Individuals with a snoring problem who were referred to our sleep center with clinical suspicion of OSA were enrolled in our study from July 2013 to February 2015. All subjects were required to undergo full-night polysomnography (PSG). Exclusion criteria were as follows: (1) patients with nasal, oral, pharyngeal, or mandibular diseases; (2) previous CPAP treatment, use of oral appliance, or upper airway surgery; (3) other diseases including apparent respiratory, cardiac, or renal disease, and hypothyroidism. All patients were scheduled to undergo an assessment of the upper airway through CT scanning during Müller's maneuver (described below). Before the study, informed consent was obtained from all subjects and the local institutional review board approved the study protocol.
2.2. Medical history and anthropometric measurements
Demographic data including sex, age, and body mass index (BMI), as well as a detailed questionnaire on sleep symptoms, Epworth sleepiness scale, and history of alcohol consumption and smoking were collected from all subjects. Body weight and height were measured without shoes while lightly clothed in the morning, and BMI was calculated as weight (kg)/height2 (m). Waist circumference was measured around the middle between the 12th rib and the iliac crest, and neck circumference at the level of the laryngeal prominence, using a measuring tape.
2.3. Polysomnography and CPAP titration
Overnight PSG (P Series Sleep System; Compumedics; Melbourne, Australia) was performed. The parameters recorded included electroencephalography, electrooculography, electromyography, airflow by nasal and oral thermistors, respiratory effort by thoracic and abdominal impedance belts, arterial oxyhemoglobin saturation by pulse oximetry, snoring by tracheal microphone, and change of body position during sleep by a sensor. PSG recordings were manually scored for sleep stages by a physician according to the criteria of the American Academy of Sleep Medicine published in 2012. The apnea–hypopnea index (AHI) was defined as the number of apneic and hypopneic episodes per hour of polysomnographically recorded sleep time, and the oxygen desaturation index (ODI) was defined as the number of dips in oxygen saturation (SpO2) 4% or greater per hour of polysomnographically recorded sleep time. Other polysomnographic parameters included the number of apneic episodes per hour of polysomnographically recorded sleep time (AI), the percentage of total sleep time spent with SpO2 <90% (T90%), and lowest O2 saturation (LaSO2).
Patients whose AHI was ≥15.0 per hour after undergoing full-night PSG received automatic CPAP titration. When the average usage time of CPAP treatment was more than 4.0 h per night and the AHI decreased to <5 per hour, this was defined as good compliance, and the optimal level of CPAP titration was defined as the lowest effective pressure. The data were collected and analyzed electronically by a smartcard embedded in the CPAP devices.
2.4. CT evaluation
CT scanning was performed to measure the following parameters in all patients while under Müller's maneuver: anteroposterior (AP) and transverse (T) diameters at the level of the nasopharynx (NP) (upper limit: cranial base; lower limit: tip of uvula), oropharynx (OP) (upper limit: tip of uvula; lower limit: tip of the epiglottis), and hypopharynx (HP) (upper limit: at the level of vallecula; lower limit: at the level of cricoid cartilage). All the patients remained awake in the supine position with the Frankfort plane perpendicular to the floor. Slices in the axial plane extending from the skull base to the hypopharynx, below the level of the cricoid cartilage, were collected at 5-mm intervals, with a total scan time of 10 s. A lateral scout view was firstly taken to determine and standardize the level of the scans during quiet tidal breathing. The minimal cross-sectional area within the NP, OP, and HP regions (mCSA-NP, mCSA-OP, and mCSA-HP) was obtained through measuring AP and T, and the shape at each mCSA level was expressed as the ratio of AP to T (AP/T-NP, AP/T-OP, and AP/T-HP) during Müller's maneuver. Upper airway length (UAL) was defined as the vertical distance from the hard palate to the hyoid in the mid-sagittal plane. We also calculated the distance from the mandibular plane to hyoid bone (MPH). UAL and MPH were obtained from the lateral scout view during quiet respiration within the first 5 s (Fig. 1a). We did not recognize any problematic images (e.g., not in the neutral anatomic position while reviewing the CT scan). To avoid bias, every subject was taught several times how to perform Müller’s maneuver until they were able to perform the standard maneuver. All measurements were done manually by one clinician who was blinded to the PSG data.
2.5. Statistical analysis
SPSS v.17.0 for Windows (SPSS, Chicago, IL) was used to analyze all data. Continuous variables were tested for normal distribution prior to subsequent statistical analysis. Data were presented as mean ± standard deviation, median (interquartile range), and number (percentage) for normally distributed, skewed, and categorical data, respectively. Correlations between PSG variables and anthropometric parameters and CPAP titration level were assessed by the Pearson correlation test. Stepwise logistic regression analyses were performed to determine the independent predictors of AHI, LaSO2, and CPAP titration level. Differences were considered significant when the p value was less than 0.05.