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ABSTRACT |
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ABSTRACT
INTRODUCTION
METHODS
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DISCUSSION
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Obesity and craniofacial abnormalities may contribute to the pathogenesis of obstructive sleep apnea. The purpose of this study was to evaluate the influence of body habitus and craniofacial characteristics on types of pharyngeal closure. The types of pharyngeal closure were determined by endoscopic evaluations of closing pressures of the passive pharynx in 54 paralyzed and anesthetized patients with sleep-disordered breathing (SDB). Assessment of craniofacial characteristics of the SDB patients and 24 normal subjects were made by lateral cephalometry. As compared with normal subjects, SDB patients demonstrated receded mandibles and long lower faces with downward mandible development. SDB patients with positive closing pressures at both the velopharynx and oropharynx (VP + OP group) demonstrated smaller maxillas and mandibles than those with positive closing pressures at the velopharynx only (VP-only group). Obesity was more prominent in the VP-only group than in the VP + OP group. Our results suggest that obesity and craniofacial abnormalities contribute synergistically to increases in collapsibility of the passive pharyngeal airway in patients with SDB. Furthermore, the relative contribution of obesity and craniofacial anomaly appears to determine the type of pharyngeal closure in SDB.
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INTRODUCTION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Keywords: obstructive sleep apnea; pharynx; closing pressure; obesity; cephalometry
Current concepts of pharyngeal airway maintenance suggest that pharyngeal airway size is determined by an interaction between neural regulation of pharyngeal airway dilator muscle activity and structural properties of the pharyngeal airway (1). Therefore, pharyngeal collapse during sleep in patients with obstructive sleep apnea (OSA) may be caused by structural abnormalities of the pharyngeal airway (anatomic hypothesis) and/or by an impairment in neural regulation of pharyngeal muscle activation (neural hypothesis). Through elimination of the neural control mechanism, we recently demonstrated that closing pressures of the passive pharynx were distinctively higher in apneic patients than in age- and body mass index (BMI)-matched normal subjects (2). Although this provides conclusive evidence for the anatomic hypothesis, the specific structural abnormalities that increase pharyngeal closing pressures remain unclear. A wide variation in pharyngeal closing pressures was revealed in apneic patients, with positive closing pressures demonstrated in only 50% of the patients at the retroglossal airway but in almost 100% at the retropalatal airway (2). It would be reasonable to hypothesize that the heterogeneity of collapsibility along the pharynx is due to varying anatomic abnormalities, such as obesity and craniofacial anomalies, which have been repeatedly reported to have strong associations with the development of OSA (3). However, studies of the influence of body habitus and craniofacial characteristics on closing pressures of the passive pharynx have yet to be reported. Accordingly, we undertook a study to evaluate the effects of craniofacial characteristics and body habitus on the segmental collapsibility of the passive pharynx.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects
The study involved 54 patients with sleep-disordered breathing (SDB) and 24 normal subjects. The aim and potential risks of the study were fully explained to each subject, and informed consent was obtained from each. The investigation was approved by the hospital ethics committee of our institution.
Pharyngeal Endoscopy under General Anesthesia
Detailed description of our endoscopic technique and evaluations of
static pharyngeal mechanics are available in previous reports (2, 8, 9).
Evaluations were performed only in patients with SDB. Briefly, static
pressure-area relationships for the velopharynx (VP) and oropharynx
(OP) were obtained under general anesthesia with complete paralysis.
The pressure-area relationship of each pharyngeal segment was denoted as the exponential function A = Amax 
B · e
K · PAW, where B
and K are constants and A is cross-sectional area. A nonlinear least-squares technique was used for the curve fitting (SigmaPlot version
2.0; Jandel Scientific Software, San Rafael, CA), which allowed estimation of closing pressure (P'close) from the following equation for
each pharyngeal segment: P'close = ln(B/Amax)K1.
Determination of Pharyngeal Closure Types
On the basis of our previous findings, abnormal collapsibility of each
pharyngeal segment was defined as P'close 
0 cm H2O, which led to
determination of pharyngeal closure types. Accordingly, patients with
P'close 0 cm H2O exclusively at the velopharynx were classified as
the VP-only group, and those with P'close 0 cm H2O at both the velopharynx and oropharynx were classified as the VP + OP group.
Evaluation of Craniofacial Characteristics
A lateral cephalogram was obtained for each patient in the upright position at the end of expiration. The cephalometric parameters reflecting position and size of the maxilla and mandible were measured as illustrated in Figure 1. The following variables were determined:
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1. Maxillary-mandibular discrepancy (
ANB): the angle between
the line from A (subspinale) to N (nasion) and the line from B (supramentale) to N.
2. Maxillary length (Cd-A): the distance between Cd (the medial condylar point of the mandible) and A.
3. Mandibular length (Cd-Po): the distance between Cd and Po (pogonion).
4. Perpendicular size of the lower face (ANS-Me): the distance between ANS (the tip of the anterior nasal spine) and Me (menton).
5. Developmental direction of the mandible ( ACdPo): the angle
between Cd-A and Cd-Po.
6. Hyoid position (MP-H): the perpendicular distance from H (anterosuperior point of the hyoid) to MP (the mandibular plane).
Statistical Analysis
Statistical differences among the groups were assessed through Kruskal-Wallis one-way analysis of variance on ranks, and all pairwise multiple comparison procedures were performed with Dunn's methods. Differences between subgroups were assessed with the Mann-Whitney rank-sum test. Spearman's rank order correlation test was applied to the variables for all SDB patients. A value of p < 0.05 was considered significant. All values are expressed as medians (10th and 90th percentiles).
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Of the 54 male patients with SDB, 27 patients had a P'close 
0 cm H2O at the velopharynx only (50%), and 24 patients had a
P'close 0 cm H2O at both the velopharynx and oropharynx
(44%) as determined by pharyngeal endoscopy under general
anesthesia. The former patients were classified as the VP-only
group and the latter patients as the VP + OP group according
to our definitions of pharyngeal closure types. Two patients
had negative closing pressures, and one patient had a positive
closing pressure only at the oropharynx.
Figure 2 presents examples of static pressure-area relationships of the pharynx, along with cephalograms for each patient group (Figure 2, upper panel: VP-only group; lower panel, VP + OP group). The VP-only group patient was obese (BMI = 29.1 kg/m2), and demonstrated 22 oxygen desaturation events
per hour during the night. The maxilla (Cd-A = 96 mm) and
mandible (Cd-Po = 125 mm) appeared adequate in size, but
showed downward development ( ACdPo = 35.5 degrees).
By contrast, the VP + OP group patient was not obese (BMI = 24.2 kg/m2) and demonstrated 38 oxygen desaturation events
per hour. The maxilla (Cd-A = 93 mm) and mandible (Cd-Po = 117 mm) were extremely small, and significant downward development of the mandible ( ACdPo = 40.5 degrees) was revealed,
resulting in posterior positioning of the mandible ( ANB = 8.0 degrees).
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Body Habitus and Nocturnal Oxygenation
Table 1 presents the body habitus and nocturnal oxygenation
data for the normal and patient groups. No statistically significant differences were found in age, height, or severity of nocturnal desaturation for the two patient groups. None of the
normal subjects demonstrated an oxygen desaturation index
(ODI) greater than 5 h1 or percent of time with SaO2 < 90%
(CT90) that exceeded 1%. Body weight, BMI, and neck circumference of the VP-only group were significantly greater
than those of the VP + OP group. Although no significant difference in height was evident in normal subjects as compared
with SDB patients, body weight, BMI, and neck circumference were significantly greater in SDB patients.
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Static Pharyngeal Mechanics of the Passive Pharynx
Comparison of static mechanical variables of the passive pharynx in the study patient groups is presented in Table 2. With the patient classification method used in the study, P'close at the velopharynx was found to be above atmospheric pressure in all SDB patients. No differences between the patient groups were demonstrated in the mechanical properties of the velopharyngeal airway. In contrast, the VP + OP group had a significantly smaller Amax, higher K value, and higher P'close at the oropharynx than did the VP-only group.
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Craniofacial Characteristics
Craniofacial characteristics of each group are presented in Table 3. In comparison with normal subjects, both patient groups had a greater maxillary-mandibular discrepancy ( ANB)
and longer lower face (ANS-Me) with greater downward direction development ( ACdPo). Furthermore, the VP + OP
group had significantly shorter maxillas and mandibles than
did either the VP-only group or the normal subjects. The hyoid bone was located more caudally (MP-H) in the patient
groups than in the normal subjects, and this caudal displacement was more prominent in the VP + OP group than in the VP-only group.
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Despite significant differences in the distribution of BMI among the groups, subgroup analyses were performed to compare craniofacial characteristics, with degree of obesity used as a control. Comparison of the cephalometric variables of BMI-matched normal and VP + OP subgroups (Table 4) indicated that the VP + OP subgroup had significant craniofacial anomaly as compared with normal subjects, independent of body habitus. Furthermore, comparison of the cephalometric variables of BMI-matched VP-only and VP + OP subgroups (Table 5) indicated that the VP + OP subgroup had more prominent craniofacial anomaly and nocturnal hypoxemia than did the VP-only subgroup. This suggests that craniofacial anomaly may facilitate the development of SDB independent of body habitus.
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Results of Spearman's Correlation Analyses among the Variables of Body Habitus, Pharyngeal Mechanics, and Cephalometry
Table 6 presents the results of correlation analyses of BMI, neck circumference, ODI, P'close(VP), P'close(OP), Cd-ANS, Cd-Po, and MP-H for all SDB patients (n = 54). BMI and neck circumference were significantly correlated with the number of episodes of nocturnal desaturation, P'close(VP), and maxillary length. BMI and neck circumference were inversely correlated with P'close(OP). Nocturnal desaturation was significantly correlated with hyoid bone position. P'close(OP) was significantly correlated with position of the hyoid bone and inversely correlated with both the maxillary and mandibular lengths. The maxillary length was correlated with the mandibular length. Shorter mandibular length was significantly associated with a lower position of the hyoid bone. Although our arbitrary classification of SDB patients does not take into consideration the distribution of P'close(OP) within the groups, these results are in agreement with the findings in our comparisons of the SDB patient groups.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In this study, we evaluated the contribution of body habitus and craniofacial characteristics to closing pressures of the passive pharynx. Patients with SDB had receded mandibles and long lower faces with downward mandibular development. In addition, SDB patients with positive closing pressures at both the velopharynx and oropharynx had small maxillas and mandibles. Obesity was prominent in SDB patients with positive closing pressures at the velopharynx only.
Limitations of the Study
Lateral cephalometry is apparently an incomplete evaluation of three-dimensional structures of the lower face. Abnormal lateral dimensions of the bony enclosure of the pharynx cannot be determined by lateral cephalometry. Furthermore, the six cephalometric variables used in our study are too simple when compared with previous cephalometric analyses of OSA patients (6, 7, 10), and may not represent the rather complicated lower facial structures. However, we selected these six variables because we believe that the position and size of the maxilla and mandible have physiologic importance in determining collapsibility of the pharyngeal airway. The amount of soft tissue was not directly measured in this study, although we believe it has a significant effect on pharyngeal collapsibility. BMI and neck circumference are indirect measures of the amount of soft tissue surrounding the pharyngeal airway. Another limitation of our study were the differences in body position and upper airway muscle activity between cephalometry and closing-pressure measurements. The maxillomandibular configurations of the anesthetized and paralyzed subjects in the supine position may have been different from those during cephalometry. Although recent studies have suggested the importance of lung volume in pharyngeal collapsibility (11), we did not evaluate the effect of obesity, which may influence pharyngeal collapsibility through a reduction of lung volume.
Since we did not perform polysomnography, our classification of the study subjects would to some extent be questionable. However, in order to minimize error in classification, we selected our normal subjects from a population with a presumably low prevalence of SDB, and we selected our SDB patients from symptomatic patients who consulted our sleep clinic. ODI is known to be a highly specific but insensitive parameter in the diagnosis of SDB (12). Therefore, although all subjects included in our two SDB groups are likely to have had SDB, some subjects in the normal group may also have had SDB. The use of CT90 in addition to ODI, however, is reported to increase sensitivity in the diagnosis of SDB (13). All subjects in the normal group had a CT90 below 1%. Accordingly, we believe that our grouping of subjects was reasonably accurate, with the caveat that some normal subjects may have had SDB, although we did not examine the nature of SDB in the present study.
Contribution of Obesity and Craniofacial Abnormalities to Closing Pressures of the Passive Pharynx
Obesity and craniofacial abnormalities are common features of patients with OSA, as described in many previous reports (3). In accordance with the findings of Ferguson and colleagues (10), our results suggested the presence of a spectrum of body habitus and craniofacial characteristics among the apneic population. We further found that this spectrum is also associated with the distribution of pharyngeal closure sites. It should be noted that our data do not indicate that a single class of anomaly, whether obesity or craniofacial abnormalities, causes OSA. Both obesity and craniofacial abnormalities were revealed in our SDB patients. Patients with closure of the VP-only type were more obese and had less severe craniofacial abnormalities. In contrast, patients with closure of the combined VP + OP type were less obese and had more severe craniofacial abnormalities. This suggests that obesity and craniofacial abnormalities contribute synergistically to an increase in collapsibility of the passive pharyngeal airway. Although our study did not directly address the mechanisms by which these anatomic abnormalities increase pharyngeal collapsibility, obesity and craniofacial abnormalities can be speculated to have a common fundamental effect on the passive pharyngeal airway.
Craniofacial Abnormalities in SDB Patients
As also reported in many previous studies, we found craniofacial abnormalities in patients with SDB (6, 7). Downward development of the mandible in patients in our VP-only group reduced the anteroposterior dimensions of the lower face and increased the vertical dimensions of the lower face, even though the maxillary and mandibular lengths of these patients did not differ from those of normal subjects. The shorter maxilla and mandible of patients in the VP + OP group also reduced the anteroposterior dimensions of the lower face in this group. This indicates that patients with SDB have varying degrees of a smaller than average cross-section of the bony enclosures of the lower face at the pharyngeal airway level, in accordance with the findings of Shelton and colleagues (14) and further indicates that pharyngeal closure types depend on the size of these bony enclosures.
Possible Effects of Craniofacial Abnormalities on Collapsibility of the Passive Pharynx
Structurally, the pharyngeal airway is surrounded by soft tissue, which is enclosed by bony structures. Therefore, a simple mechanical model for the pharyngeal airway is a collapsible tube surrounded by soft material within a rigid box (Figure 3). In this mechanical model, the luminal size of the collapsible tube is determined by mechanical properties of the tube and transmural pressure (Ptm) (1). Ptm is defined as the pressure difference between pressures inside the tube (Plumen) and outside the tube (Ptissue). For a given Plumen, lumen closure is caused by an increase in Ptissue. Our previous finding (2) that OSA patients have higher closing pressures than normal subjects suggests that Ptissue is higher in OSA patients than in normal subjects. This is also in accordance with the finding of Gleadhill and coworkers (15) that critical closing pressures (Pcrit), which are considered to reflect the pressure surrounding a Starling resistor, are higher in OSA patients than in normal subjects although their Pcrit was determined under conditions of active contraction of pharyngeal airway muscles. Ptissue is determined by the balance between the amount of soft material inside the enclosure and the size of the surrounding rigid box. As illustrated in Figure 4, obesity can be a condition in which an excess of soft material is present inside the rigid box. In contrast, a small bony enclosure at the level of the pharynx may be analogous to the condition in which the size of the rigid box is small. Accordingly, an imbalance between body habitus and craniofacial abnormalities may result in increased tissue pressure surrounding the pharyngeal airway, leading to closure of this airway.
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This suggestion can be supported by several previous findings. Winter and associates (16) increased Ptissue by inflating a balloon inside the bony enclosure at the level of the pharynx in anesthetized pigs, and found that upper airway resistance increased during balloon inflation. Furthermore, Schwartz and coworkers reported that Pcrit decreased after weight reduction in OSA patients (17). These results imply that an excessive amount of soft tissue caused by obesity may increase Ptissue. We recently reported that stepwise incremental advancement of the position of the mandible resulted in a corresponding, increment-dependent reduction of closing pressures at both the velopharynx and oropharynx in SDB patients (18), implying that a small bony enclosure increases Ptissue. Accordingly, both obesity and craniofacial abnormalities may synergistically increase the tissue pressure surrounding the pharynx, and therefore increase the closing pressure of the passive pharynx in SDB patients. One limitation of our model is its lack of longitudinal forces acting on the airway, which may also influence airway collapsibility, as reported by van de Graaff (19) and Thut and colleagues (20).
Possible Mechanisms of Hyoid Bone Displacement
Although soft tissue expansion is limited at the base of the cranium, the submandibular area does not have the bony structure of this region, thereby permitting caudal expansion of excessive soft tissue. We found the hyoid bone to be located more caudally in SDB patients than in normal subjects, in accordance with previous findings (6, 7, 10). Obese OSA patients are described as having an increased volume of parapharyngeal fat (21, 22) and a larger than average soft palate (10) and tongue (23, 24). Excessive soft tissue may shift the hyoid bone caudally, in addition to increasing Ptissue around the pharyngeal airway. Similarly, a relative excess of soft tissue in patients with a small mandibular enclosure may also cause a downward shift of the hyoid bone. We therefore believe that displacement of the hyoid bone in OSA patients results from an excess of soft tissue and a reduction in size of the bony enclosure of the pharynx. In fact, mandibular advancement with an oral appliance was found to decrease the mandible-to-hyoid distance (25).
In conclusion, our results suggest that obesity and craniofacial anomaly may synergistically increase tissue pressure surrounding the pharynx, leading to an increase in closing pressure of the passive pharyngeal airway. The relative contribution to this of obesity and craniofacial anomaly appears to determine the different types of pharyngeal closure.
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Footnotes |
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Correspondence and requests for reprints should be addressed to Shiroh Isono, M.D., Department of Anesthesiology, Chiba University School of Medicine, 1-8-1 Inohana-cho, Chuo-ku, Chiba, 260-8670, Japan. E-mail: isonos{at}ho.chiba-u.ac.jp
(Received in original form September 22, 2000 and accepted in revised form August 27, 2001).
This article has an online data supplement, which is accessible from this issue's table of contents online at www.atsjournals.orgAcknowledgments: The authors express great appreciation to Dr. H. Kitahara and Mr. Y. Hanazawa of the Division of Radiology, Chiba University School of Medicine, for their help in filming cephalograms of normal subjects. Sara Shimizu, M.D., greatly helped to improve this manuscript.
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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S. Isono, A. Tanaka, Y. Tagaito, T. Ishikawa, and T. Nishino Influences of head positions and bite opening on collapsibility of the passive pharynx J Appl Physiol, July 1, 2004; 97(1): 339 - 346. [Abstract] [Full Text] [PDF]
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 Y K Wing, S H Hui, W M Pak, C K Ho, A Cheung, A M Li, and T F Fok A controlled study of sleep related disordered breathing in obese children Arch. Dis. Child., December 1, 2003; 88(12): 1043 - 1047. [Abstract] [Full Text] [PDF]
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A. R. Schwartz, H. Schneider, and P. L. Smith Upper airway surface tension: is it a significant cause of airflow obstruction during sleep? J Appl Physiol, November 1, 2003; 95(5): 1759 - 1760. [Full Text] [PDF]
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A. Oliven, D. J. O'Hearn, A. Boudewyns, M. Odeh, W. De Backer, P. van de Heyning, P. L. Smith, D. W. Eisele, L. Allan, H. Schneider, et al. Upper airway response to electrical stimulation of the genioglossus in obstructive sleep apnea J Appl Physiol, November 1, 2003; 95(5): 2023 - 2029. [Abstract] [Full Text] [PDF]
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M. T. Naughton Cycling Sleep Apnea: The Balance of Compensated and Decompensated Breathing Am. J. Respir. Crit. Care Med., September 15, 2003; 168(6): 624 - 625. [Full Text] [PDF]
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M. Younes Contributions of Upper Airway Mechanics and Control Mechanisms to Severity of Obstructive Apnea Am. J. Respir. Crit. Care Med., September 15, 2003; 168(6): 645 - 658. [Abstract] [Full Text] [PDF]
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M. J. Tobin Sleep-Disordered Breathing, Control of Breathing, Respiratory Muscles, and Pulmonary Function Testing in AJRCCM 2002 Am. J. Respir. Crit. Care Med., February 1, 2003; 167(3): 306 - 318. [Full Text] [PDF]
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T. Ishikawa, S. Isono, J. Aiba, A. Tanaka, and T. Nishino Prone Position Increases Collapsibility of the Passive Pharynx in Infants and Small Children Am. J. Respir. Crit. Care Med., September 1, 2002; 166(5): 760 - 764. [Abstract] [Full Text] [PDF]
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