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Validation of the Snore Outcomes Survey for Patients With Sleep-Disordered Breathing
Richard E. Gliklich, MD;
Pa-Chun Wang, MD, MSc
Arch Otolaryngol Head Neck Surg. 2002;128:819-824.
ABSTRACT
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Objective To develop and validate a self-reported outcomes measure for patients
with sleep-disordered breathing—the Snore Outcomes Survey.
Design Item areas of the SOS were developed by an expert panel. Consecutive
patients were enrolled into the study in a prospective manner. Patients received
the SOS, the Epworth Sleepiness Scale, the Pittsburgh Sleep Quality Index,
the Medical Outcomes Study 36-Item Short-Form Health Survey, and standard
overnight polysomnography at baseline and after 4 months of continuous positive
airway pressure therapy.
Setting A tertiary care, academic otolaryngology and sleep disorders referral
center.
Patients One hundred fifty-six adult patients presenting with a chief complaint
of snoring or sleep-disordered breathing to the Massachusetts Eye and Ear
Infirmary, Boston.
Main Outcome Measures Test-retest reliability, intrasurvey reliability, internal consistency,
validity, and standardized response means of the SOS.
Results Overall, reliability of the SOS was excellent (test-retest reliability r = 0.86; P<.001; Cronbach 
coefficient, 0.85). The SOS index significantly correlated with the Epworth
Sleepiness Scale (r = - 0.42; P<.001) and the global Pittsburgh Sleep Quality Index score (r = - 0.38; P<.001),
as well as with the number of recorded arterial oxygen saturation levels below
85% (r = - 0.46; P
= .02). The SOS index was sensitive to clinical changes after intervention
(standardized response mean, 0.57).
Conclusion The SOS is a reliable and valid instrument for assessing sleep-related
health status for patients with snoring and sleep-disordered breathing and
for measuring change in health status following therapy.
INTRODUCTION
SLEEP-DISORDERED breathing (SDB) represents a continuum of sleep disorders
from simple snoring to severe obstructive sleep apnea syndrome. While 2% to
4% of the middle-aged workforce are reportedly affected by obstructive sleep
apnea syndrome, SDB affects 3 to 6 times this number.1-5
Recent advances in treatment of SDB in patients with and without obstructive
sleep apnea syndrome have raised questions regarding treatment efficacy and
how to determine the best practice.6-11
While the polysomnogram (PSG) provides reliable data on respiratory behavior
during sleep, it does not fully address the problem of SDB from either a patient's
or a spouse's perspective and is prohibitively expensive and burdensome to
be used for long-term, multiple follow-ups in these patients.12-13
In view of this, we developed the Snore Outcomes Survey (SOS) to be a patient-based
measure for the full range of SDB patients in whom snoring is a primary symptom,14-16 where the goal is
to measure the snoring component of SDB. Approaches to develop and validate
quality-of-life measures as performed in this study have been previously well
described.17
PARTICIPANTS AND METHODS
CONSTRUCTION OF SOS
The item areas for the SOS were developed by an expert panel. The initial
questionnaires were piloted with open-ended responses. Actual survey items
were constructed according to a Likert scaling model. The SOS (Figure 1) contains 8 items that evaluate the duration, severity,
frequency, and consequences of problems associated with SDB, snoring in particular.
Because of the impact of SDB on others, a separate Spouse/Bed Partner Survey
(SBPS) containing 3 Likert-type items was also developed. Scores on the SOS
and SBPS are normalized on a scale ranging from 0 (worst) to 100 (best), similar
to other measures such as the Medical Outcomes Study 36-Item Short-Form Health
Survey (SF-36).
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The Snore Outcomes Survey. (Printed with permission of Outcome Sciences
Inc, Boston, Mass.)
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STUDY POPULATION
One hundred fifty-six consecutive patients with chief complaints of
snoring and SDB who were evaluated by otolaryngologists at the Sleep and Snoring
Center of Massachusetts Eye and Ear Infirmary, Boston, were enrolled. All
patients underwent overnight PSG.
SLEEP STUDY
Measurements of height and weight were obtained to calculate the body
mass index as weight in kilograms divided by the square of height in meters.
All patients underwent standard overnight PSG to record the pulse oxygen saturation,
chest and abdominal wall movement, electroencephalogram, oronasal airflow,
electrocardiogram, electrooculogram, and submental electromyogram. Respiratory
Distress Index was defined as the sum of total apnea and hypopnea episodes
per hour of sleep. An apnea episode is defined as cessation of airflow lasting
longer than 10 seconds, whereas a hypopnea episode is defined as a 33% or
greater reduction in combined oral and nasal flow lasting longer than 10 seconds.
SURVEY FORMS
At entry, patients were administered the SOS, the Epworth Sleepiness
Scale15, 18 (ESS), the Pittsburgh
Sleep Quality Index19 (PSQI), and the SF-36.20-21 Permission to use these forms were
obtained in each case. The 8-item ESS evaluates daytime somnolence and generates
a total score ranging from 0 (best) to 24 (worst). The PSQI is a 19-item,
self-reported, global sleep questionnaire that evaluates 7 dimensions of sleep
quality. Each domain is scored on a scale from 0 (best) to 3 (worst), and
these subscores are summed to yield a total score ranging from 0 (best) to
21 (worst). The SF-36 is a widely used generic quality-of-life measure that
divides general health into 8 domains. The subscales and definition of SF-36
and PSQI are listed in Table 1.
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Table 1. SF-36 and PSQI Subscales*
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RELIABILITY AND VALIDITY OF SOS
Fifty-nine patients who were deemed clinically stable were retested
after a 2- to 4-week interval. This cohort received no medical or surgical
intervention during this interval. Test-retest, intersurvey, and intrasurvey
reliability were calculated using Spearman rank order correlation coefficients
for individual items and for total survey score. Internal consistency of the
SOS was calculated using Cronbach correlation coefficients.
In addition to its face validity, the SOS was assessed for convergent
validity through correlations to concurrent PSG data. In addition, scores
on the SOS were compared with 3 previously validated measures that were completed
by the patients at the same time as the SOS. These measures were the ESS,
PSQI, and SF-36. One hundred forty-nine patients completed all 3 surveys.
REPONSIVENESS OF SOS
Instrument responsiveness or sensitivity to clinical change was assessed
in the following manner. Of the 149 patients used in the validity analysis,
21 were retested with the SOS after being treated for 4 months with continuous
positive airway pressure. These patients were evaluated on an intention-to-treat
basis. Actual in-home monitoring of continuous positive airway pressure compliance
was not performed. However, all patients reported that continuous positive
airway pressure resolved their snoring when it was used. Longitudinal sensitivity
to clinical change was calculated as the standardized response mean, according
to method described by Liang et al.22
STATISTICAL ANALYSIS
All data were stored in an Access 7.0 database (Microsoft, Redmond,
Wash). Analyses were conducted using the SAS software package (SAS Institute,
Cary, NC).
RESULTS
STUDY POPULATION
There were 130 men and 26 women (mean ± SD age, 46.2 ±
11.6 years). The mean Respiratory Distress Index was 32.66 ± 28.34.
The mean awake oxygen saturation was 94.97% ± 2.4%, and the minimum
oxygen saturation during sleep was 77.85% ± 16.04%.
RELIABILITY OF SOS
1. Test-Retest Reliability: mean test and retest
total scores varied from 27.54 ± 15.89 on the first administration
to 27.59 ± 17.44 on the second administration. The test-retest correlation
coefficients for individual items (r = 0.54-0.88; P<.001) and total score (r
= 0.86; P<.001) were statistically significant
(Table 2).
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Table 2. Spearman Correlations for Test-Retest Reliability of SOS*
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2. Intersurvey Reliability: the correlation
between SOS and SBPS was not significant (r = 0.18; P = .07).
3. Intrasurvey Reliability: the item-item,
item-total correlations for SOS are presented in Table 3. The item-item correlations for SBPS were very high.
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Table 3. Spearman Correlations for Intrasurvey Reliability of SOS and
SBPS*
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4. Internal Consistency: Cronbach correlation
coefficients were calculated as 0.85 for the SOS and 0.96 for the SBPS.
VALIDITY OF SOS
1. Correlation With PSG Data: the correlations
between the SOS and the PSG parameters are shown in Table 4. Four Respiratory Distress Index elements, awake oxygen
saturation, number of arterial oxygen saturation values less than 85%, and
minimum arterial oxygen saturation were collected from a computerized PSG
database. The number of desaturation values to less than 85% was most strongly
correlated with the SOS (r = - 0.46; P = .02).
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Table 4. Spearman Correlations Between SOS and Polysomnogram Data*
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2. Correlation With ESS: a significant correlation
was observed between the SOS and the ESS (r = -
0.42; P<.001). The correlation between the SBPS
and ESS was not significant (Table 5).
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Table 5. Spearman Correlations Between SOS With ESS, SF-36, and PSQI*
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3. Correlation With SF-36: the 8 subscale scores
of the SF-36 were compared with the SOS total score, and the correlation coefficient
was derived. The SOS demonstrated significant correlations with all SF-36
subscales except for the mental health subscale. The SBPS did not correlate
well with any SF-36 subscale (Table 5).
4. Correlation With PSQI: component and total
scores were calculated according to published guidelines.19
The PSQI evaluates 7 dimensions of sleep quality: subjective quality, latency,
duration, efficiency, disturbance, use of sleep medication, and daytime dysfunction
(Table 1). The SOS was found to
be significantly (P<.01) correlated with the total
PSQI score, subjective sleep quality, habitual sleep efficiency, sleep disturbance,
and daytime dysfunction (Table 5).
RESPONSIVENESS OF SOS
The standardized response mean for the total SOS score was 0.57, indicating
that the SOS is responsive to clinical change. The SBPS, on the other hand,
was not sensitive to change.
COMMENT
Sleep-disordered breathing1 is a condition
characterized by repeated pauses in breathing during sleep, which lead to
the fragmentation of sleep and a decrease in arterial oxygenation. The spectrum
of SDB includes simple snoring, obstructive sleep apnea, and upper airway
resistance syndrome. Several studies have shown that patients with SDB carry
higher risks of daytime somnolence and cardiovascular morbidities.2-3 The objective evaluation of SDB has
relied on standard PSG, which requires a patient to sleep in a laboratory
overnight. From a cost and burden perspective, overnight PSG is not an efficient
means to track patients over time. Although snoring is only one component
of SDB, a valid and reliable patient-based measure for snoring could contribute
to the tracking of patients with SDB and certainly those whose SDB is limited
to snoring alone.
Use of patient-based questionnaires to assess patients with SDB has
become a widely accepted approach.14-16,18-19
In this study, we introduce the SOS as a reliable, valid, and sensitive disease-specific
outcomes measure that adds another dimension to our understanding of the impact
of SDB to patients' quality of life. While other measures focus on sleep quality
and somnolence, the SOS focuses on snoring with the idea that it is the presenting
complaint for many patients.
In terms of reliability, the SOS demonstrated good test-retest reliability
for individual test questions and for total score. Improvement in wording
may further enhance the reliability of items 1, 4, and 8. The measured item
and total score test-retest reliabilities ranging from 0.54 to 0.88 were comparable
with those of well-studied SF-36, which had correlations ranging from 0.6
to 0.81 for its subscales.23 The poor correlation
between the SOS and SBPS show a significant discrepancy between patient and
bed-partner evaluations in terms of the patient's snoring. This result was
not expected. The Cronbach correlation coefficients of 0.85 and 0.96
for SOS and SBPS, respectively, exceed the commonly accepted threshold (0.7)
for a reliable measure.17
The overall comparison between the SOS and the PSG data suggests that
snoring may be more strongly related to oxygen desaturation than previously
assumed. Previous studies have also shown oxygen desaturation to be more closely
related to quality of life than the Respiratory Distress Index.24
Although the SOS and PSG were correlated, the correlations were moderate,
again suggesting that they are measuring different aspects of sleep disturbance.
The SOS was rigorously compared with other sleep-specific questionnaires
such as the ESS and PSQI. The correlation coefficient of - 0.42 between
the SOS and ESS was similar to that between the SOS and PSQI daytime dysfunction
component (r = -0.47) and confirms the predicted
convergent validity. The weak but statistically significant correlation between
the SOS and several SF-36 subscales again supports convergence and suggests
that SDB may have an impact on general health.24
Responsiveness, or sensitivity to longitudinal change, is the ability
of a health measure to detect clinical change over time. The standardized
response mean measures this sensitivity. The SOS (standardized response mean,
0.57) demonstrates moderate responsivenesss.22, 25
Hence, the SOS can be used as an evaluative instrument in clinical studies
of interventions for snoring and other forms of SDB when the principal complaint
is snoring.
The SOS is short, easy to understand, and easy to administer in a busy
clinical setting without sacrificing its evaluative power. In addition to
providing a quality-of-life dimension to the overall assessment of SDB, the
SOS can be used to follow-up patients with SDB over time, with and without
intervention.
To summarize, the SOS is a patient-based, disease-specific outcomes
measure for SDB. It is valid, reliable, and sensitive to clinical change.
The SOS also provides an inexpensive and accurate measure to follow up patients
with SDB, especially when snoring is the primary symptom.
AUTHOR INFORMATION
Accepted for publication November 21, 2001.
Corresponding author and reprints: Richard E. Gliklich, MD, Department
of Otolaryngology and Clinical Outcomes Research Unit, Massachusetts Eye and
Ear Infirmary, 243 Charles St, Boston, MA 02114.
From the Department of Otolaryngology and the Clinical Outcomes Research
Unit, Massachusetts Eye and Ear Infirmary, and the Department of Otology and
Laryngology, Harvard Medical School, Boston, Mass. Dr Wang is now with the
Department of Otolaryngology, Cathay General Hospital, and the Department
of Public Health, China Medical College, Taiwan.
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