The Prevalence of Functional Gastrointestinal Disorders in the Chinese Air Force Population

Background. Functional gastrointestinal disorders (FGIDs) are common in the general population worldwide. However, there is paucity of large sale surveys for prevalence of FGID in the military personnel. Methods. It is a cross-sectional study, using Rome III criteria for the diagnosis of FGID among the Chinese Air Force (CAF) workers. Results. Of 4633 registered male subjects, there were 818 (16.4%) air crew and 4170 (83.6%) ground personnel. FGIDs were identified in 1088 (23.48%) of cases. It was more prevalent in the ground personnel than air crew (24.02% versus 20.33%; P = 0.022). Based on Rome III criteria, the commonest disease category was functional gastroduodenal disorder (37.4%), whereas functional nausea and vomiting disorder (FNV) was the most frequent overall diagnosis. Functional dyspepsia (FD) with irritable bowel syndrome (IBS) was the leading FGIDs' overlap (3.9%). Conclusion. FGIDs in CAF population are rather underestimated. This necessitates preventive strategies according to job characteristics.


Introduction
FGIDs comprise idiopathic disorders, characterized by chronic and bizarre complaints arising from dysmotility and hypersensitivity of the digestive system. The diagnosis is made by symptom-based approach using Rome III criteria [1]. These functional disorders render a negative impact on patients' quality of life and induce substantial cost on healthcare system through diagnostic evaluation [2,3]. The prevalence of FGIDs varies enormously on account of differences in environmental factors, population, and ascertainment criteria. There is considerable overlap between different functional disorders of the digestive tract in general population [4]. This aspect is of special interest in the air force personnel, considering their occupations, arduous working environment, and exposure to various chemicals including jet fuel [5]. These personnel are also subjected to air sickness [6], altitude flying [7], combat stress, and prolonged separation from home [8]. Their job requires an all time high level of physical and mental performance [9]. The scarcity of large scale surveys about FGIDs in military service has prompted us to undertake this study in the CAF personnel. Our aim is to assess the prevalence and overlap of FGIDs in these subjects with a focus on their service requirements.

Patients and Methods
Between September 2008 and March 2009, a cross-sectional survey was performed in the representative CAF population ( = 5800), located at six different regions of China. A randomized, multistage sampling methodology was used for this purpose. All respondents were required to complete a questionnaire, which consisted of two parts: Part 1 pertained to demography, that is, age, marital status, ethnicity, education, and job description; Part 2 was a modified Chinese version of the FGIDs self-report questionnaire (FGIDs-Q) based on Rome III criteria and comprising 93 questions [10]. The sampled population was interviewed by squadrons' flight surgeon and the information was recorded on a computer using software EPIDATA3.02. The present study was approved by Institutional Review Board of the Chinese People's Liberation Army General Hospital. Written informed consent was obtained from all participants.
Statistical analysis was performed with SPSS software version 13.0 (SPSS Inc, Chicago, IL, USA). Chi-square test and Fisher exact test were used for detecting significant differences in the percentage of categorical data. value <0.05 was taken as statistically significant.
The categories of FGIDs occurred in different combinations in our air force workers: two-way combination 0.92%-4.06%, three-way combination 0.35%-0.87%, and four-way combination 0.23%. Categories B and C of FGIDs were the most prevalent categories in these complexes. FD-IBS 2.89% was the most frequent combination whereas four-way complexes, comprising FH + FD + IBS + FAP, constituted only 0.08% of persons (Table 3).

Discussion
This is the first population-based military survey using Rome III criteria to evaluate FGIDs in the CAF workers. According to our observation, FGIDs is a rather low prevalent functional disorder in healthy military personnel. On the contrary, prevalence of FGIDs in the Chinese citizens ranges from 5.67% to 55.24% [11,12]. There are several possible explanations for this difference in the prevalence of FGIDs. The most plausible reason may be the induction of a male predominant group of respondents in this survey. Our participants are healthy military men, who do not represent the real Chinese general population. The gender predilection in majority of FGIDs exists. There is an obvious female predominance on account of psychosocial factors and difference in the hormonal ratios [13]. The diversity in prevalence rates may also be due to wide variation in study designs and sampling methods. The majority of aircrew in this survey are pilots. It is not clear why the aircrew reported slightly lesser FGID than ground personnel. However, we presume that pilots mainly belong to the officer class, who possess higher level of technical education than workers in the support services. The job of aircrew requires a highly efficient and error-free performance. They undergo regular physical and psychiatric evaluation by the flight surgeon for routine flying and flight training. Only qualified candidates with higher psychophysical score are selected as aircrew [14]. On the other hand, ground personnel include aeronautical engineers, fuel technicians and airfield defense troops. The enlisted ranks normally constitute a massive proportion of the ground personnel. They require lower formal education according to the occupational needs. The ground personnel are required to perform arduous tasks. They enjoy lesser autonomy and personal control as compared with the aircrew. Thus FGIDs symptoms amongst ground workers are apparently caused or aggravated by tough working environment, adverse psychological features, and nongastrointestinal conditions [15]. However, more studies should be carried out to determine the impact of specific occupational, organizational, and psychosocial factors on the clinical presentation of FGIDs.
Our results indicate that FNV was the most prevalent FGIDs overall and in the ground personnel. It was also the second most frequent functional disorder in aircrew. On account of low prevalence of these symptoms, there is no convincing explanation in the medical literature to support the underlying mechanism for FNV in the civilian population [16]. Several trigger factors can be found in approximately 80% of patients with cyclic vomiting syndrome [17]. They include infection, psychological stress, motion sickness, menstruation, lack of sleep, and physical exhaustion [18]. The triggers appear to be more common in a military setting due to increased stress of flying sophisticated combat aircraft. A skilful handling of these complicated machines involves high speed flying, constant exposure to rapid acceleration/deceleration (i.e., "g" forces), vibration, noise pollution, spatial disorientation and risk of rapid decompression [19]. Nausea and vomiting are common manifestations of high altitude flying [20,21]. Motion sickness, nausea, and vomiting are also caused by combined lateral and roll oscillation during flying [22,23]. The aerotechnicians are at risk of developing the above symptoms when subjected to irritant jet fuel during accidental spills, degreasing, and fuel storage. The aeronautical engineers are also at similar risk during general maintenance or operation of the military aircraft and vehicles. The data for epidemiological survey of the Spanish Gastroenterology Research and Practice 5 Ministry of Health & Consumer Affairs shows that nausea and vomiting were major complaints by the workers engaged in clean-up of the Prestige oil-spill [24]. However, few related manuscripts are available about the influence of these factors on FNV in specific settings because the exact underlying mechanism for this finding remains to be established.
As shown in the previous studies, several other highly prevalent FGIDs, including IBS, FD, and FAB, are associated with diverse pathophysiological mechanisms. The data of Defense Medical Surveillance System (1999-2007) describes the distribution of main FGID as FC 55%, dyspepsia 21.2%, FD 2.1%, IBS 28.5% [25]. The ratio of FC and IBS in this data is higher than our study. The reason for this incompatibility may be due to more subcategories of FGID. It is also suggested that infectious gastroenteritis during deployment increases the risk of FGID in these workers [26]. The same system further suggests that dysmotility may result from deploymentrelated travellers' diarrhea although these findings need to be confirmed. According to them, lower military ranks are more vulnerable to develop IBS (OR: 3.70; = 0.02) [26]. This observation is consistent with our results. Besides infection, noise and other occupational exposures have significant association with IBS [27]. The available evidence indicates that aircraft noise increases IBS prevalence in residents around the military airbase in Pyeongtaek city [28].
Several studies demonstrate that FGIDs' overlap is a common occurrence [29,30]. The prevalence rates for different combinations of FGIDs range from 0.23% to 4.06% in our survey. These figures appear small but the overall rate of FGIDs complexes actually accounts for >50% of total participants in the present study. This observation is coincident with a previous survey in the civilian population [29]. Some studies advocate that various combinations of FGIDs may result from a common pathogenesis [30]. Psychosocial abnormalities seem to play an important role in the development or exacerbation of FGIDs' overlap. Depression, anxiety, and posttraumatic stress disorder tend to favor the development of IBS with dyspepsia in women receiving primary care at the Veteran Affairs Medical Center. The aforementioned mechanisms thus provide some clue to high incidence of overlap in a particular setting.
There are some limitations in the current survey. Our sample was derived from frontline troops. The females could not be included due to their small percentage in the CAF population. The study was based on self-reported data so that organic diseases were not ruled out in our subjects.

Conclusion
In summary, our study provides an objective evidence to examine the impact of several military service-related factors on the prevalence of FGIDs. The preventive strategy should be rationally planned according to the occupational characteristics.