To test a hypothesis that a child's anatomy – specifically, their individual pharyngeal geometry and/or soft tissue anatomy -- correlates with the severity of SDB, a team of researchers has examined MRI images and other data gathered from a group of children 7-12 years of age with sleep-related respiratory disturbances of varying severity. The researchers found that children with a narrow "retropalatal air space," defined as the ratio of the retropalatal airway cross-sectional area (CSA) to the CSA of the soft palate, had significantly more apneas and hypopneas (abnormally slow, shallow breathing) during sleep than did the children with relatively unobstructed airways.
A New Study
The authors of the study, entitled "Sleep-Disordered Breathing, Pharyngeal Size, and Soft Tissue Anatomy in Children," are R.F. Fregosi, S.F. Quan, K.L. Kaemingk, W.J. Morgan, J.L. Goodwin, R. Cabrera, and A. Gmitro, all of the University of Arizona, Tucson, AZ. Their findings appear in the online edition of November 2003 edition of the Journal of Applied Physiology. The Journal is one of 14 journals published each month by the American Physiological Society (APS.)
Methodology
To test their premise that pharyngeal geometry and soft tissue dimensions correlate with the severity of sleep-disordered breathing in children, the researchers used the following methodology:
Participants: From an earlier study, the researchers randomly chose 10 children with a respiratory disturbance index (RDI) of <5 and 10 children with RDI values of >5. RDI was defined as the number of respiratory events (apneas and hypopneas) per hour of the total sleep time, irrespective of any associated oxygen desaturation or arousal. The final sample comprised 18 subjects, 7-12 years of age. Of this number, four snored and three of those four also had witnessed apnea. Four other subjects had excessive daytime sleepiness, and two of those also snored. All subjects with symptoms were placed in the high-OAHI (obstructive apnea-hypopnea index) group.
Polysomnography: All participants underwent unattended home polysomnography from which key data were obtained: C3/A2 and C4/A1 electroencephalogram, right and left electrooculogram, a bipolar submental electromyogram, thoracic and abdominal displacement, airflow, nasal pressure, electrocardiogram, snoring, body position, pulse oximetry, and ambient light exposure.
Airway Imaging: In order to be able to study airway imaging, participants had to be positioned within an MRI machine so that their entire pharynx was clearly visible. Axilal images were obtained from just above the orbital cavity to just below the larynx. Sagittal images were obtained from the midline to the ears bilaterally. To highlight the air spaces, longitudinal relaxation time (T1)-weighed axial and sagittal spin-echo images were obtained in two separate series. To highlight the pharyngeal soft tissues, a third sequence consisting of transverse relaxation time (T2)-weighted sagittal fast spin-echo images was obtained.
Image Analysis: Because air-filled spaces are black on MRI, the research team used a threshold method to differentiate the oropharyngeal airway from surrounding tissue. Bony and soft tissues were analyzed using standard radiologic landmarks. T-1--weighted axial slices were used to measure the CSA of the tonsils at their widest point, the pharyngeal fat pad CSA, and the intermandibular distance. Midline T-1--weighted saggital slices were used to obtain the CSA of nasopharynx. T-2--weighted saggital images were used to obtain the CSA of the soft palate and adenoids. Measuring the CSA of each relevant axial slice and multiplying the CSA by the slice thickness to obtain a volume for each slice yielded the volume of the oropharynx. Boundaries defining the oropharynx included the tongue or soft palate anteriorly, the pharyngeal constrictor muscle posteriorly, and the pharyngeal tonsils laterally. A horizontal line extending from the junction of the hard and soft palate to the posterior wall of the pharynx defined the rostral limit of the oropharynx, while the tip of the epiglottis defined the caudal margin.
Data Analysis: Subjects were divided into two groups: one with high- (>7) and one with low- (<4) OAHI values. Researchers used ANOVA, followed by the Student-Neuman-Keuls post hoc procedure, to determine which variables were significantly different between the groups, identifying a post hoc P value of <0.05 as significant.
Results
Highlights of the researchers' findings with respect to pharyngeal geometry and soft tissue dimensions in high- and low-OAHI groups included the following:
(1) the CSAs of the tonsils and soft palate were significantly larger in the high- compared with the low-OAHI group; (2) although there was a trend toward a larger adenoid volume and pharyngeal fat pad CSA in the high-OAHI group, the differences were not significant; and both tonsil and soft palate CSA correlated significantly with the OAHI, accounting for 44 and 39 percent of the variability in the OAHI, respectively; (3) significant differences between the high- and low-OAHI groups were found for the sum of tonsil and soft palate CSA, tonsil plus adenoid CSA, and the sum of tonsil, adenoid, and soft palate CSA; (4) analysis of pharyngeal airway dimensions did not reveal differences in the CSA of the retropalatal or nasopharyngeal air spaces, but did show that the volume of the oropharynx was smaller in the high- compared with the low-OAHI group; (5) a significant correlation between the OAHI and the oropharyngeal volume was found; and (6) researchers found that the narrowest diameter measured at any point in the oropharynx of children with high-OAHI values was significantly smaller than that measured in the group with low-OAHI values. There was also a significant correlation between this variable and the OAHI.
With respect to the airway CSA-length relation, researchers found:
- (1) the airway was significantly narrower in the region where the soft palate, adenoids, and tonsils were maximally overlapped in both high- and low-OAHI groups; (2) area-length curves of the low- and high-OAHI groups were significantly different; and (3) subjects with narrow retropalatal air spaces tended to have higher OAHI values.
Conclusions
From these findings, the research team has concluded that:
- the severity of SBD correlates significantly with the oropharyngeal volume and the size of the tonsils and soft palate in children 7-12 years of age;
- the pharynx of children with high-OAHI values is significantly narrower where the adenoids, tonsils, and soft palate overlap; and
- the OAHI is inversely and significantly related to the size of the retropalatal air space.
Although this study has several limitations, including the fact that a true control group composed of children with a total absence of respiratory disturbances during sleep was not used and the researchers' decision not to sedate the children for the imaging protocol.
Nevertheless, these findings add to the growing body of knowledge of how all children may someday get a good night's sleep, despite their biology.
Source: November 2003 edition of the Journal of Applied Physiology.
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.
Journal
Journal of Applied Physiology