Low-level cognitive ability in young adulthood and other risk factors of depression in an observational cohort study among deployed Danish soldiers

Lars Ravnborg Nissen,Merete Osler,Søren Bo Andersen,Anni Brit Sternhagen Nielsen,Erik Lykke Mortensen,Mia Sadowa Vedtofte,Karen-Inge Karstoft,Gunhild Tidemann Christensen

doi:10.1007/s00127-018-1648-1

Abstract

PurposeEvidence 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.MethodsAn 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.ResultsStudy 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.ConclusionsLow 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.

Highlights

Armed conflicts are increasingly recognized as having longterm detrimental consequences for the mental health of combatants [1]
To evaluate if the cognitive ability score could be used as an indicator of potential mental vulnerability, we examined the performance of the score as a predictive test for postdeployment depression by conducting Receiver Operating Characteristics (ROC) curve analysis for both outcomes
We found that young adult cognitive ability score was significantly associated with probable depression measured post-deployment when controlled for other factors known at baseline such as age, educational level, and gender

Summary

Introduction

Armed conflicts are increasingly recognized as having longterm detrimental consequences for the mental health of combatants [1]. There is substantial evidence for a link between participation in combat and post-traumatic stress disorder (PTSD) [2, 3], while the influence of military deployment on depression is less well studied. There is evidence of a PTSD-depression co-morbidity both in general [4] and when PTSD is combat-related [5]. Whether there are risk factors associated with depression in a military context has only been addressed in very few studies [6, 7]. That deployment with combat exposure is a risk factor for new-onset major depression [8,9,10]. In a study of 1560 marines deployed to Iraq or

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