Introduction
Snoring as an isolated phenomenon is very common. The symptom is generated by vibration of the soft tissue of the upper airway during sleep. According to the International Classification of Sleep Disorders (ICSD-3), isolated snoring is categorized under sleep-related breathing disorders, but must be differentiated from obstructive sleep apnea, either by home sleep testing (HST) or polysomnography (PSG) [
1,
2]. According to the National Sleep Foundation, 90 million Americans report isolated snoring, of which 37 million declare to snore routinely [
3]. In general, epidemiological investigations on the prevalence of isolated snoring are difficult due to the fact that the necessary differentiation from snoring as part of obstructive sleep apnea (OSA) is not routinely performed. Additionally, a universal definition of this acoustic phenomenon is not available and therefore the widely reported incidence ranges from 2 to 86% [
4,
5]. Standardized telephone interviews resulted in a frequency of snoring of 20% in women and 26% in men up to the age of 24 years with the highest frequency between 45 and 55 years [
6]. It is proven that snoring is more frequent in men than in women [
7]. Besides gender and age, elevated body weight, impaired nasal passage, velar hyperplasia, smoking, and alcohol consumption are further risk factors.
Due to the high incidence of snoring and the substantial burden for the patient and the bed partner, a thorough examination and appropriate therapy for snoring are required. It is still part of ongoing discussion and investigation, whether or not isolated snoring is a cardiovascular risk factor. In a comparison of 377 subjects with isolated snoring after PSG exclusion of OSA with 264 healthy subjects, no difference regarding fatal and non-fatal cardiovascular events “could be detected after adjustment for the common risk factors [
8]. However, Lee et al. observed a correlation between arteriosclerosis of the carotid artery and snoring intensity in a collection of 110 subjects with isolated snoring and excluded OSA via PSG [
9].
The role of isolated snoring as potential risk factor for cardiovascular diseases remains unclear—also in the diagnosis and especially in the treatment of snoring, many recommendations are either not evidence-based or derived on recommendations for the management of patients with OSA, despite the fact that snoring in the context of OSA is not equivalent to snoring as an isolated phenomenon. Therefore, the discrimination between isolated snoring and snoring as part of OSA is essential in order to indicate the appropriate treatment. Thus, the aim of this review is the identification and description of current gaps in the evidence of diagnosis and treatment of isolated snoring and the illustration of areas for further research.
Material and methods
In the context of developing the new version of the German S3 guideline on “diagnosis and therapy of snoring in adults,” a multidisciplinary team of ten experts defined multiple topics in the management of snoring, for which the evidence should be evaluated [
10,
11]. The panel contained members with extensive clinical and scientific experience in the fields of otorhinolaryngology/head and neck surgery, maxillofacial surgery, and sleep medicine. The following study questions were defined:
Diagnosis of isolated snoring
Therapy of isolated snoring
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What is the evidence for the effectiveness of positional treatment of isolated snoring?
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What is the evidence for the effectiveness of myofunctional treatment of isolated snoring?
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What is the evidence for the effectiveness of weight loss in the treatment of isolated snoring?
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What is the evidence for the effectiveness of a treatment with a mandibular advancement device (MAD) in isolated snoring?
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Is the application of individual MAD superior compared to ready-for-use MAD in the treatment of isolated snoring?
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Does the effectiveness of MAD in the treatment of isolated snoring depend on the amount of advancement/protrusion?
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What is the evidence for the effectiveness of soft palate surgery in the treatment of isolated snoring?
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What is the evidence for the effectiveness of nasal surgery in the treatment of isolated snoring?
Systematic literature research
In January 2018, a literature search was conducted for existing Cochrane reviews in the Cochrane library under the term of “snoring” without date restriction. Additionally, a literature research for existing guidelines or systematic reviews under the term of “snoring” and the limits of “guideline,” “systematic reviews,” “human,” “adults,” “English,” and “German” was performed. In February 2018, a systematic literature research by a certified librarian (Maurizio Grilli, MLIS, Library of the Medical Faculty of Mannheim, University of Heidelberg, Germany) in the databases of PubMed, Cochrane Library, Web of Science Core Collection, and
ClinicalTrial.gov, starting from the year 2000, has been conducted. This time, restriction has been chosen since a differentiation between isolated snoring and OSA has not been done consequently in earlier publications.
Review of the literature
The results from the systematic literature research have been thematically separated and allocated to the authors. In the first step, all abstracts were screened by at least two authors and excluded if obviously irrelevant for the single topics. Abstracts were included, if at least one author defined them as relevant. In the second step, all relevant abstracts were reevaluated on the basis of the full-texts and were either included, if again at least one of two authors defined the articles as relevant, or excluded, if still irrelevant for the single topics. At this stage, the reason for exclusion was documented. The selection was limited to English or German articles, adult patients in whom OSA was objectively excluded (AHI < 5/h) and, in the case of articles evaluating therapeutic modalities, to studies with at least ten patients. The information flow through the different phases of the systematic review is depicted as flow diagram according to the PRISMA recommendations [
12]. The articles that were finally identified were evaluated independently by two authors and evaluated according to Oxford Level of Evidence (
https://www.cebm.net/2009/06/oxford-centre-evidence-based-medicine-levels-evidence-march-2009/).
Identification of current gaps in the literature
Questions on the diagnosis and treatment of isolated snoring, which could not be answered by the existing literature, were identified and described as current gaps and areas for future research.
Discussion
As part of the development of the current version of the German guideline on the diagnosis and treatment of isolated snoring in adults and the associated systematic literature research, striking and partially unexpected gaps in the related literature were identified. This review was conducted in order to illustrate the areas of missing evidence, both in diagnosis and treatment of this common disorder, and thereby stimulate prospective clinical trials.
Although in the first round of the evaluation many abstracts were identified, screened and numerous full-texts were evaluated, only few studies met the inclusion criteria of this evaluation. The most frequent reason for exclusion of many articles was the insufficient differentiation between isolated snoring and snoring as part of OSA. Although a number of studies performed a home sleep test or polysomnography to rule out OSA, cutoff values that did not sufficiently differentiate isolated snoring from OSA were frequently used, which led to the inclusion of patients with snoring in combination with mild to moderate sleep apnea. Therefore, it is highly recommended, in accordance with the ICSD-3, to conduct either a home sleep test or polysomnography with an AHI of < 5 events per hour as cutoff at baseline as inclusion criterion both for diagnostic and therapeutic studies in isolated snoring [
1].
The multidisciplinary working group further defined relevant outcome parameters which should receive special attention (the ranking corresponds to the importance of the parameter):
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Subjective snoring intensity including subjective scores
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Unwanted effects/morbidity
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Quality of life
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Acoustic snoring analysis including objective methods of investigation and classification
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Costs of the procedure
With regard to the most important outcome parameter, the subjective snoring intensity, a particular characteristic, has to be taken into account. Usually, the primary outcome parameters are patient-relevant endpoints and therefore in the focus of guidelines or the evaluation of the effectiveness of diagnostic or therapeutic procedures. In isolated snoring, however, the subjective evaluation of the snoring intensity is not evaluated by the patients for obvious reasons, but by the bed partners. The affected individuals usually do not suffer from their own snoring, but from the extent of the annoyance of the environment. This justifies why, in the present case, the subjective harassment of the bed partners was selected as the most important endpoint, as it is also customary in the international context. However, evaluation with subjective scores is prone to bias and only the reflection of the current bed partner. The authors around De Meyer et al. demand the development of a model for assessing snoring, which includes both properties of the sound, but also physiological aspects, such as the annoyance influenced by personality aspects, sensitivity to noise, and environmental factors [
5].
Visual analogue scales offer the ability to classify snoring by volume and frequency. Similar to questionnaires, such as the Pittsburgh Sleep Quality Index (PSQI) and the Epworth Sleepiness Scale, visual analogue scales do not enable the differential diagnosis of isolated snoring and OSA, but allow the longitudinal documentation (e.g., after therapeutic intervention) [
33,
34]. The Snore Outcome Survey (SOS), the Snoring Scale Score (SSS), and the Snoring Symptoms Inventory (SSI) represent other validated questionnaires for the assessment of the snoring intensity at baseline and also following therapeutic interventions [
35‐
37], but are rarely used in clinical trials today. It must be emphasized that these questionnaires can only be answered by the bed partner, since the snoring patient is not able to assess the own snoring intensity.
Snoring is an acoustic phenomenon that can be described by objective parameters. The acoustic detection of snoring sounds should be done by air conduction (sound transmission through the air), since this is the only way to ensure that frequencies above 1000 Hz are adequately reproduced. The measurement of snoring sounds by body contact microphones or dynamic pressure measurement (sound transmission through the body), as it usually is the case in HST and PSG, leads to a reduction of the intensity spectra above 1000 Hz [
38]. The acoustic analysis of snoring sounds, however, may contribute to the differential diagnosis and objective assessment of snoring in the near future, if the recording quality is standardized and improved. The scores usually provided in standard outpatient recording/polysomnography are currently not validated and can only be used to a limited extent for the qualitative or quantitative assessment of snoring, both intra- and inter-individually. Despite the positive data, the acoustic analysis of snoring sounds is therefore currently not suitable for routine diagnostics of isolated snoring. Pevernagie et al. have summarized suggestions for future research on the acoustics of snoring [
39].
In conclusion, several gaps in the literature regarding the diagnosis and treatment of isolated snoring, and therefore areas for further research, have been identified. Future studies on these topics should pay attention to differentiate between isolated snoring and snoring in the context of obstructive sleep apnea with the help of sufficient objective testing. According to current standards, patients with an AHI above 5 events per hour should not be included in studies on isolated snoring. Another difficulty is the selection of the optimal endpoint in these studies. Usually, visual analogue scales are applied to evaluate the baseline snoring intensity and the effect of the particular treatment. No distinct objective evaluation method has gained general acceptance to date, even though the evaluation of the snoring intensity is usually done by the bed partner and therefore prone to multifactorial bias. With this regard, the authors (a) emphasize the particular need of randomized-controlled trials, since the majority of the existing literature is of low evidence, (b) indicate the reliable exclusion of OSA in studies on isolated snoring, and (c) encourage the application/establishment of validated outcome parameters.
Compliance with ethical standards
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