The main findings of this study are that as hypothesised, sustaining an injury on deployment is associated with over four times the odds of developing PTSD. Contrary to our hypothesis, having an illness on deployment that results in attendance at a field hospital is associated with post deployment mental health problems. The strength of this association is similar to, if not more than, the association with having a physical injury. Being returned to unit following attendance at a field hospital was not associated with any adverse effects on post deployment mental health. Attending a field hospital on deployment for either an illness or an injury was not associated with reported alcohol misuse post deployment.
It is widely accepted that serious injury increases the risk of probable PTSD
[6, 8, 19] particularly injuries resulting from hostile action
 and this has been corroborated in this current study. On the other hand, the finding that being medically evacuated for an illness was strongly associated with having probable PTSD and common mental disorders is a novel finding within the military literature. Evidence from civilian populations indicates that patients with chronic illnesses report symptoms of PTSD as do those requiring treatment in intensive care
[21, 22]. The perceived, and often actual, threat to life during episodes of these illnesses may trigger PTSD
. Illness has also been identified as a risk-factor for depression and anxiety in civilian populations, thought to be due to an increased pain and, or in addition, to it disabling the individual
However, the illness events requiring medical air-evacuation are often not life-threatening or chronic; patients requiring prolonged treatment are routinely evacuated as field hospital space is limited and a substantially wider range of treatment options are available in the UK. It may be, instead, that the social environment faced by those returning home due to an illness puts them at greater risk of mental health problems in two ways. First, whereas injured personnel are recognised as being vulnerable to mental health problems, to the extent they have mandatory mental health monitoring, personnel with an illness are less visible and may feel more isolated; low levels of social support is a known risk factor for PTSD and other mental health conditions
Second, personnel with illnesses may be subject to a greater degree of stigma than people with injuries. Society tends to treat personnel seriously injured on deployment as war heroes, with an ‘honourable’ reason for leaving, while those leaving the deployment due to illness may not receive the same degree of reverence and may be, or at least feel, stigmatised.
The other main finding is that personnel returned directly to their unit after attending an emergency department are not at increased risk of mental health problems post deployment. This fits with the UK military’s policy on treating psychiatric injuries known as ‘forward psychiatry’; that is, to treat them within the proximity of where the event is presented, to deliver care immediately and with the expectancy of occupational recovery
. By avoiding evacuation too readily and keeping personnel with their unit, evidence suggests that psychiatric problems are less likely to develop
. Military patients with general medical problems may also realise benefits to their mental health in the long term if, where their medical state allows, they are returned quickly to the support of colleagues and allowed to remain operationally effective. However, it should be considered that the severity of the injury or illness for those returned to unit is likely to be low, and it may be this factor making them less vulnerable to post deployment mental health problems.
This study also found that attending an emergency department for any reason, including a hostile injury, was not associated with an increased risk of alcohol misuse among UK Army personnel. Higher rates of alcohol misuse have been observed among US personnel exposed to ‘threatening situations’, one of which was ‘being injured or wounded’
 and mild traumatic brain injury (mTBI) has been associated with increased risk of alcohol misuse
. It may be that specific injuries, such as mTBI, increase the risk of alcohol misuse, but when combining all injuries no effect is found. The lack of association with illness may be explained by the illness itself not being conducive with drinking alcohol. Despite the questionnaire being self-reported and anonymous, alcohol use may be under-reported as excessive use is socially undesirable.
One of the main strengths of this study has been the ability to distinguish between illness and injury, allowing the impact of becoming ill during a military operation on post deployment mental health to be studied for the first time. The study also benefits from using routinely collected data on injuries and illnesses, rather than self-reported data, which many studies looking at injury and mental health rely on
[6, 27]. This reduces the potential of recall bias; specifically that those with and without mental health problems report injuries or illnesses experienced on deployment differently.
The current study has certain limitations. Injuries or illnesses occurring on deployment treated in primary care settings are unlikely to be captured by OpEDAR; further, accessibility to the field hospitals is likely to impact on field hospital attendance. This may explain the lack of association seen between injuries and role on deployment, since intuitively those in combat roles would be expected to have more injuries. Another limitation of the data is that severity was only assessed through a proxy (disposal type); some studies have found the risk of PTSD increases with the severity of the combat injury
. Additionally, the study is limited to Army personnel meaning the results are not generalisable to the entire UK military; this was justified as the Army are the largest group to deploy and non-Army personnel had a very small number of OpEDAR events. Identifying the direction of any bias introduced from 10% of the study sample being excluded due to non-consent of use of medical records is not possible. As the proportion excluded is small, it is unlikely to have a significant effect on the results reported here. Furthermore, although a sensitivity analysis was run where illness took precedence over injury, it is acknowledged that the associations found here may be affected by the choice of the most severe event. The authors also acknowledge that the illnesses experienced whilst on deployment may have been psychosomatic manifestations of the stress response. If a physician believes an illness to be psychosomatic, it may have been recorded as a psychiatric illness on OpEDAR, though it is appreciated that distinguishing physical illness from somatic symptoms caused by distress is challenging. However we believe this is unlikely to account for all the association between physical illness on deployment and post-deployment mental health problems observed here. Finally, it is acknowledged that the conclusions regarding the mental health consequences of air-evacuations for medical reasons rely on small numbers and must thus be treated with caution.