Cephalometric Analysis of Nonobese Snorers Either with or Without Obstructive Sleep Apnea Syndrome

Obstructive sleep apnea syndrome (OSAS) is caused by recurrent upper airway obstruction during sleep, and it manifests as loud snoring, arterial oxygen desaturation, sleep fragmentation, and excessive daytime sleepiness.1 OSAS affects 2% to 4% of the adult general population2 and may be even more frequent in specific subgroups such as hypertensive3 or heart failure patients.4 

Several causes for OSAS have been suggested. It appears to result from a variable combination of anatomical and pathophysiological factors, some of which may be under genetic control.5 Relaxation of the upper airway musculature has been studied in relation to OSAS.6 Anatomic narrowing of the upper airway as a result of alterations in the craniofacial morphology or soft tissue enlargement, the Bernoulli effect, sleep posture, age, male gender, nasal obstruction, and adipose tissue in the pharynx have been suggested as etiologies of OSAS.7–11 

There are several risk factors for OSAS, with the strongest being obesity and age.10,11 The prevalence of OSAS increases with age, with a twofold to threefold higher prevalence in individuals older than 65 years compared with those in middle age.12 There is a relationship between a body mass index (BMI) of >26 kg/m² and OSAS.13 Previous research suggested that there may be differences in the degree to which obesity and craniofacial anatomy serve as risk factors between Asians and Caucasians14 and that the etiology of OSAS in obese patients may differ from that in nonobese patients.15 It is still unclear whether there are anatomical differences in cephalometric measurements between severe-OSAS patients and patients simply suffering from snoring among nonobese young Asians.

This retrospective study was undertaken to compare the craniofacial and pharyngeal airway structures of severe OSAS patients with those of simple snorers using lateral cephalometric radiographs and to determine the risk factors for OSAS in a nonobese Taiwanese young adult population.

The subjects of this retrospective study were Taiwanese adult male patients who visited the Ear, Nose, and Throat Department of National Taiwan University Hospital with complaints of snoring and/or sleep apnea. An institutional review board approval for the study was obtained at National Taiwan University Hospital before the study. Weight (kilograms) and height (meters) of all patients were recorded, and the BMI was calculated using the following formula: BMI (kg/ m²) = weight (kg)/height² (m²). Only the patients whose BMI was <25 kg/m² and whose age was younger than 40 years were selected.

The subjects' sleep was monitored by overnight polysomnography. Apnea was defined as cessation of breathing for at least 10 seconds. Hypopnea is a decreased effort to breathe of at least 50% less than the baseline and with at least a 4% decrease in oxygen saturation. The respiratory disturbance index (RDI)16,17 was calculated as the sum of the total events (apnea and hypopnea) per hour ([apnea + hypopnea]/sleep time). A diagnosis of OSAS was based on the RDI. Patients who snored but had an RDI of <5 were grouped into the simple-snoring group, while patients whose RDI was >40 were grouped into the severe OSAS group.

A total of 82 patients (46 severe OSAS and 36 simple snoring) were selected for this study. All subjects included in this study had a full complement of permanent teeth except for third molars, no apparent craniofacial deformity, no history of orthodontic treatment, no history of pharyngeal airway surgery, and a clinically acceptable symmetry of the dental arches. Lateral cephalometric radiographs were obtained for all subjects using a standardized technique. The patient was seated with a natural head position and instructed to bring the posterior teeth into contact, close the mouth, and refrain from swallowing during cephalometry. One of the authors traced all the cephalometric radiographs and identified the 35 reference points (Figure 1) with no prior knowledge of the polysomnographic results. Tracing error was evaluated by retracing 10 randomly selected patients. The variance of the error was less than 2% of the total variance in the entire series.

The reference points were digitized as coordinates. The straight line that passes point Po and Or was designated the x-axis. The straight line vertical to the x-axis and passing at a right angle through point S was designated the y-axis. Twenty-four linear and 34 angular measurements were calculated from the coordinate values. Mean values of the x and y coordinates of all reference points as well as the mean values and standard deviations for all measurements were calculated for both groups. An unpaired Student's t-test was used to determine whether significant differences were present between severe OSAS patients and simple snorers. To build a model of the severe OSAS group and simple-snoring group, multivariate discriminate analyses were carried out. All statistical analyses were performed using the Microsoft Excel statistical software package, and a 5% level of significance (P < .05) was used.

The mean age and BMI were 33 years and 24 kg/ m² in the severe OSAS group and 32 years and 23 kg/m² in the simple-snoring group, respectively. The comparisons of angular and linear measurements between the severe OSAS and simple-snoring groups are given in Tables 1 and 2. No statistically significant difference was observed in age, BMI, or angular measurements between the severe OSAS and simple-snoring groups. However, the soft palate length, mandibular body length, distance from the base of the epiglottis to the tongue dorsum, tongue thickness, distance from the hyoid bone to the mandibular plane, and distance from the hyoid bone to C3-Me were significantly larger in the severe OSAS group than in the simple-snoring group.

Discriminate analysis was performed using the above-described five variables, and significant differences between groups were exhibited. Table 3 presents Fisher's linear discriminate function coefficients of the variables and constants and the classification results of the discriminate analysis.

The mean composite facial polygons are shown in Figure 2. Outlines of the soft palate and tongue are also shown with the lines that connect the related reference points. No statistically significant difference was observed in the x-coordinate values of any of the reference points; however, the y-coordinate values of the soft palate, epiglottis, and hyoid bone were significantly smaller in the severe OSAS group compared with the simple snoring group (Table 4).

Craniofacial anatomic risk factors are said to play a role in OSAS, together with the mechanism of upper airway compliance and muscle function. Several studies have recommended the use of cephalometric radiographs to characterize the craniofacial hard- and soft-tissue structures of patients with and without OSAS.15,18–21 

Most patients with OSAS are middle-aged men with evidence of obesity or craniofacial abnormalities.22 In general, the facial skeleton is reduced in depth anteroposteriorly due to shortening of the skull base, maxilla, and mandible.15,23,24 One study15 reported that anatomic abnormalities may be more of an underlying problem in nonobese OSAS patients, while soft-tissue variables have a much greater influence in obese OSAS patients. However, data in the literature are inconsistent, such that studies evaluating cephalometric anomalies in patients with OSAS have found no clear-cut morphological characteristics.25 In addition, a recent study showed that gender and racial variations are present in cephalometric parameters.26 

Because the structural relationship between the hard and soft tissues of the upper airway is confounded by obesity, which independently remains an important factor for increasing apneic activity, it is important to know which craniofacial parameters are risk factors for nonobese young adults. For Asian men, a BMI exceeding 27 kg/m² is defined as being obese.27 This study demonstrated the cephalometric parameters of craniofacial characteristics that can be used as indicators of OSAS severity in nonobese young Taiwanese male adults.

Since narrowing may be present in various segments of the upper airway, knowledge of its location is central to an understanding of the pathogenesis of OSAS.28,29 In contrast to previous studies, our results indicated that there was no statistically significant difference in any portion of the upper pharyngeal airway space between severe OSAS patients and simple snorers. Therefore, the cephalometric craniofacial features that possibly reflect structural narrowing of the upper airway, which contributes directly to the pathogenesis of upper airway obstruction in OSAS, were not found in the nonobese subjects of this study.

In comparison with simple snorers in this study, nonobese patients with severe OSAS were characterized by a greater tongue thickness, an inferiorly positioned hyoid bone, a greater mandibular body length, and a longer soft palate. In the supine position, the tongue falls back posteriorly due to gravity and obstructs the oropharyngeal space. This posterior encroachment by the tongue is counteracted only by the tone of the genioglossal muscle. Thus, the thicker the tongue is, the more likely obstruction of the airway will occur during sleep.

The position of the hyoid bone serves as a central anchorage for the tongue muscles and determines the position of the tongue. A lower hyoid bone might be a compensatory mechanism to alleviate the increased airway resistance caused by a reduced airway space, or it might be the result of a greater tongue mass. A downward and forward position of the hyoid was noted in children with enlarged tonsils and adenoids.30 Children with an obstruction in the upper airway region adopt a more anterior head position with increased craniocervical stretching.31,32 With this altered habitual head posture, the position and the tone of the supra- and infrahyoid muscles change; the hyoid adopts a downward and forward position relative to the mandible.33 

A longer mandibular body length in the OSAS group in this study might be the result of a greater tongue mass, because growth of the mandibular body is related to development of the tongue.34 There is a tendency for the tone of the lingual and pharyngeal musculature to decrease during sleep and for the tongue and soft palate to fall back in the supine position. Therefore, elongation of the soft palate in this study probably resulted from long-term vibration of the soft tissue by recurrent obstruction of the upper pharyngeal airway during sleep.

Because the tongue dorsum, epiglottis, and soft palate are soft tissues that are sometimes unclear on routine cephalograms, a hard-tissue hyoid bone that can be easily identified on radiographs might make a better indicator for differentiating the two groups. A previous study35 reported that the position of the hyoid bone was consistently above and near the line from the C3 to the Me from the primary to the early permanent dentition in normal Taiwanese subjects. A longitudinal study36 investigating the hyoid bone position of adult men revealed that the hyoid bone changes to a more inferior position with increasing age.

Another longitudinal study37 indicated that gonion and the second cervical vertebra body were anatomically interrelated. Therefore, to understand the positional changes in the same patient and to compare the hyoid bone position among different patients, it seems that the use of the relative position of the hyoid bone with both the mandible and vertebra may be a more accurate and easier method than using the distance from the hyoid bone to the mandibular plane. In addition, the perpendicular distance from the hyoid bone to the mandibular plane may be influenced by the anteroposterior position of the hyoid bone: it increases when the hyoid bone is positioned posteriorly and decreases when the hyoid bone is positioned anteriorly. Figure 3 shows the superimposition of the standard deviations of the mean position and y-coordinate values of the hyoid bone relative to the C3 and the mandible for both groups. Figure 4 shows that the perpendicular distances from the hyoid bone to the mandibular plane were comparable in both groups; however, the position of the hyoid bone in simple snorers was below but near the straight line from the C3 to the Me, whereas the position of the hyoid bone in severe OSAS patients was far below this line. To summarize our results, it is suggested that the line from the C3 to the Me on lateral cephalometric radiographs may be used as a clinical guideline for diagnosing severe OSAS in nonobese young Taiwanese men.

• The position of the hyoid bone relative to a line from the third vertebra to menton can be used as an indicator for a diagnosis of severe OSAS in nonobese young male Taiwanese adults.