Sixty-nine patients, who underwent a full-night respiratory recording in a sleep research laboratory because of snoring during sleep, were evaluated by an olfactory function test from January 2010 to December 2013. The inclusion criteria were as follows: age above 20 years; absence of neurologic, cardiovascular, pulmonary, or other chronic illness; no history of prior surgery on the nose and palate; no current history of drug intake; and no prior history of treatment of sleep-disordered breathing. Exclusion criteria were as follows: any previous history of treatment for snoring or OSAS, conductive olfactory dysfunction identified by computed tomography and endoscopic findings, sinonasal disease, central sleep apnea, previous history of oropharyngeal surgery excluding remote tonsillectomy, previous history of cerebrovascular accident, diabetes, any neuropathy or active neurologic disease, and a recent history of upper respiratory tract infection at the time of proposed investigation. The moderate-severe group included patients with AHI≥15/hr and the mild group included patients with habitual snoring verified by partners and AHI<15/hr [16].

This study was approved by the Ethics Committee of Konkuk University Hospital (KUH1110018).

All of the subjects with an initial evaluation who met the inclusion criteria underwent nocturnal polysomnography (PSG). PSG study was performed using an Embla N7000 (Embla, Reykjavik, Iceland). The following variables were monitored: electroencephalogram, electrooculogram, chin electromyogram, leg electromyogram, heart rate (modified V2 lead), and body position. Respiration was evaluated with a nasal cannula/pressure transducer system, polygraphic recording with measurement of esophageal pressure (PES), thoracic bands, abdominal bands, pulse oximetry, and neck microphone. Apnea was defined as the complete cessation of airflow for at least 10 seconds, whereas hypopnea was defined as a substantial reduction in airflow (50%) for at least 10 seconds or a moderate reduction in airflow (30%) for at least 10 seconds associated with electroencephalographic arousal or oxygen desaturation (>4%). The AHI was defined as the total number of apneas and hypopneas per hour of sleep [16]. Snoring components included relative snoring time (%), average snoring episode duration (minutes), longest snoring episode duration (minutes), total snoring time (minutes), and number of snoring episodes. These components were automatically calculated during PSG.

Olfactory function test included Korean version of Sniffin’s stick test I, II (KVSS I, II), which was performed by the same examiner. KVSS I test is an identification test which is performed with 8 different odorant pens. There are tampons with an odorant substance in each pen so that when the examiner opens the lid and shakes it horizontally, it is set up to make the odor with equal concentration. The KVSS II offers an extensive test of olfactory deficits. It consists of three different sets, namely KVSS II threshold, discrimination, and identification tests. In brief, threshold is defined as the concentration at which n-butanol (highest concentration 4%, 1:2 serial dilutions to 16 steps) is correctly identified four times in a row from two blanks. For discrimination testing, triplets of odorants (two of them are identical, one is different) are presented and subjects have to choose the odd odorant. KVSS II identification test is a 16-item odor identification test [17,18]. All of the odors used in the test are familiar to Koreans. Subjects are supposed to choose one out of the four odors even though they do not know the correct one, and the number of correct answers is recorded. The result of each test is recorded as a score, ranging from 0 to 16, and the sum of the three tests is presented as a composite KVSS II score. Total scores of 0–20 are defined as anosmia, 21–27 as hyposmia, and 28–48 as normosmia [19]. Patients with a KVSS II score <28 were diagnosed as having olfactory dysfunction in this study. PSG and olfactory function test were performed on the same day.

Analysis were performed using IBM SPSS ver. 20.0 (IBM Co., Armonk, NY, USA). The Wilcoxon signed rank test and Fisher exact test were used to evaluate the differences between the moderate-severe group and the mild group. Univariate analyses were performed using logistic regression to identify risk factors for olfactory dysfunction. A value of P<0.05 was considered to indicate a statistically significant result.

Sixty-nine patients were recruited for this study (56 men, 13 women; mean age, 43.86±14.16 years). The number of patients in the mild group was 19 (28%), and the number of patients in the moderate-severe group was 50 (73%). The variables such as demographic distribution, body mass index (BMI), basic PSG parameters, and olfactory function test are summarized in Table 1. BMI, sex, average oxygen saturation (SaO2), lowest SaO2, average snoring episode duration, and KVSS II score were significantly different between the two groups. Otherwise, no statistically significant difference was observed in age, snoring time, longest snoring duration, and KVSS I. Regarding olfaction, 88% of patients (61/69) demonstrated olfactory dysfunction on testing. Among all patients, 68% of patients in the mild group (13/19) and 96% of patients in the moderate-severe group (48/50) demonstrated olfactory dysfunction on testing.

In KVSS II, odor threshold test and discrimination test scores were significantly worse in moderate-severe group. Otherwise, odor identification score showed no difference between the two groups.

KVSS I test score was significantly correlated with age (P<0.001). Also, KVSS II test score (threshold, discrimination, identification [TDI]) was significantly associated with AHI (P=0.022) and average SaO2 (P=0.024). We found that AHI and average SaO2 were more closely related to the olfactory function test scores than snoring time and longest snoring episode (Table 2, Fig. 1). Snoring time and longest snoring episode showed no significant association with olfactory function test scores. In KVSS II test, threshold (T) test score was significantly correlated with AHI (P=0.005). Therefore, AHI was more strongly associated with odor threshold score than average SaO2. Discrimination (D) test score was significantly correlated with average SaO2 (P=0.042). Average SaO2 was related to odor discrimination score than AHI. Identification (I) test score was significantly associated with age (P<0.001). Odor identification score showed no significant relation with AHI and average SaO2 (Table 3, Fig. 2).