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Objective Mental health problems (MHP) are a relatively common consequence of deployment to war zones. Early identification of those at risk of post-deployment MHP would improve prevention efforts. However, screening instruments based on linear models have not been successful. Machine learning (ML) has shown promise for providing the methodological frame for better prognostic models. Methods The study population was all Danish military personnel deployed for the first time between January 1, 1992 and December 31, 2013. From extensive registry data, 21 pre- or at-deployment predictors comprising early adversity, social, clinical and demographic variables were used to predict psychiatric contacts (psychiatric diagnosis and/or use of psychotropic medicine) occurring within 6.5 years after homecoming. Four supervised ML methods (penalized logistic regression, random forests, support vector machines and gradient boosting machines) were compared in ability to classify those with high risk of post-deployment MHP and those without. Results Of 27594 subjects, 2175 (8%) had a psychiatric contact. All four ML methods applied had performances well above chance (Area under the Receiver-operating Curve 0.62-0.68). Positive predictive value for the best model was 0.16. A range of pre-deployment factors were found to be predictive of post-deployment psychiatric contacts. Conclusions ML methods can be useful in early identification of soldiers with high risk of MPH in the years following their first deployment. However, performances were modest and positive predictive values were low, limiting the applicability of the models for pre-deployment screening. Future studies should include neurobiological data and deployment experiences to increase accuracy of the models.

Military personnel may withhold information on mental health problems (MHPs) for fear of not being permitted to deploy. Past or current MHPs may, however, increase the risk of postdeployment MHPs. Using psychiatric diagnoses rather than self‐report assessments in predeployment screening may be a more effective screening strategy for determining deployment fitness. This retrospective follow‐up study investigated (a) the extent to which predeployment childhood and adult psychiatric diagnoses predicted postdeployment MHPs, measured as psychiatric diagnosis and the purchase of psychiatric drugs, and long‐term sickness absence among formerly deployed Danish military personnel and (b) whether perceived combat exposure moderated or mediated the effect of predeployment psychiatric diagnoses. Complete data were available for 7,514 Danish military personnel who answered questions on perceived combat exposure between 6–8 months after returning from their first deployment to the Balkans, Iraq, or Afghanistan. Data on all psychiatric diagnoses given at Danish hospitals, all medicine purchases, and all sickness absences were retrieved from nationwide research registers. Personnel with predeployment psychiatric diagnoses had a statistically significant higher risk for both postdeployment long‐term sickness absence, hazard ratio (HR) = 2.06, 95% CI [1.52, 2.80]; and postdeployment MHPs, HR = 2.38, 95% CI [1.73, 3.27], than personnel without a predeployment psychiatric diagnosis. Personnel with a predeployment psychiatric diagnosis demonstrated a higher risk of reporting high levels of perceived combat exposure. Perceived combat exposure was not found to moderate or mediate the effect of a predeployment psychiatric diagnosis on the two outcomes. Additional findings, limitations, and implications are discussed.

Objective: The aim of this study was to estimate the association between self-reported perceived danger during deployment, measured as combat exposure or witnessing the consequences of war, and post-deployment suicide attempts among military personnel. Furthermore, the effect of post-deployment symptoms of posttraumatic stress disorder (PTSD) and/or depression on the risk of suicide attempts was also evaluated.Methods: This observational cohort study included Danish Army military personnel who returned from deployment in international missions from 1998 to 2016 and had completed a post-deployment questionnaire. Perceived exposure to danger was ascertained by self-report. Data on suicide attempt were retrieved from national registers. Adjusted Cox regression analyses were used to evaluate if military personnel indicating high level of combat exposure were more likely to have attempted suicides post-deployment than military personnel with lower levels of combat exposure.Results: Eighty-three suicide attempts were registered after homecoming among 12,218 military personnel. Perceived higher exposure to combat was associated with the risk of suicide attempt (hazard ratio = 1.08; 95% CI, 1.01-1.16). Furthermore, the association between combat exposure and suicide attempt was fully mediated by post-deployment symptoms of PTSD and/or depression. No association was found between witnessing consequences of war and the risk of post-deployment suicide attempt.Conclusions: This nationwide study found that combat exposure was associated with an increased risk of suicide attempt among military personnel. This association was, however, fully mediated by mental disorders (PTSD and/or depression). These findings suggest that better psychological follow-up of military personnel identified as having PTSD and/or depression may be warranted.

Purpose: Evidence exists of an association between pre-morbid lower cognitive ability and higher risk of hospitalization for depressive disorder in civilian cohorts. The purpose of this study was to examine the relationship of cognitive ability at conscription with post-deployment depression and the influence of (1) baseline factors: age, gender, and pre-deployment educational level, (2) deployment-related factors: e.g., war-zone stress and social support, and (3) co-morbid PTSD. Methods: An observational cohort study linking conscription board registry data with post-deployment self-report data. The study population consisted of Danish Army military personnel deployed to different war zones from 1997 to 2015. The association between cognitive ability at conscription and post-deployment depression was analyzed using repeated-measure logistic regression models. Results: Study population totaled 9716 with a total of 13,371 deployments. Low-level cognitive ability at conscription was found to be weakly associated with post-deployment probable depression after adjustment for more important risk factors like gender, education, and deployment-related factors [odds ratio (OR) 0.93, 95% confidence interval (CI) 0.88–0.99]. The co-occurrence rate with PTSD was nearly 60%. When adding co-morbid PTSD as an independent variable, the association between cognitive ability and probable depression became insignificant, OR 0.95, CI 0.89–1.02. Conclusions: Low cognitive ability at conscription is a risk factor for depression among returning military personnel, but unimportant compared to gender, education, and deployment-related factors. Part of this effect may be related to co-morbid PTSD. Use of cognitive ability score as an isolated selection tool cannot be recommended because of low predictive performance.

Objectives Little is known about the employment prospects of formerly deployed soldiers (FDS) after returning from military deployment. The few studies that exist reported mixed results, and even fewer undertook comparisons with a civilian control population. This study compared labor market transitions of FDS within five years of returning from their first international deployment with those of a closely matched general-population control group. Methods Danish FDS (N=6653) returning from their first ever peacekeeping in Kosovo or Iraq, or more intense combat in Afghanistan (period 2002–2012), were matched with non-deployed controls from the general population (N=62 281). We modelled time-to-event using Cox models, for transitioning from employed to unemployed and back, and from work to long-term sickness absence and back. Each analysis adjusted for age and level of education and was stratified for the region of residence and the underlying period. Results Independent of deployment country, FDS had a lower risk of becoming unemployed [hazard ratio (HR) 0.55–0.73] and a higher chance of obtaining employment (HR 1.19–1.31) than matched controls. FDS returning from Afghanistan had a higher risk of long-term sickness absence (HR 1.66), while those returning from Kosovo had a higher chance of returning to work (HR 1.24). Conclusion Independent of deployment country, FDS fared better in the labor market within five years of returning home compared to non-deployed controls. However, deployment to Afghanistan was related to a higher risk of long-term sickness absence, suggesting that some soldiers have worse outcomes than the general population.

Gulf War veterans (GWVs) have an elevated risk of reporting symptoms of mental disorders as compared with nondeployed military controls. A difficulty in the Gulf War health research is that most health outcomes are self-reported; therefore, it is highly relevant to study objective outcomes in this line of research. The Danish National Prescription Registry provides an opportunity to use the prescription of drugs as an objective evaluation of the impact of mental health disorders at the individual level. In this study, we investigated the prescription of drugs and postdeployment hospitalizations for mental disorders among GWVs compared with a control population of nonveterans (NVs).A prospective registry study including a cohort of 721 GWVs and a control cohort of 3,629 NVs. Main outcome measures were incidence of (1) use of antidepressants, (2) use of anxiolytic/hypnotic medication, and (3) number of postdeployment psychiatric contacts. The association between outcomes and GWVs status was studied by using time-to-event analysis. The index date was the return date from the last deployment to the Gulf. The follow-up period was the time from index date until December 31, 2014.GWVs had an elevated average risk over time for use of both types of medication compared with NV. For use of antidepressants the average hazard rate (HR) was 2.56, with 95% confidence interval (CI) = 2.04-3.21 (p0.0001); for use of anxiolytic/hypnotic medication the corresponding results were HR = 1.78, CI = 1.37-2.31 (p0.0001). The interaction with time was statistically significant with HR increasing with time for both outcomes. Incident use of antidepressants in GWVs after 10 years was two times higher than among NV, after 20 years it was nearly four times higher than among NV. Incident use of anxiolytic/hypnotic medication was one and a half that of NV after 10 years, but nearly three times that of NV after 20 years. There was no difference in rate of postdeployment psychiatric contacts.The findings of increased use of antidepressants and anxiolytic or hypnotic medicine among GWVs compared with NVs were rather surprising since we recently, by using the same study population, found that deployment to the Persian Gulf was not associated with increased sickness absence or reduced labor market attachment. However, our results indicate that the mental health of the Danish GWVs is worse than in NV, and that this unfavorable difference increased with time. A possible explanation is that veterans have a high motivation for being in work, and that the deployment-related mental problems they may have acquired do not impair their ability to work, when treated properly. Furthermore, registry-based research in GWVs could include other outcomes, e.g., the use of pain medication, and other military comparison groups, e.g., veterans deployed to other areas than the Persian Gulf in addition to NV. The method of surveillance of military personnel with register data pertinent to health and monitoring outcomes compared with suitable control populations is highly recommended as a tool in the prevention of deployment-related health problems.

BACKGROUND Posttraumatic stress disorder (PTSD) is a relatively common consequence of deployment to war zones. Early postdeployment screening with the aim of identifying those at risk for PTSD in the years following deployment will help deliver interventions to those in need but have so far proved unsuccessful. OBJECTIVE This study aimed to test the applicability of automated model selection and the ability of automated machine learning prediction models to transfer across cohorts and predict screening-level PTSD 2.5 years and 6.5 years after deployment. METHODS Automated machine learning was applied to data routinely collected 6-8 months after return from deployment from 3 different cohorts of Danish soldiers deployed to Afghanistan in 2009 (cohort 1, N=287 or N=261 depending on the timing of the outcome assessment), 2010 (cohort 2, N=352), and 2013 (cohort 3, N=232). RESULTS Models transferred well between cohorts. For screening-level PTSD 2.5 and 6.5 years after deployment, random forest models provided the highest accuracy as measured by area under the receiver operating characteristic curve (AUC): 2.5 years, AUC=0.77, 95% CI 0.71-0.83; 6.5 years, AUC=0.78, 95% CI 0.73-0.83. Linear models performed equally well. Military rank, hyperarousal symptoms, and total level of PTSD symptoms were highly predictive. CONCLUSIONS Automated machine learning provided validated models that can be readily implemented in future deployment cohorts in the Danish Defense with the aim of targeting postdeployment support interventions to those at highest risk for developing PTSD, provided the cohorts are deployed on similar missions.

Background Posttraumatic stress disorder (PTSD) is a relatively common consequence of deployment to war zones. Early postdeployment screening with the aim of identifying those at risk for PTSD in the years following deployment will help deliver interventions to those in need but have so far proved unsuccessful. Objective This study aimed to test the applicability of automated model selection and the ability of automated machine learning prediction models to transfer across cohorts and predict screening-level PTSD 2.5 years and 6.5 years after deployment. Methods Automated machine learning was applied to data routinely collected 6-8 months after return from deployment from 3 different cohorts of Danish soldiers deployed to Afghanistan in 2009 (cohort 1, N=287 or N=261 depending on the timing of the outcome assessment), 2010 (cohort 2, N=352), and 2013 (cohort 3, N=232). Results Models transferred well between cohorts. For screening-level PTSD 2.5 and 6.5 years after deployment, random forest models provided the highest accuracy as measured by area under the receiver operating characteristic curve (AUC): 2.5 years, AUC=0.77, 95% CI 0.71-0.83; 6.5 years, AUC=0.78, 95% CI 0.73-0.83. Linear models performed equally well. Military rank, hyperarousal symptoms, and total level of PTSD symptoms were highly predictive. Conclusions Automated machine learning provided validated models that can be readily implemented in future deployment cohorts in the Danish Defense with the aim of targeting postdeployment support interventions to those at highest risk for developing PTSD, provided the cohorts are deployed on similar missions.

BackgroundStudies of the association between pre-deployment cognitive ability and post-deployment post-traumatic stress disorder (PTSD) have shown mixed results.AimsTo study the inflence of pre-deployment cognitive ability on PTSD symptoms 6–8 months post-deployment in a large population while controlling for pre-deployment education and deployment-related variables.MethodStudy linking prospective pre-deployment conscription board data with post-deployment self-reported data in 9695 Danish Army personnel deployed to different war zones in 1997–2013. The association between pre-deployment cognitive ability and post-deployment PTSD was investigated using repeated-measure logistic regression models. Two models with cognitive ability score as the main exposure variable were created (model 1 and model 2). Model 1 was only adjusted for pre-deployment variables, while model 2 was adjusted for both pre-deployment and deployment-related variables.ResultsWhen including only variables recorded pre-deployment (cognitive ability score and educational level) and gender (model 1), all variables predicted post-deployment PTSD. When deployment-related variables were added (model 2), this was no longer the case for cognitive ability score. However, when educational level was removed from the model adjusted for deployment-related variables, the association between cognitive ability and post-deployment PTSD became significant.ConclusionsPre-deployment lower cognitive ability did not predict post-deployment PTSD independently of educational level after adjustment for deployment-related variables.

To examine the assumption that postdeployment incidence of sickness and other absence from work are higher among Gulf War Veterans compared with nonveterans.A prospective registry study including a cohort of 721 Danish Gulf War Veterans and a control cohort of 3,629 nonveterans selected from the general Danish population. Outcome measures were up to 23 years postdeployment incidence of (1) long-term sickness absence and (2) long-term all types of absence from work. Long term with regard to sickness and other absence was defined as exceeding 8 weeks. The association between outcomes and information on deployment history was studied using time-to-event analysis. The index date was the return date from the last deployment to the Gulf. The follow-up period was the time from index date until April 27, 2014.As the main finding, no difference was found between veterans and nonveterans in the incidence rate of long-term sickness absence. After an initial short period (3 months) with elevated incidence rate of long-term absence from work among veterans, there was no difference between the cohorts.Among Danish Gulf War Veterans, no postdeployment increased risk of long-term sickness absence or long-term absence from work was found as compared with nonveterans.

Using data from an occupational medical health surveillance program, we studied the associations between mental stressors and social support and the two outcome measures postdeployment psychological distress and multiple physical symptoms among Danish soldiers deployed to Iraq. The study was cross-sectional and questionnaire-based with soldiers returning from the mission as the target group. Witnessing atrocities, fear of being physically harmed, feeling of insecurity, feeling of meaninglessness, and having been in touch with prisoners were associated with both outcome measures. In conclusion, our findings support the assumption that other factors than combat exposure—psychosocial and cultural—are of importance in increasing psychological distress among soldiers deployed to Iraq. Additionally, we have shown that the reporting of multiple physical symptoms among the deployed soldiers is closely related to increased psychological discomfort and certain mental stressors.

To compare the prevalence of musculoskeletal disorders of personnel in the main battle tank (MBT) units in the Danish army with those of personnel in other types of army units, and to investigate associations between job function in the tank, military rank, and musculoskeletal problems.An epidemiologic cross-sectional questionnaire-based examination of an exposed group (MBT personnel) and a reference group (army personnel from other units than MBT units) was conducted. Outcome measures were 1-year prevalence of pain in the anatomical locations of neck, shoulder, low back, knee, and ankle.There were only 4 women in the MBT group; as a consequence, female personnel were excluded from the study. The participation rate was 58.0% (n = 184) in the MBT group and 56.3% (n = 333) in the reference group. The pattern of musculoskeletal disorders among personnel in the main battle tank units of the Danish army was not significantly different from that in other types of units (infantry, signal, combat service support, engineers, and artillery). Working as a gunner less than 2 years increased the risk of reporting neck pain (p = 0.011) and working as a loader increased the risk of reporting shoulder pain (p = 0.017).

Where much is known about the consequences of spinal and low back pain (LBP) during military deployments, there is lesser knowledge of risk factors for LBP among the deployed forces. The objective of this study was to identify deployment-related exposures associated with LBP. The study was a questionnaire-based cohort study among 1,931 Danish soldiers deployed to Iraq. Of the 680 respondents, 175 (26%) reported LBP. The population of respondents was adjusted for potential nonresponse bias. The associations between LBP and explanatory variables were analyzed using ordinal logistic regression models. Older age (p = 0.016), support from leaders (odds ratio [OR] = 1.69, p = 0.019), psychological stress (OR = 1.71, p = 0.009), awkward working positions (OR = 1.98, p = 0.001), and working in depots or storehouses (OR = 2.60, p = 0.041) were found to be associated with LBP after adjustment of all other variables. Combat and exposure to work, sport, or traffic accidents were not associated with LBP in this study, which was attributed to the characteristics of the actual mission. Preventive measures should include predeployment preparation of leaders to cope with LBP and other musculoskeletal trouble among their subordinates and involve medical personnel, especially deployed physiotherapists, by giving advice to soldiers of different military occupational specialties on how to optimize ergonomics at work.

On Denmark's decision to join the war in Iraq, it was decided to immunise all personnel to be deployed there with anthrax vaccine. This paper describes the types and frequency of adverse reactions to the vaccine.Between March 2003 and February 2004, 1,899 immunisations against anthrax in 755 personnel were registered at the health care facilities of the Danish Defence.No serious or life-threatening adverse reactions were reported. The data showed a statistically significant decrease in adverse reactions between the four immunisation series. For males, adverse reactions after the second immunisation depended on possible adverse reactions to the first immunisation. In females, this correlation was not found. No differences in the frequency of adverse reactions between males and females were found.None of the vaccinated persons suffered serious or life-threatening adverse reactions. Compared to other studies on self-reported adverse reactions, the overall frequency was low. The data showed a statistically significant decrease in adverse reactions between the four immunisation series. This has been described in comparable studies. The correlation between adverse reactions in the second immunisation being dependent on adverse reactions to the first immunisation has been partly confirmed by other studies. No differences between the sexes were found, probably because only a few females were included in the study. In case of a significant level of threat concerning biological weapons, immunisation against anthrax is considered appropriate.