoarytenoid muscle) and vocal cord tensor (cricothyroid muscle) activity 8and may have influenced vocal cord positions in the children we studied.

In 1951, Eckenhoff 4published a seminal article on characteristics of the infant larynx and their influence on endotracheal anesthesia. This article was one of the first to emphasize that the cricoid cartilage is functionally the narrowest point of the upper respiratory tract of the child. Eckenhoff described the cricoid plate as “inclined posteriorly at its superior aspect, so that the larynx is funnel shaped with the narrowest point of the funnel at the laryngeal exit.” This narrowest point is described as possibly smaller than more cephalad portions of the trachea. Eckenhoff stated that as the child grows, the cricoid plate becomes vertical, and the larynx becomes more cylindrical shaped. He derived this information from Bayeux, 1who used moulages and anatomic sections of 15 children, aged 4 months to 14 yr. Bayeux documented that the circumference of the cricoid ring was narrower than that of the trachea or the glottis. Our results do not allow us to make direct comparisons with the findings of Eckenhoff and Bayeux since we measured dimensions in the tonically active larynx.

The clinical importance of our findings is speculative. On one hand, our results indicate that choosing an endotracheal tube based on the size of the cricoid ring may not prevent mucosal damage to the larynx cephalad to the cricoid ring in unparalyzed children. On the other hand, these more cephalad portions of the larynx consist of yielding structures that distend with placement of a relatively larger endotracheal tube. The cricoid ring is unyielding and prone to development of edema and scarring in response to excessive mucosal pressures.

Autopsy studies have described the laryngeal pathology that occurs with prolonged intubation and mechanical ventilation in children. The entire larynx is often involved, and the most severe damage often occurs at the level of the vocal cords. 9–14Our findings are consistent with these pathologic observations. Although it is not specifically mentioned in these studies, one can speculate that the children included in these autopsy studies did not consistently receive neuromuscular blockade, and therefore, the glottic regions above the cricoid were the most narrow and bore the brunt of endotracheal tube irritation and mucosal damage.

In summary, we found that in sedated, unparalyzed children, the narrowest portions of the larynx are the glottic opening (vocal cord level) and the immediate sub–vocal cord level, and there is no change in the relationships of these dimensions, relative to cricoid ring dimensions, throughout childhood. In addition, we found that the shape of the cricoid ring does not change during development. Our findings offer a plausible mechanism for the more cephalad laryngeal (glottic) damage that occurs in chronically intubated children.

The authors thank Traci Salter, R.T.R., for assistance and the team of magnetic resonance imaging technicians at Strong Memorial Hospital, Rochester, New York, for their valuable assistance with this project and their dedication to the care of children.