Study design and participants
A cross-sectional study was conducted in Golmud (average altitude: 2,890m), Qinghai province and Fuzhou (average altitude: 84m), Fujian province, China during July and August 2016. 200 plateau drivers were selected from the Golmud motor transport base as HA group, and 200 drivers were selected from Fuzhou motor transport base as LA group. All selected drivers were male Han Chinese. Selected drivers with the following conditions were excluded: (1) suffering acute physical conditions; (2) born, growing or living at HA (> 500m) before enlisted; (3) for LA group, had gone to HA (> 500m) in the recent year; (4) for HA group, left the plateau in the recent 3 months. Self-administered questionnaires were directly distributed to the remained participants who were asked to complete the questionnaires in 15 minutes. Finally, 194 HA group participants and 190 LA group participants completed the questionnaires (see Fig. 1).
The protocols were approved by the Review Board of the Second Military Medical University. Written informed consent was obtained from each subject before participating in this study.
Measures
Demographic and working characteristics
Age, altitude, deployment duration at the present base, service time, educational level, marital status, only child or not, and military rank were obtained in this study. ‘Altitude’ was categorized as ‘High altitude’ and ‘Low altitude’. ‘Education’ was categorized as ‘Secondary school’, ‘High school’, ‘Junior college’, and ‘Undergraduate or above’. ‘Marital status’ was categorized as ‘Single’, ‘Married’. ‘Only child or not’ was categorized as ‘Yes’ and ‘No’. ‘Military rank’ was categorized as ‘Private’ and ‘Non-commissioned officer (NCO)’.
Depression
Depression was measured with the Self Rating Depression Scale (SDS) [22]. SDS consists of 20 items, and each item describes a symptom of depression scored on a four-point Likert scale according to the frequency of the symptoms in the previous week, with anchors 1 (none or a little of the time), 2 (some of the time), 3 (a good part of the time), 4 (most or all of the time). The total score was multiplied by 1.25 to obtain the standard score [23. Higher standard score represented higher intensity of depression symptoms. According to the Chinese norm of the SDS, the severity of depression was calculated by dividing the total score by 80. The cut-off score was set at 0.5 in this study [24]. The Chinese version of SDS has been repeatedly validated and widely used to examine depression status among Chinese adults [24-26]. The Cronbach's α of SDS in this study was 0.854.
Symptoms of CMS
Due to the time constraint, we measured four major symptoms of CMS in this study, namely fatigue, migraine, breathlessness, and sleep disturbances.
Fatigue was measured using 14-item Fatigue Scale (FS-14) [27]. The FS-14 consists of 14 yes-or-not questions (0 = no; 1 = yes) in order to reflects the severity of physical fatigue and mental fatigue. The total score was calculated and a higher score suggested severer fatigue. The Chinese version of FS-14 has been proved to have high reliability and widely used in China [28, 29].
Migraine was measured with Global Assessment of Migraine Severity (GAMS) [30]. GAMS contains a single question with seven response categories: “How severe is your migraine?”, 1 = Not at all severe, 2 = a little severe, 3 = somewhat severe, 4 = moderately severe, 5 = quite severe, 6 = very severe, 7 = extremely severe. GAMS has been proved to be a brief and valid tool to assess migraine severity [30].
Breathlessness was measured with numeric rating scale (NRS) [31]. NRS contains a single question “Are you short of breath now?” which is anchored at 0 and 10 with verbal anchors “nothing at all” to “maximal”. The NRS has been proved to be a valid measure of present dyspnea with low response bias [31, 32].
Sleep disturbance was measured with item three of the Patient Health Questionnaire-9-item (PHQ-9), which was an efficient brief screener for sleep disturbance with relatively high sensitivity and specificity [33]. The item asks the participant to rate how often he/she has had “Trouble falling or staying asleep, or sleeping too much” over the past two weeks, range from 0 (not at all) to 3 (nearly every day).
Statistical analysis
SPSS (version 22.0) was used for statistical analysis. All tests were two-tailed, and statistical significance level was set at a p < 0.05. HA group participants (plateau drivers) and LA (lowland drivers) were included in the analyses. Sociodemographic and working characteristics were recorded and reported as Mean ± SD for continuous variables and number (percentage) for categorical variables. Comparisons of depression, symptoms of CMS, and demographic data between HA and LA group were tested by independent sample t-tests and χ2 tests. Comparisons of depression and symptoms of CMS among different categorical demographic and working characteristics in HA group were tested by independent sample t-tests and ANOVA. Multivariate logistic regression was used to calculate the crude and adjusted odds ratio (OR) with 95% confidence interval (CI) to assess the factors associated with depression among plateau drivers.