10.14639/0392-100X-N1202
Objective. This study aimed to describe the sound frequency of snoring in relation to the site of upper airway vibration among snorers.Methods. 383 snores from 40 participants who complained of snoring were digitally re- corded during natural and induced sleep using a level III polysomnography monitor with a built-in microphone. During drug-induced sleep endoscopy (DISE), the real-time site of upper airway obstruction was assessed, and the sound frequency of snoring was recorded synchronously.Results. The mean peak of snoring frequency for unilevel palatal, oropharynx and epiglot- tis obstruction were 522.5, 482.4 and 300.0 Hz, respectively. Most participants showed multilevel obstruction at the palate and oropharynx, in which the mean for bi-peak snoring frequency were 402.90 Hz and 1086.96 Hz, respectively. Severity of OSA was significantly associated with multilevel obstruction.Conclusions. There was a significant association between the snoring sound frequency and site of unilevel obstruction. Palatal or oropharyngeal obstruction produced sound at mid- frequency range, while the epiglottis produced a low frequency range. Multilevel obstruction documented a bi-peak snoring frequency.
text/xml
1827-675X
1202
en
Marina Mat Baki
http://creativecommons.org/licenses/by-nc-nd/4.0
ACTA Otorhinolaryngologica Italica
snoring
Sound frequency spectra of snore in relation to the site of obstruction among snorers
Text.Serial.Journal
https://www.actaitalica.it/
41
OSAHS
2021-09-14
2020-10-25
2021-09-15
2021-09-15
348-355
1
Marina Mat Baki
2021/09/14
10.14639/0392-100X-N1202
348
Department of Otorhinolaryngology, Head and Neck Surgery, Pusat Perubatan Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia
1827-675X
snoring
355
Sound frequency spectra of snore in relation to the site of obstruction among snorers
https://www.actaitalica.it/article/download/1202/654
41
ACTA OTORHINOLARYNGOLOGICA ITALICA 2021;41:348-355; doi: 10.14639/0392-100X-N1202




OSAHS


Sound frequency spectra of snore in relation
to the site of obstruction among snorers
Tipi di tracciato spettrografico nel russamento in relazione al sito ostruttivo
Nor Hafiza Qualickuz Zanan1, Mawaddah Azman1, Khairulamir Zainuddin2, Sharifah Ezat Wan Puteh3,
Abdullah Sani Mohamed1, Marina Mat Baki1
1
 Department of Otorhinolaryngology, Head and Neck Surgery, Pusat Perubatan Universiti Kebangsaan Malaysia, Kuala Lumpur,
Malaysia; 2 Department of Anaesthesiology, Pusat Perubatan Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia; 3 Department
of Community Health, Pusat Perubatan Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia


SUMMARY
Objective. This study aimed to describe the sound frequency of snoring in relation to the
site of upper airway vibration among snorers.
Methods. 383 snores from 40 participants who complained of snoring were digitally re-
corded during natural and induced sleep using a level III polysomnography monitor with
a built-in microphone. During drug-induced sleep endoscopy (DISE), the real-time site of
upper airway obstruction was assessed, and the sound frequency of snoring was recorded
synchronously.
Results. The mean peak of snoring frequency for unilevel palatal, oropharynx and epiglot-         Received: October 25, 2020
tis obstruction were 522.5, 482.4 and 300.0 Hz, respectively. Most participants showed            Accepted: February 9, 2021
multilevel obstruction at the palate and oropharynx, in which the mean for bi-peak snoring
frequency were 402.90 Hz and 1086.96 Hz, respectively. Severity of OSA was significantly          Correspondence
associated with multilevel obstruction.                                                           Marina Mat Baki
                                                                                                  Department of Otorhinolaryngology Head and Neck
Conclusions. There was a significant association between the snoring sound frequency and
                                                                                                  Surgery, Universiti Kebangsaan Malaysia Medical
site of unilevel obstruction. Palatal or oropharyngeal obstruction produced sound at mid-         Centre, Malaysia, Jalan Yaacob Latif, 56000 Che-
frequency range, while the epiglottis produced a low frequency range. Multilevel obstruc-         ras, Kuala Lumpur
tion documented a bi-peak snoring frequency.                                                      Tel. +60 3 91456045. Fax +60 3 91456647
                                                                                                  E-mail: marinamatbaki@ppukm.ukm.edu.my
KEY WORDS: drug-induced sleep endoscopy, obstructive sleep apnoea, frequency
analysis, snoring
                                                                                                  Funding
                                                                                                  This work is not an industry-supported study. This
RIASSUNTO                                                                                         study was supported by the Universiti Kebangsaan
Obiettivo. Questo studio mira a descrivere la frequenza del suono del russamento in rela-         Malaysia Fundamental Grant (FF-2016-326).
zione al sito di ostruzione delle vie aeree superiori.
Metodi. È stata ottenuta la registrazione di 383 russamenti da 40 partecipanti affetti da         Conflict of interest
russamento cronico. Il russamento è stato registrato digitalmente durante il sonno naturale       The Authors declare no conflict of interest.
e indotto, utilizzando un monitor per polisonnografia di livello III con microfono incor-
porato. Durante la sleep endoscopy a sonno indotto farmacologicamente (DISE), è stato               How to cite this article: Qualickuz Zanan
valutato il sito, in tempo reale, dell’ostruzione delle vie aeree superiori, registrando conte-     NH, Azman M, Zainuddin K, et al. Sound fre-
stualmente la frequenza del russamento.                                                             quency spectra of snore in relation to the site
Risultati. Il picco medio della frequenza del russamento per l’ostruzione ad un singolo             of obstruction among snorers. Acta Otorhi-
livello, palatale, orofaringeo o epiglottico, era rispettivamente di 522,5, 482,4 e 300,0 Hz.       nolaryngol Ital 2021;41:348-355. https://doi.
La maggior parte dei partecipanti ha mostrato un’ostruzione a più livelli (palato e orofa-          org/10.14639/0392-100X-N1202
ringe), in cui la media per la frequenza del russamento a due picchi era rispettivamente di
                                                                                                  © Società Italiana di Otorinolaringoiatria
402,90 Hz e 1086,96 Hz. La gravità dell’OSA era significativamente associata all’ostru-
                                                                                                  e Chirurgia Cervico-Facciale
zione multilivello.
Conclusioni. È risultata un’associazione significativa tra la frequenza del suono del rus-
                                                                                                                 OPEN ACCESS
samento e il sito di ostruzione ad un singolo livello. L’ostruzione palatale o orofaringea
produceva un suono a una gamma di frequenze medie, mentre l’epiglottide produceva una             This is an open access article distributed in accordance with
                                                                                                  the CC-BY-NC-ND (Creative Commons Attribution-Non-
gamma di basse frequenze. L’ostruzione multilivello ha documentato una frequenza di rus-          Commercial-NoDerivatives 4.0 International) license. The
samento bi-picco.                                                                                 article can be used by giving appropriate credit and mentio-
                                                                                                  ning the license, but only for non-commercial purposes and
PAROLE CHIAVE: sleep endoscopy, apnea ostruttiva del sonno, analisi della frequenza,              only in the original version. For further information: https://
russamento                                                                                        creativecommons.org/licenses/by-nc-nd/4.0/deed.en




348
                                                           Relationship between sound frequency of snore and site of obstruction among snorers




Introduction                                                       tongue and epiglottis and assessed their relationship with
                                                                   the severity of apnoea-hypopnoea index (AHI).
Snoring is a social nuisance to bed partners and everyone
who hears this unpleasant sound. Produced while sleeping,
snoring is due to upper airway narrowing and increased             Materials and methods
resistance to airflow, which results from the following
mechanisms: 1) lax pharyngeal muscles with reduced tone;           Study design and patient recruitment
2) reduced airflow in the nose; and 3) crowded airway due          A cross-sectional study was conducted over 15 months on
to enlarged tonsils or tumours 1. The craniofacial profile of      40 patients above the age of 18 years who attended the oto-
Asians has been shown to cause more severe snoring than            rhinolaryngology (ORL) clinic in the Universiti Kebang-
that of Caucasians 2,3.                                            saan Malaysia Medical Centre (UKMMC) with complaints
The clinical significance of snoring is when the snorer falls      of snoring with or without daytime somnolence. Informed
into the category of obstructive sleep apnoea (OSA). These         consent was obtained from all participants before enrol-
snorers are at an increased risk of hypertension, stroke,          ment in the study. The study protocol was approved by the
myocardial infarction and sudden death 1,4-7.                      ethics and research committee of UKMMC (FF–2016–
The current standard for clinical practice is to confirm diag-     326), which adheres to the 1964 Helsinki Declaration and
nosis of OSA with in-laboratory polysomnography (PSG).             its later amendments or comparable ethical standards.
However, this is relatively expensive and technically com-         Patients were excluded from this study if they had undergone
plex. Therefore, level III PSG monitors have been used as          previous treatments for sleep apnoea with continuous positive
an alternative diagnostic test for OSA. These utilise biosen-      airway pressure, corrective upper airway surgery, or a mandib-
sors to monitor airflow, respiratory effort, blood oxygena-        ular advancement device. Patients with significant signs and
tion, cardiac activity and sleep/wake activity 4.                  symptoms of nasal obstruction amenable to medical therapy
Identifying the site of obstruction and pattern of upper air-      or nasal surgery; those with a known underlying decompen-
way changes during sleep is pertinent in the management            sated cardiorespiratory disease or a neurological or psychiatric
of OSA. The awake state findings of the Müller manoeuvre           disease that could account for sleepiness; and those with other
may differ quite significantly from the sleep-breathing situ-      known sleep disorders, such as narcolepsy or restless leg syn-
ation 5. Conversely, drug-induced sleep endoscopy (DISE)           drome, were also excluded from this study.
provides a good objective assessment of the site of obstruc-       Other exclusion criteria included patients who were known
tion in the sleep state. It helps in tailoring therapy on an       to be on medication, such as stimulants, hypnotics, benzo-
individual basis, leading to a higher rates of surgical suc-       diazepines, antidepressants, or had a history of excessive
cess 6. Therefore, it would be particularly useful if a level      alcohol consumption; those with a body mass index (BMI)
III PSG monitor could provide clinicians with information          of more than 35; those with an American Society of Anaes-
on the site of obstruction in snorers.                             thesiologists score of 3 or more; and those who are allergic
The site and mechanism of upper airway obstruction and             to propofol.
vibration among Caucasian and East Asian populations               The patients were seen in the clinic for a comprehensive
have been found to produce snoring sounds with specific            sleep evaluation, which included history taking, physical
acoustic characteristics. Palatal snoring has been found to        examination (nasal septum, inferior turbinates, tonsil size,
have a lower centre frequency than tongue base snoring,            Friedman tongue position and type of jaw malocclusion)
although the actual frequencies reported varied substan-           and answering the Epworth Sleepiness Scale. Awake flex-
tially 7-9. However, none of these studies has used a level        ible nasopharyngolaryngoscopy was conducted and video-
III PSG monitor with a built-in microphone, which can car-         recorded for all participants. Participants were then sched-
ry out an acoustic analysis concurrently. Furthermore, no          uled for a level III PSG monitoring at night and DISE on
study has investigated snoring sound frequency in South-           the next day.
east Asian countries, which comprise multi-ethnic Asian
populations.                                                       Study instruments
Therefore, the present study aimed to describe the acoustic
characteristics of snores using NOX-T3 (a level III PSG            A. Polysomnography
monitor with a built-in microphone) among participants of          All participants were subjected to a level III PSG moni-
various local Asian ethnicities. The study also investigated       toring using the NOX–T3 (Nox Medical, Iceland) device,
the acoustic characteristics of uni- and multilevel upper air-     which is a type 3 monitor with a position sensor, pressure
way obstruction involving the palate, oropharynx, base of          transducer, pulse oximeter and audio recording from a



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N.H. Qualickuz Zanan et al.




built-in microphone. It can analyse the acoustic charac-        C. Snoring frequency analysis
teristics of snores. The microphone on the T3 device has        The snoring sounds were analysed by fast Fourier transfor-
a sampling frequency of 8 kHz. It was checked against           mation (FFT) available in the NOX- T3 device. Peak fre-
calibrated industrial sound level metres with a frequency       quency, which is the sound frequency of the highest mag-
sweep measured in a specially set up insulated sound box        nitude in an analysed snoring sample, was used for data
with a speaker. The participants were not allowed to con-       analysis. The upper limit for the range of frequencies in
sume caffeine or alcohol on the night and day of testing.       which sound occurs was fixed at 2,000 Hz, as most snoring
The NOX- T3 device was adjusted to their chest using a          sounds are produced at frequencies below 2,000 Hz. The
belt placed 10 cm from the lower lip. Oxygen saturation         spectra produced were grouped into three different bands,
and heart rate were recorded using a pulse oximeter. The        namely B1 (low-frequency, 40-300 Hz), B2 (mid-frequen-
snore events were scored automatically by the Noxtur-           cy, 301-850 Hz) and B3 (high-frequency, 851-2,000 Hz),
nal version 5.1 software algorithm (Nox Medical). The           as proposed by Lee et al. 12,13. The snoring samples during
mean of the snoring frequency during the supine position        DISE were compared with the same participant’s natural
was selected randomly. The frequency and severity of ob-        snore samples in the supine position for a valid statistical
structive events were reported as AHI. Automatic scoring        analysis. The selection of peak frequency was performed
was used to obtain the parameters from the level III PSG        manually, and thus all non-snore sounds were excluded
monitor. OSA severity was defined as mild for AHI ≥ 5           (e.g. coughing, groaning, high-pitch squeaks and duvet
and < 15, moderate for AHI ≥ 15 and ≤ 30 and severe for         noise).
AHI > 30 10.
                                                                Statistical analysis
B. Drug-induced sleep endoscopy                                 All data were computerised and analysed using Statistical
Participants underwent DISE the following morning. The          Package for Social Sciences version 23.0 software. The
procedure was conducted at the General Operating Thea-          spectrum of snoring frequency was documented as the
tre of UKMMC. Participants were placed in the supine            mean and median. The comparison between the frequency
position on the operating table. They received continuous       of snoring and the origin of unilevel vibration was made
oxygen (0.5 L/min) and underwent pulse oximetry, elec-          using the Kruskal-Wallis test. The chi-square test of in-
trocardiography and blood pressure monitoring throughout        dependence was used to analyse the association between:
the procedure. Unconscious sedation was achieved using          1) the uni- or multilevel site of obstruction and severity of
intravenous propofol infusion. The propofol titration was       OSA (normal or mild, moderate or severe OSA) and 2) the
                                                                snoring acoustic frequency spectra (B1, B2 and B3) and the
done through the target control infusion algorithm in the
                                                                site of unilevel obstruction. The Wilcoxon signed-rank test
machine, in which the patient’s age, height, weight and
                                                                was used to compare the difference in the mean frequency
gender were keyed in and titration started at 1.5 mcg/ml.
                                                                of snoring acoustic between natural and induced sleep.
Sedation was given by a qualified anaesthetist. Once a fa-
vourable level of sedation [bispectral index level (BIS) 50-
70] was reached, flexible endoscopy was performed. If the       Results
BIS level went out of range, the scope was withdrawn until
another steady state was achieved after adjusting the dose      Patient demographics
of propofol. The velum, oropharynx, tongue base and epi-        A total of 383 snores were collected from 40 patients (28
glottis were examined and continuously video-document-          males and 12 females) who were included in the study. The
ed. Parallel to the visual recording, the NOX-T3 unit was       mean age was 39 years (standard deviation (SD): 10.7;
adjusted to the participant’s chest using a belt placed 10 cm   minimum: 23; maximum: 65). The mean BMI was 28.4
from the lower lip. A minimum of 10 snores were captured        (SD: 4.8; minimum: 16.4; maximum: 35). Among these,
from each patient. The observation from sleep endoscopy         7 were primary snorers, and 9, 9 and 15 had mild, moder-
was recorded using the VOTE classification system 11, while     ate and severe OSA, respectively. A summary of the demo-
the acoustic recordings were traced from the NOX-T3. The        graphic data is presented in Table I.
level of obstruction was evaluated alongside the frequency
of snoring by incorporating the flexible endoscopy video        Snoring acoustic frequency and pattern of upper airway
into the NOX-T3 recording (Fig. 1). The beginning of the        obstruction
video recording was captured by NOX-T3 through a verbal         In the 40 participants, multilevel upper airway obstruc-
cue given by the endoscopist.                                   tion (62.5%) was more common than unilevel obstruction



350
                                                                         Relationship between sound frequency of snore and site of obstruction among snorers




Figure 1. Oropharynx obstruction. Demonstration of drug-induced sleep endoscopy and concurrent recording of snoring of a 50-year-old Indian man who had
severe OSA (AHI = 53 events/hour) and a unilevel obstruction in the oropharynx. (A) FFT shows the peak frequency at 450 Hz. (B) The power spectra of the snore
are in the mid-frequency band (B2: 301-850 Hz).


Table I. Summary of the demographic data of the study participants.              Table II. Summary of the level and site of obstruction contributing to snoring.
 Characteristics                          Number           Percentage             Description                                  Number           Percentage
                                         (total 40)            (%)                                                             (n = 40)              (%)
 Ethnicity                                                                        Level of obstruction
   Malay                                        27               70.0                Unilevel                                      15               37.5
   Chinese                                      10               25.0                Multilevel                                    25               62.5
   Indian                                        3                5.0             Site of obstruction
 Gender                                                                              Epiglottis only                                3                7.5
   Male                                         28               70.0                Oropharynx only                                9               22.5
   Female                                       12               30.0                Palate only                                    3                7.5
 Body Mass Index (BMI)                                                               Palate and epiglottis                          2                5.0
   Underweight (< 18.5)                         1                 2.5                Palate and oropharynx                         21               52.5
   Normal (18.5-24.9)                            7               17.5                Palate and tongue base                         2                5.0
   Overweight (25-29.9)                         14               35.0
   Obese (30-34.9)                              14               35.0
   Extremely obese (35)                          4               10.0            struction was ‘palate and oropharynx’ (21 patients), fol-
 Apnoea-Hypopnea Index (AHI)                                                     lowed by ‘palate and tongue base’ and ‘palate and epiglot-
   None/Minimal (< 5 per hour)                  7                17.5            tis’ with 2 participants each (Tab. II).
   Mild (5-14.9 per hour)                        9               22.5            In the analysis of the data of 383 snoring samples, uni-peak
   Moderate (15-29.9 per hour)                   9               22.5            and bi-peak frequencies were detected in unilevel and mul-
   Severe (≥ 30 per hour)                       15               37.5            tilevel obstruction, as shown in Figures 1 and 2, respective-
                                                                                 ly. For the unilevel obstruction, the ranges of the uni-peak
                                                                                 sound frequency obtained for each site of obstruction were
(37.5%). For the unilevel obstruction, 3, 9 and 3 partici-                       as follows: palate (lowest: 200 Hz, highest: 1300 Hz), oro-
pants had palatal, oropharynx or epiglottis obstruction, re-                     pharynx (lowest: 150 Hz, highest: 750 Hz) and epiglottis
spectively. None of the patients had tongue base collapse                        (lowest: 100 Hz, highest: 600 Hz). The bi-peak frequency
alone. The most common combination for multilevel ob-                            of snoring acoustic was identified in the participants with



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N.H. Qualickuz Zanan et al.




Figure 2. Multilevel obstruction. Demonstration of drug-induced sleep endoscopy and concurrent recording of snoring of a 31-year-old Malay man who had
moderate OSA (AHI = 17 events/hour) and a multilevel obstruction in the palate and oropharynx. (A) FFT shows the bi-peak frequency at 150 Hz and 600 Hz. (B)
The power spectra of the snore are in the low- and mid-frequency bands (B1: 40-300 Hz; B2: 301-850 Hz).


Table III. Frequency of snore in relation to the site of obstruction.
 Level of obstruction             Site of occurrence                    n                   Frequency (Hz)                     X2            p-value
                                                                                   Median                 IQR
         Unilevel                          Epiglottis only              26         300.00             100.0-350.0            18.929          < 0.001
                                          Oropharynx only               91         500.00             350.0-650.0
                                            Palate only                 20         450.00             200.0-537.5
P-value is based on the Kruskal-Wallis test.



multilevel obstruction. For the bi-peak frequency, the                          the site of unilevel obstruction, (X2 (4, N = 125) = 32.611,
means of the lower and upper peaks were 402.90 Hz and                           p < 0.0001) (Tab. IV). The epiglottis was found to produce
1086.96 Hz for ‘palate and oropharynx’, 445.83 Hz and                           snores in the low-frequency band (B1), and the oropharynx
833.33 Hz for ‘palate and tongue base’ and 668.75 Hz and                        and palate produced snores in the mid-frequency band (B2).
1275.00 Hz for ‘palate and epiglottis’, respectively.
                                                                                Association between the site of obstruction and the severity
Comparison between the frequency of snoring acoustics                           of OSA
and the site of unilevel obstruction                                            The analysis between the site of obstruction (uni- and mul-
The comparison of the median of the snoring acoustic fre-                       tilevel) and the severity of OSA (normal, mild, moderate
quency produced at the palate, oropharynx or epiglottis                         or severe) showed a significant association between both
revealed a significant difference in snoring frequency be-                      variables (X2 (3, N = 383) = 35.199, p < 0.0001). Multilevel
tween the different levels (p < 0.0001) (Tab. III).                             obstruction was significantly associated with higher AHI,
                                                                                in which the participants with moderate and severe OSA
Association between the snoring acoustic frequency bands                        were more likely to have multilevel obstruction (Tab. V).
and the site of unilevel obstruction
The relationship between the snoring acoustic frequency                         Comparison of snoring acoustic frequency between natural
bands (B1, B2 and B3) and the origin of vibration revealed                      and induced sleep
a significant association between the different bands and                       The comparison between the mean snoring acoustic fre-



352
                                                                         Relationship between sound frequency of snore and site of obstruction among snorers




Table IV. Association between snoring acoustic frequency bands and unilevel obstruction.
                                                           Site of obstruction (unilevel)                              Chi-Square (X2)        p-value
                                                Epiglottis             Oropharynx                   Palate
 Overall, n (%)
 B1 (40-300 Hz)                                 14 (36.8%)             17 (44.7%)                7 (18.4%)
 B2 (301-850 Hz)                                12 (14.5%)             62 (74.7%)                 9 (10.8%)                 32.611            < 0.0001
 B3 (851-2000 Hz)                                0 (0.0%)               0 (0.0%)                 4 (100.0%)
 Total                                          26 (20.8%)             79 (63.2%)                20 (16.0%)
P-value is based on the chi-square test.



Table V. Association between the severity of OSA and the site of obstruction.
 Site of obstruction                          AHI (transform refer to cut-off point)                              Total %       Chi-Square       p-value
                               None/Minimal             Mild            Moderate              Severe             (by row)           (X2)
                               (< 5 per hour)     (5-14.9 per hour) (15-29.9 per hour)    (≥ 30 per hour)
 Unilevel                        38 (27.7%)           44 (32.1%)        24 (17.5%)          31 (22.6%)        137 (100.0%)
                                                                                                                                     35.199     < 0.0001
 Multilevel                      31 (12.6%)           41 (16.7%)        63 (25.6%)          111 (45.1%)       246 (100.0%)
P-value is based on the chi-square test.



quency recorded during natural sleep at night and in-                            tion; 2) there is a significant difference in the mean of snore
duced sleep during DISE showed no significant difference                         frequency at different levels of obstruction (X2 = 18.929,
(p = 0.464).                                                                     p ≤ 0.001); 3) snorers with a multilevel obstruction have
                                                                                 higher AHI (p ≤ 0.0001); and 4) a similar snoring acoustic
Discussion                                                                       frequency was recorded for both natural and induced sleep.
                                                                                 Unlike earlier studies which found that palatal snore lies in
The main diagnostic challenge in patients with OSA is to                         the low-frequency band 7,8,14 the present study shows that
determine the site of the upper airway obstruction during                        the mean peak frequencies of palatal snore (522.50 Hz) and
sleep. DISE has the advantage of identifying the collapse of                     oropharyngeal snore (482.42 Hz) fell into the mid-frequen-
the upper airway, which is not visible when awake, result-                       cy band of the power spectra, while epiglottal snore (300.0
ing in a better selection of a specific surgical procedure for                   Hz) fell into the low-frequency band. The authors attribut-
patients. However, due to time and personnel constraints in                      ed these findings to the difference in the craniofacial profile
many centres, DISE cannot be offered to all OSA patients.                        of the participants compared with that in previous studies.
Thus, it is useful to know the pattern of upper airway ob-                       Despite suffering from a similar degree of OSA severity,
struction among snorers as well as the frequency of snor-                        Asian patients are mostly less overweight than Caucasians,
ing acoustic that they produce. This knowledge can enable                        suggesting that OSA risk factors may be influenced by
doctors to use sound frequency analysis as an evaluation                         ethnicity 3. Obese OSA patients have features of increased
tool to ascertain the site of vibration or obstruction during                    upper airway soft tissue structures, whereas non-obese pa-
sleep without the need for DISE. Patients can then receive                       tients have abnormal craniofacial structures that increase
therapy targeted to the identified level of upper airway ob-                     the risk of upper airway collapse during sleep. Li et al.
struction, which may result in greater treatment success.                        reported that unlike in Caucasians, obesity is a less sig-
To the authors’ knowledge, previous studies evaluating the                       nificant risk factor for OSA in Asian patients. Asians with
acoustic characteristics of snores were conducted among                          OSA have features of a maxillomandibular protrusion,
Caucasian and East Asian populations. The present study is                       narrower cranial base angle, larger posterior airway space
the first to involve participants of various ethnicities Malay:                  and more superiorly positioned hyoid bone compared with
n = 27, Chinese: n = 10, Indian: n = 3). This study included                     Caucasians 15. In this study, more than half of the recruit-
habitual snorers and OSA patients of various severities, un-                     ed patients were non-obese. A local study on craniofacial
like other studies that recruited habitual snorers alone or                      morphology among OSA patients revealed that differences
those with mild to moderate OSA 7-9,14.                                          exist in the craniofacial profile among patients of different
The key findings of the present study are as follows: 1) mul-                    ethnicities but with the same severity of disease. Among
tilevel obstruction is more common than unilevel obstruc-                        those with moderate to severe OSA, Malay patients had



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N.H. Qualickuz Zanan et al.




shorter maxillary and mandibular length, whereas Chinese         of snoring frequency in relation to the site of obstruction
patients were found to have a more caudally located hyoid        among snoring patients of a local multi-ethnic Asian popu-
bone 16. The snoring acoustic is a result of the vibration       lation, was achieved.
of the upper airway structures, namely the palate, pharynx,
tongue base and epiglottis. Therefore, the length of the soft    Conclusions
palate, dimension of the pharyngeal airway, position of the
hyoid bone and other cephalometric differences between           This study demonstrated that a unilevel obstruction has a
Asians and Caucasians may translate into different findings      uni-peak frequency, whereas a multilevel obstruction has
of snoring acoustic frequency as depicted by the partici-        a bi-peak snoring acoustic frequency. The mean and range
pants in the present study.                                      of the snoring acoustic frequency were documented, with
This study has several potential limitations. First, the level   the palatal and oropharyngeal snoring recording a mid-fre-
of sedation was not necessarily equal to sleep stages. Al-       quency band of the snoring acoustic spectrum. The multi-
though an increasing depth of sleep was associated with          level site of obstruction was significantly associated with a
a gradual decrease in BIS, many overlapping scores were          higher severity of OSA.
found between different sleep stages 17. Nonetheless, the
monitoring of BIS to ensure that DISE was performed              References
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                                                                      Liam CK. Is snoring a health risk? Med J Malaysia 1999;54:155-159.
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                                                                      Lam B, Ip MSM, Tench E, et al. Craniofacial profile in Asian and
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this BIS level 17-19. Second, obtaining a large sample of di-    3
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ability of this study to compare the snoring acoustic fre-       4
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equipped with personnel and facilities for resuscitation in      6
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                                                                 7
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study uses FFT for the analysis of snoring acoustic. Note        8
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                                                                 9
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built-in microphone that uses a different method of acous-            can Academy of Sleep Medicine Clinical Practice Guideline. J Clin
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With respect to these limitations, this study asserts that the        endoscopy in adults with sleep-disordered breathing: technique and
acoustical analysis of snoring is a non-invasive, convenient          the VOTE Classification system. Oper Tech Otolayngol Head Neck
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cal analysis should be applied. Nonetheless, the primary         13
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objective of this study, which is to determine the spectrum           sites and surgical response in patients with obstructive sleep apnea




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