OSA significantly increases the risk of cardiovascular complications and mortality. The prevalence of OSA is 9–38%, and it is increasing annually.32,33 OSA is typically associated with nighttime snoring in patients. At a personal level, snoring is a serious symptom. It is also a social disorder that not only reduces the quality of one’s sleep but also interferes with the sleep of their partner. However, this is not considered medically important. The apnea-hypopnea index (AHI) used to diagnose OSA is a rather specialized and complex concept for patients to comprehend. However, because snoring is a symptom that can be easily understood by both patients and doctors, establishing a definition can be an important means of doctor-patient communication in diagnosing and treating OSA. Snoring properties have also been reported to be potentially useful in characterizing OSA. This approach could provide the opportunity for the development of technologies for the long-term monitoring of OSA and home sleep tests that do not require patient attachment to a sensor.
OSA should be diagnosed easily and inexpensively.34 To diagnose OSA, the use of attended PSG is the gold standard. PSG can be classified into four types: type 1, which incorporates standard PSG; type 2, which uses both sleep phases and respiration measurements with a minimum of seven channels; and types 3 and 4, which use only respiration measurements with no provision for sleep phases. In general, the severity of OSA is measured based on the AHI using type 1 PSG.35 However, considering the costs of the sites of PSG installation, monitoring devices, and personnel, among others, and the fact that it is not a place where patients are accustomed to sleep, several limitations arise. Most importantly, because it is an overnight test, the reproducibility of OSA patterns is poor. In contrast, when measuring snoring using a microphone, the location of the microphone and intensity threshold of the snoring sound varied. In 1990, the American Sleep Disorders Association defined snoring as “loud upper airway breathing caused by vibrations of the pharyngeal tissues.”36 As such, snoring has been generally recognized as a sound for several decades. Furthermore, once the concept is established, it can be easily developed using a smartphone equipped with a microphone. Compared with the piezoelectric sensor or nasal transducer, it also carries the advantage of having fewer factors that interfere with the patient’s sleep. However, because it is affected by the noise of the surrounding environment and microphone’s position, unifying the standardized control of ambient noise and microphone’s position in order to measure snoring using a microphone is necessary.
Snoring is characterized by repetitive packets of energy that are responsible for creating the vibratory sound peculiar to snorers.37 Measuring snoring using a piezoelectric sensor is a measurement method that is close to the most fundamental concept of snoring in this respect. However, the location of vibration measurement varies with the skin area of the suprasternal notch or pharyngeal region. In addition, this method is disadvantageous in that it is greatly affected by the skin’s condition, such as obesity, skin thickness, and sweating during sleep.
A nasal transducer is a device that measures airflow through the nasal cavity. Because it is a device worn on the nose, its location does not vary. However, since snoring is generally referred to as a sound or vibration energy caused by the fluttering of the soft palate, the measurement of nasal airflow is not strictly based on snoring. As the device is worn on the nose, it potentially interferes with sleep by affecting nasal breathing. If abnormalities exist in the nasal cavity, such as a severely deviated nasal septum, nasal polyps, or nasal tumors, the evaluation of snoring using a nasal transducer may not be accurate.
In modern society, sound, movement, and oximeter data are collected by using smartphones. The most notable advantage of snoring measurement using smartphones is patient accessibility. In modern society, most people use smartphones; therefore, if a method of measuring snoring in connection with a sleep application is established, patient access to OSA treatment will improve. When measuring snoring sounds using a smartphone, we can evaluate the natural sleep state because it is contactless. Furthermore, it can be developed as a home sleep test. If OSA can be diagnosed more easily by measuring snoring sounds, this will lead to a reduction in the socioeconomic costs associated with OSA.
Based on our study, the concept of snoring episode index has been confirmed to be highly correlated with AHI.24 It appears to have a considerably strong relationship with apneic or hypopneic situations through the measurement of successive snoring episodes compared with counting each snoring episode individually. In this study, snoring was measured using a nasal transducer. If this concept could be measured using a microphone, it would be tremendously beneficial in predicting OSA because measuring snoring using a microphone is most easily linked to the home sleep test in terms of interlocking with sleep applications and convenience of patient use.
To our knowledge, this is the first systematic review that examines measurement of snoring by listing measurement devices, definitions, and device locations proposed in previous literatures. However, No studies have focused on how to measure snoring from a methodological point of view. In other words, this is a review that only analyzed how snoring was described in studies addressing OSA. Therefore, in the future, studies should focus on analyzing the advantages and disadvantages of different kinds of snoring measurement from a methodological point of view. Furthermore, a consensus on the selection of snoring measurement device in studying OSA should be determined.
Numerous studies have investigated OSA and snoring. However, the methods of measuring snoring and defining snoring-related concepts also differed across studies. To understand the pathophysiology of OSA and address sleep-related diseases, a consensus in the academic community on how to measure and define snoring is required.