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Volume 09 No. 07
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Scientific Investigations

Inter-Observer Reliability of Candidate Predictive Morphometric Measurements for Women with Suspected Obstructive Sleep Apnea

http://dx.doi.org/10.5664/jcsm.2842

John A. Gjevre, M.D., M.Sc.1; Regina M. Taylor-Gjevre, M.D., M.Sc.2; John K. Reid, M.D., F.A.A.S.M.1; Robert Skomro, M.D., F.A.A.S.M.1; David Cotton, M.D.1
1Division of Respiratory, Critical Care and Sleep Medicine; 2Department of Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

ABSTRACT

Objective:

Obstructive sleep apnea (OSA) is increasingly recognized as a public health concern. Definitive diagnosis is by overnight polysomnographic (PSG) examination. Identification of clinical predictors would be beneficial in helping prioritize high-risk patients for assessment. Practical application of morphometric predictive variables would require a high level of reproducibility in a clinical setting. In this study, our objective was to evaluate reliability between observers in measurements of candidate morphometric parameters in women.

Design and Methods:

This was a prospective study of 71 women who had been referred for PSG with suspected OSA. Selected morphometric parameters were measured independently in the sleep laboratory by two trained sleep physicians.

Results:

Neck circumference and truncal measurements for lower costal, midabdominal, and hip circumferences had higher reliability coefficients (intraclass correlation coefficients [ICC] of 0.78, 0.95, 0.95, and 0.81) than the smaller dimension measurements, including cricomental distance or retrognathia (ICC of 0.04 and 0.17). Of the women participating in this study, 50 of 71 had apnea-hypopnea indexes (AHI) ≥ 5. Body mass index (BMI), neck circumference, lower costal girth, midabdominal girth, and hip girth were all significantly higher (p < 0.001-0.004) in women with AHI ≥ 5.

Conclusions:

There was wide variation in inter-observer reliability for different physical dimensions. We propose that any clinical morphologic measurement employed in predictive modeling should be reliably reproducible in clinical setting conditions. Our findings support the use of several truncal measures, BMI, and neck circumference as predictive measures in women undergoing evaluation for OSA.

Citation:

Gjevre JA; Taylor-Gjevre RM; Reid JK; Skomro R; Cotton D. Inter-observer reliability of candidate predictive morphometric measurements for women with suspected obstructive sleep apnea. J Clin Sleep Med 2013;9(7):695-699.


There is growing recognition of obstructive sleep apnea (OSA) as a public health concern, with up to 20% of adults estimated to have at least mild OSA.1 Untreated, OSA may pose substantial risks, contributing to development of systemic hypertension and mortality from cardio/cerebrovascular disease.2 If associated with hypersomnolence, OSA may contribute to an increase in motor vehicle or work-related accidents.3,4

The definitive diagnostic tool for OSA is the overnight polysomnogram (PSG). Increasing awareness of this disorder among both physicians and the general public is leading to greater numbers of referrals for sleep physician assessment and for PSG. Timely access to PSG varies by geographic region. Because of PSG access issues, alternative home-based sleep diagnostic testing has been validated for use.5 Clinical predictors for significant OSA are useful tools enabling the physician to prioritize referrals and diagnostic test scheduling by degree of urgency. In men, increased BMI (body mass index) and neck circumference are strong predictors of OSA.6 However, gender differences in body fat distribution and OSA associated symptoms have been reported, and it is unclear that recognized predictors of OSA in men would also carry the same significance in women.710

BRIEF SUMMARY

Current Knowledge/Study Rationale: Physical parameters may serve as clinical predictors of obstructive sleep apnea, assisting physicians in the diagnostic process and in prioritization of patient referrals for polysomnography. A high level of reproducibility of such physical measurements would be required for practical application; this study evaluates level of agreement between physicians in such measurements.

Study Impact: The results from this study demonstrate wide variation in inter-observer reliability for different physical measurements. Our findings support the utilization of body mass index, neck circumference and several truncal measures in predictive modeling for women undergoing evaluation for obstructive sleep apnea.

There have been efforts made to identify specific morpho-metric measurements, which could be employed as clinical predictors for OSA, either alone or in conjunction with other parameters.11,12

In order for such dimensions to be employed as practical screening tools, it is crucial to understand the reliability of measurements in a clinical environment. A physical dimension measurement that in a particular range is predictive for OSA may prove to be misleading should the measurement itself be subject to substantial variation between physicians. In this study, we address this concern by examination of inter-observer agreement in candidate predictive morphometric measures in women who are undergoing polysomnography for suspected OSA.

METHODS

Consecutive women scheduled for routine PSG testing for evaluation of clinically suspected OSA were invited to participate in this study. Informed consent was obtained. This study was approved by the institutional research ethics board and is in accordance with the Helsinki Declaration.

Inclusion criteria included age ≥ 21 years and ability to provide informed consent. Exclusion criteria were: referring sleep physician's strong suspicion of another primary sleep disorder (primary insomnia, narcolepsy, restless legs syndrome, a parasomnia, or nocturnal seizures) as indicated on the patient's referral form.

Morphometric variables were assessed by standardized, focused upper airway examination and general examination. For each participant, this was performed by 2 independent sleep physician observers to assess inter-observer reliability. Measurements included: neck circumference, lower costal or chest circumference (lower margin of the antero-lateral ribs, while standing at functional residual capacity [FRC]), umbilical abdominal (mid-abdominal) circumference (peri-umbilical circumference of the abdomen with abdominal muscles relaxed at FRC while standing), hip circumference (widest circumference of the buttocks while standing), lateral pharyngeal space narrowing (grading by Tsai et al.11), vertical pharyngeal space narrowing (modified Mallampati score),12 cricomental space (in mm), maxillary over jet (in mm), and retrognathia (in mm). The presence or absence of tongue ridging, macroglossia, and tonsillar enlargement were also recorded.11,13

The pharyngeal grading system described by Tsai included a 4-class categorization, with class I designated when the palatopharyngeal arch intersects at the edge of the tongue, class II when the palatopharyngeal arch intersects at ≥ 25% of the tongue diameter, class III when the palatopharyngeal arch intersects at ≥ 50% of the tongue diameter, and class IV when the palatopharyngeal arch intersects at ≥ 75% of the tongue diameter.11

All 5 sleep physicians assessing patients for this study participated in a standardized training session on morphometric measurement techniques prior to study initiation.

Patients were studied overnight in the sleep lab using the standard 15-channel PSG (Sandman Elite version 8.0 sleep diagnostic software, Ottawa, Canada). Established protocols were used for all PSG studies.14 This included electroencephalogram (EEG, 3-channel), electrooculogram (2-channel), electromyo-gram (chin and leg), electrocardiogram, heart rate, snoring, thermistor airflow, nasal pressure airflow, oxygen saturation, chest wall motion, and abdominal motion.

Statistical Analysis

SPSS v.17.0 was employed for data entry and analysis. Means and standard deviations were calculated for continuous data. Proportions were calculated for categorical data. Between group comparisons of continuous data were evaluated with independent 2-tailed t-tests. Between group comparisons of categorical data were evaluated with χ2 testing and Fisher exact test when the cell size was < 5. Measures of agreement between observers were calculated for morphometric measurements, κ coefficients for categorical data, and intraclass correlation coefficients for continuous data.15,16

For this reliability study using intraclass correlations, we used conventional values for α of 0.05 and β of 0.20. As each patient underwent separate measurements by 2 different observers, using a minimum acceptable level of agreement of ρ0 = 0.4, and an expected level of agreement of ρ1 = 0.9, then approximately 7 subjects would be the estimated sample required per reliability assessment.17

For 2 group comparisons based on an apnea hypopnea index (AHI) cutoff of 5, mean morphometric values derived from the 2 observers were utilized. Receiver operator characteristic (ROC) curves were plotted for the predictive relationships between abnormal range AHI (≥ 5) and morphometric parameters.

RESULTS

Of 95 consecutive female patients who attended the sleep lab during the study period, 71 consented to participate. The means and standard deviations of morphometric measurements by 2 physicians and the measures of agreement are detailed in Table 1. A greater degree of inter-observer agreement, as represented by the intra-class correlation coefficients and κ coefficients,15,16 was observed for some measurements compared to others. The greatest agreement was observed for the truncal measures of lower costal girth, midabdominal girth, and hip circumferences. The lowest degree of agreement was evident for the smaller dimension measurements for cricomental distance and retrognathia. Subjective dichotomous observations for the presence or absence of tongue enlargement, tongue ridging, or tonsillar enlargement also had lower measures of agreement between observers.

Comparison of morphometric measurements between observers

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Table 1

Comparison of morphometric measurements between observers

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Of the 71 participants, 50 had AHI ≥ 5. Comparisons of morphometric continuous measurements between the 2 groups are detailed in Table 2. Mean measurement scores derived from the 2 observers were employed for this 2-group comparison. There were no significant differences in proportions of participants designated to have tongue ridging, macroglossia, or tonsillar abnormalities between those with elevated AHI and those with normal AHI values. Predictive relationships for abnormal AHI (≥ 5) with morphometric measures are described in Figures 1 and 2 and Table 3.

Comparison of morphometric measures between groups based on AHI

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Table 2

Comparison of morphometric measures between groups based on AHI

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Area under the curve (AUC) for prediction of abnormal AHI

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Table 3

Area under the curve (AUC) for prediction of abnormal AHI

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ROC curve for prediction of AHI ≥ 5 from truncal measures

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Figure 1

ROC curve for prediction of AHI ≥ 5 from truncal measures

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ROC curve for prediction of AHI ≥ 5 from upper airway/mandibular measures

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Figure 2

ROC curve for prediction of AHI ≥ 5 from upper airway/mandibular measures

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DISCUSSION

Although obesity has been linked to OSA and there are comparable frequencies of obesity between genders, the prevalence of OSA in women has been lower than in men.1 In men, increased BMI and increased neck circumference are predictive of OSA.6 It is less clear that these variables are predictive in women with OSA.810,18 Whittle et al. have demonstrated in magnetic resonance imaging (MRI) studies of men and women that there are differences in neck fat deposition distribution between the genders and greater overall soft tissue volume around the airway in men. They speculate that these or other anatomic factors may contribute to the gender disparity in OSA prevalence.7 Identification of clinical predictors for OSA in women would help prioritize PSG evaluation. Morphometric measures have been examined for identification of candidate variables to aid in the screening process. In this group of female patients who had been referred for polysomnography we found significantly different mean values for a number of measures (BMI, neck circumference, lower costal girth, midabdominal girth, hip girth, and the cricomental distance) between groups based on AHI category. However, of these potentially predictive measurements, the cricomental distance has a quite low measure of inter-observer agreement.

As illustrated by Figure 1 and 2 ROC curves, there is substantial overlap in the predictive relationship between truncal measures for an abnormal AHI, whereas the upper airway/ mandibular measures have a lesser area under the curve and are observed to be clustered about the reference line, which implies lack of predictive contribution. The truncal parameters each provide a statistically significant predictive measure, with BMI and neck circumferences having the highest area under the curve. It is possible that greater predictive capacity may be achieved by an additive combination of physical measures, or alternatively by ratio (through adjustment for height as an example). However, identification of such predictive models was outside the scope of this study.

The extent of inter-observer reproducibility would be expected to influence utilization of any predictive morphometric parameter. In this study evaluating measure of agreement between observers in a variety of morphometric assessments, we observe the greatest degree of agreement for truncal dimensions and neck circumference, and the lowest agreement for smaller upper airway/mandibular dimensions, such as cricomental distance. We propose that any clinical morphologic measurement employed in a predictive capacity would need to be one reliably reproduced in clinical settings in order to be of practical value. Our findings support the use of truncal measures, BMI, and neck circumference as predictive measures in women undergoing evaluation for OSA.

DISCLOSURE STATEMENT

This was not an industry supported study. The authors have indicated no financial conflicts of interest.

ACKNOWLEDGMENTS

The authors thank Dr. Brian McNab for his contribution to this study and also the Saskatchewan Health Research Foundation for their support of this work. This study was supported by a grant from the Saskatchewan Health Research Foundation.

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