Landmark Study Documents the Mental Health Challenges of Military Service in Canada

Studies of military samples hold an important place in the history of psychiatric epidemiology. In fact, the first widespread screening of mental health was conducted prior to the two world wars to predict vulnerability to stress associated with deployment or combat exposure. Ironically, brief examinations by psychiatrists to screen out “psychiatrically unfit” and “mentally marginal” recruits for World War II, which rejected as many as 12% of recruits, did not avert the accumulation of “psychiatric causalities” of war. ¹ Millions of soldiers were admitted to hospitals for combat exhaustion, war neurosis, and in the European theater, as many as 250/1,000 were removed from the battlefields for such disturbances. This experience in apparently healthy young men had a profound influence on psychiatry and highlighted the need to develop systematic tools and classification methods to characterize both mental disorders and stress vulnerability. Nevertheless, subsequent systematic screening tools repeatedly failed to predict vulnerability to deployment stress. ² With the subsequent evolution of psychiatric classification, incorporation of social sciences concepts and methods, and the introduction of standardized diagnostic interviews, the application of psychiatric epidemiology to military samples across the world has reached its maturity. Prediction of stress vulnerability and postdeployment adjustment are two of the most critical goals of military cohort research.

This issue of the Canadian Journal of Psychiatry reflects the sophisticated manifestation of systematic research designed to identify the magnitude, risk factors, consequences, and service patterns of Canadian military and veterans. The Canadian Armed Forces Members and Veterans Mental Health Follow-up Survey (CAFVMHS), conducted in 2018, is the first longitudinal survey of Canadian military and veterans that tracked a representative sample of Canadian military personnel of 2,941 from 2002 to 2018. ³ With its sound design and compelling findings about the links among deployment, trauma exposure, and mental disorder, the study represents a valuable scientific resource for understanding the mental health trajectories of the Canadian military over a period of 16 years. This work adds to the growing body of research on mental health now underway in several countries across the world. ⁴ Key findings from the CAFVMHS reveal that:

Nearly 6 in 10 Canadian Armed Force active and veteran service members experience a mental disorder in their lifetime, with pervasive comorbidity among mental disorders, and a marked increase in both the occurrence and severity of mental illness over the 16-year period. ⁵

While every deployment was not associated with major depression outcomes, deployment-related traumatic experiences were significantly and consistently related to new and persistent cases of major depression. ⁶

The accumulation of exposure to trauma during childhood and military deployment represents major independent and cumulative risk factors for mental disorders. ²

Incident and persistent posttraumatic stress disorder were associated with new trauma, transitioning to veteran status, and alcohol dependence. ⁷

There are several key features of the sample, design, and methodology that distinguish it from numerous other parallel follow-up studies of military samples worldwide. These include the representative sample of Canadian full-time regular military members at inception; the large proportion who enrolled in follow-up; and the inclusion of both regular-force personnel and veterans to allow investigators to explore exposures and outcomes during deployment as well as afterward during the transition to civilian life. The other methodological strengths of the study include the 16-year period between assessments that maximizes the ability to detect both change in and incidence of mental disorders, even long after exposure; the use of the same structured diagnostic interviews that were used in the World Mental Health Surveys of nationally representative samples in more than 20 countries, thereby anchoring the findings internationally and providing comparative data with general population samples; the inclusion of measures of multiple traumas and stressors during childhood and later military employment to permit studying the independent, cumulative, and interactive effects of these exposures on mental health; and the rich set of other measures of physical and mental health and their correlates. More broadly, as reflected in Bolton et al. ⁸ there was an impressive commitment to applying methodological gold standards and practices to sampling, longitudinal weighting, retention, respondent burden, content consistency between surveys, qualitative testing, survey language, and statistical adjustments and analyses. The investigators in this complex and multiorganizational collaboration should be commended for maintaining focus on primary objectives in the face of myriad design choices, trade-offs, and budgetary realities.

Despite the many important and meritorious findings reported in this issue, it is important to consider a few limitations inherent in the study design as well as restrictions on some measures. First, despite the longitudinal design and minimal evidence of systematic attrition bias, the findings regarding childhood trauma should be interpreted in the context of the well-established lack of reliability of retrospective recall that strongly reflects the current state. ⁹ Several military cohort studies have either used electronic health records and other data sources to reduce the inherent bias in self-reported retrospective measures. ¹⁰ Second, the sole focus on mental disorders without the inclusion of physical health limits the ability of these reports to provide a comprehensive characterization of impairment and disability that are important for public health policy on service needs. Third, one of the most important limitations of this study and many other military cohort studies is the focus on the individual rather than including their families who are also at increased risk of mental disorders and critical to the life context of military personnel and veterans. In fact, there is a dose-dependent association between time of deployment and disorders in offspring.^11,12 Several ongoing cohort studies such as the Millennium Cohort Study in the US ¹⁰ have built-in studies of families. ¹² Even if there are limited resources for such derivative studies, the inclusion of familial disorders in the risk equations could improve prediction estimates of risks of deployment as well as postmilitary-service adjustment.

The reports in this issue present the first descriptive level findings from this cohort that are likely to generate substantial investment of analyses designed to develop greater insight into mechanistic processes underlying links between military service and mental health. In this regard, the use of the World Mental Health CIDI can provide information on subthreshold manifestations of disorders, as well as specific domains associated with disorders such as sleep problems, fatigue, physical activity, smoking, alcohol use, and other drug use that could constitute actionable targets of prevention or intervention. The application of this interview can also provide a broad international anchor for comparisons with community samples worldwide. Cross-national comparisons with cohort-specific military samples could also inform some of the common and unique contextual factors that may either enhance or protect against mental disorders both during and after deployment. The most important conclusion of this rich series of studies is their clear documentation that military personnel and veterans in Canada are at heightened risk for experiencing traumatic events both before and after deployment that strongly increase the risk for mental disorders. Research into the etiological nature of these relationships constitutes a critical priority that may inform effective interventions to help improve the mental health of military personnel, veterans, and their loved ones.