Systematic review of the different aspects of primary snoring

Summary

Primary snoring, also known as simple or non-apnoeic snoring, is regarded as the first stage of sleep disordered breathing without severe medical consequences for the snorer and co-sleeper. Although it is a highly prevalent phenomenon in the general population, our knowledge is limited because of the lack of a consensus on terminology. This systematic review of the aspects used in the definitions of simple/primary snoring was conducted to obtain an inventory of current practices and compare these definitions with the conceptual definition of the American Academy of Sleep Medicine. PubMed and Web of Science were searched from July 2016 onwards without any language limitations, and 362 references were obtained. After selection based on titles, 39 remained, among which 29 contained a definition or reference to a definition. In 69% of the studies, a cut-off <5 apnoea/Hypopnoea events per hour of sleep on the Apnoea–Hypopnoea Index was used. Despite this tendency, the cut-offs ranged from 0 to <15/h. Unfortunately, the cut-off and occasional requirements did not match the conceptual definition of the American Academy of Sleep Medicine. A consensus must be reached on an operational and clinically relevant definition based on the clear conceptual definition.

Introduction

The first major classification of sleep disorders, namely the diagnostic classification of sleep and arousal disorders, was published in 1979 [1]. It organised sleep disorders into symptomatic categories, forming the basis of current classification systems. This classification attempted to describe the snoring phenomenon but provided no operational definition of primary snoring (PS) (i.e., snoring without medical co-morbidity).

Recently, the 2014 revision of the International Classification of Sleep Disorders (ICSD), ICSD-3, was published by the American Academy of Sleep Medicine (AASM) [2]. The ICSD-3 moved the classification of snoring to the category of ‘sleep-related breathing disorders’, as a separate entity, namely the first of the subdivision of ‘isolated symptoms and normal variants’. The ICSD-3 describes PS as ‘audible vibrations of the upper airway during respiration in sleep’. In the same paragraph, the ICSD-3 does not repeat the adjective ‘primary’, and shifts first towards the aspect of possible malignancy. It mentions dry mouth and irritated tissues, and continues towards the presence of apnoeas, influencing factors, and treatments. This definition remains relatively vague. The approach of the ICSD-3 suggests that the absolute absence of apnoea is not maintained, indicating that to translate the concept of PS/SS into practice, a certain degree of apnoea during sleep must be allowed for.

Because of the diverse acoustical, individual, and physical aspects of snoring, several authors [2], [3], [4], ∗[5], ∗[6] have agreed only on the following points in their definitions of snoring:

• Location in the body: the upper airway or aero-digestive tract.

• Moment of respiration: mainly during inspiration, but eventually occurring during expiration or during the complete respiration cycle.

• Mental condition of the snorer during sleep.

• Causative factor: vibrations of the pharyngeal tissues during breathing causing a fluttering sound.

These aspects do not specify diagnostic criteria such as acoustical metrics and non-acoustical and social aspects. Consequently, the diagnosis of isolated snoring or PS/simple snoring (SS) is indistinct and the boundaries are open to interpretation. Therefore, investigation of the operational definitions used in research practice is essential. To our knowledge, no study has been conducted on this topic.

In 2005, Hoffstein et al. attempted to categorise the different clinical conditions that are accompanied by a specific snore signal [6]. The key criterion was the Apnoea/Hypopnoea Index (AHI), namely the number of apnoeic/hypopnoeic events per hour measured during polysomnography (PSG).

• Asymptomatic non-apnoeic snoring with an AHI ≤5/h and no daytime sleepiness

• Non-apnoeic snoring concomitant with upper airway resistance syndrome (UARS) with respiratory-effort-related arousals (RERAs) between > 5 and ≤10 and AHI ≤ 5/h and daytime sleepiness and oxygen desaturation > 90% [7], [8].

• Apnoeic snoring concomitant with obstructive sleep apnoea syndrome (OSAS) characterised by oxygen desaturation (SaO₂), AHI > 5/h, and a deviating electroencephalogram pattern.

In 2008, Stoohs classified obstructive sleep-disordered breathing (SDB) as four entities, taking into account the respiratory disturbance index (RDI) and daytime sleepiness. Patients were classified as having either PS (no sleepiness) or UARS (sleepiness) if the RDI was <5 events/hour of sleep, and if the RDI was ≥5, patients were classified as having obstructive sleep apnoea-hypopnoea (no sleepiness) and obstructive sleep apnoea-hypopnoea syndrome (sleepiness) [9]. The RDI is based on respiratory events during sleep, but unlike the AHI, it also includes RERAs [10].

Deary et al. also referred to a continuum or evolution of snoring within the spectrum of SDB from PS to UARS to OSAS [5]. Although a patient with PS is assumed to move gradually towards OSAS based on a continuum of factors (e.g., weight gain or alcohol abuse), no convincing evidence has shown this to be the case; more than 40% of habitual snorers reported resolution rather than worsening of their snoring when asked about it in a 10-year follow-up study by Lindberg et al. [11]. An early diagnosis of PS can possibly prevent progression to a more severe medical condition such as UARS or OSAS [12].

This review paper includes a draft inventory of definitions for PS/SS. The focus lies on current practice regarding the different aspects used in the operationalisation (the translation into practice) of the concept of PS/SS. The encountered definitions will be discussed in terms of the function of their conceptual angles and possible applications.