Adjustment disorder in the Armed Forces: a systematic review

Abstract Background In the UK military, adjustment disorder (AjD) is reported as one of the most diagnosed mental disorders, alongside depression, in personnel presenting to mental health services. Despite this, little is understood about what may predict AjD, common treatment or outcomes for this population. Aim The systematic review aimed to summarise existing research for AjD in Armed Forces (AF) populations, including prevalence and risk factors, and to outline clinical and occupational outcomes. Method A literature search was conducted in December 2020 to identify research that investigated AjD within an AF population (serving or veteran) following the PRISMA guidelines. Results Eighty-three studies were included in the review. The AjD prevalence estimates in AF populations with a mental disorder was considerably higher for serving AF personnel (34.9%) compared to veterans (12.8%). Childhood adversities were identified as a risk factor for AjD. AjD was found to increase the risk of suicidal ideation, with one study reporting a risk ratio of 4.70 (95% Confidence Interval: 3.50–6.20). Talking therapies were the most common treatment for AjD, however none reported on treatment effectiveness. Conclusion This review found that AjD was commonly reported across international AF. Despite heterogeneity in the results, the review identifies several literature gaps.


Background
Armed Forces (AF) personnel are often required to work in challenging environments which may impact their physical and psychological health. Research has identified several occupational stressors which may increase the risk of psychological ill-health including deployment, combat experience, lack of leadership support and role conflict (Brooks & Greenberg, 2018;Campbell & Nobel, 2009). In 2021, Defence Statistics data indicated that 1 in 10 UK AF personnel were seen by an AF clinician for mental health reasons (Ministry of Defence, 2021). Of those seen by a clinician, 33% were initially found to be experiencing adjustment disorder (AjD). Of the whole AF population AjD (0.6%) was as frequently diagnosed as depression (0.6%) and seen more frequently than Post-Traumatic Stress Disorder (PTSD) (0.2%) and substance misuse (0.1%).
The tenth edition of the International Classification of Diseases (ICD-10) identifies symptoms of AjD as: anxiety, depressed mood and worry, and an inability to cope (World Health Organization (WHO), 1993). The main distinction between AjD and other mental disorders is the exposure to a specific external stressor which triggers the symptomology and significantly impacts the individual's ability to cope (Carta et al., 2009;Casey, 2009;Kazlauskas et al., 2018;Reid, 2018). The exact nature of the external stressor can vary from a serious accident (K€ uhn et al., 2006), to a bereavement (Greis, 2012), financial issues or a divorce (Glaesmer et al., 2015). Much like PTSD, individuals are aware of the stressor, which must be identified before making a diagnosis (Casey, 2009). However, there are differences which separate PTSD from AjD . The stressful event for PTSD can be unexpected, traumatic, stressful or life-threatening event where the individual may have episodes of reliving the event (i.e. flashbacks) (World Health Organization (WHO), 1993). In comparison, for AjD the individual's response to a significant life event needs to exceed a "normal" reaction if others were experiencing the same event (Gradus et al., 2010) and symptoms can be directly linked to the event (World Health Organization (WHO), 1993).
Despite AjD's prevalence within the AF population, little research has been conducted into the disorder. This contrasts the substantial body of literature exploring risk factors, outcomes, and treatment for other mental disorders such as depression and PTSD. Research beyond the UK AF has been conducted to explore potential AjD risk factors, however, all report mixed findings (Chen et al., 2011;ex-serving, new recruits, conscripts) and separately examined the outcomes for veterans and AF personnel. There are discrepancies between the definition of a military veteran across different countries. In the United States (US), a veteran is defined as an AF personnel who has been deployed at least once, whereas in the UK a veteran is defined as an ex-serving Armed Forces personnel who served in any military Service as a regular or reserve for at least one day. Where applicable odds ratios (OR), adjusted odds ratios (AOR) or unadjusted odds ratio (UOR) and 95% confidence intervals (CI) were extracted for AjD risk and protective factors.

Study sample
Eighty-three papers published between 1984 and 2020 were included in the final analysis ( Figure 1). Of the studies included, the majority (fifty-four studies) were conducted in the United States (US). Other study locations included the United Kingdom (UK), Turkey, Taiwan, Croatia, Denmark, Finland, Germany, Greece, Israel, Korea, Sweden, and Sri Lanka. Sixty-seven studies used a cohort or observational design, and sixteen studies used a case-control design.
Fifty-three studies examined current AF personnel across military Services (Army, Navy, Air Force, Marines and Coast Guard) ( Table 1). Thirteen of these studies included personnel from two or more Service branches, twenty studies used only Army personnel in the sample and three studies were comprised of Air Force personnel only, the remaining studies did not specify which Service branch personnel served in. Thirty-one studies included both males and females in the study sample, with the frequency of male participants in each sample varying from 55% to 99%. Nineteen studies had a male-only sample, one study had a female-only sample, and two studies did not report the gender of participants. Three of the studies recruited only AF   (Elonheimo et al., 2007;Gale et al., 2010;Ristkari et al., 2006). Thirty of the studies included in the review sampled veteran participants, and all employed a cohort design ( Table 2). As all thirty veteran studies were conducted in the US, the US definition of veteran will be employed. Most veteran studies included a study sample with both males and females (N ¼ 24), although most participants in these studies were male (80-98%). Three studies included female veterans only and one study examined a male-only sample. All data for veteran studies were extracted from the US Veteran's Affairs (VA) database.

Adjustment disorder prevalence estimates
Forty-one of the 83 studies included in this review reported on the prevalence of AjD. As most studies were conducted in the US (N ¼ 30), the AjD prevalence estimates reported in this review predominantly reflect the US military population. Over half of the studies included in this review reported on participants who were serving members of the AF at the time of data collection (n ¼ 27).
Of the studies included in this review on AF personnel that reported AjD estimates (n ¼ 27), twenty-two reported AjD as either the most reported, or second most reported mental disorder for AF personnel compared to other commonly assessed mental disorders such as depression, anxiety, and PTSD. Only six studies reported AjD prevalence in a general AF population (including healthy AF personnel), with AjD prevalence ranging from 0.7 to 16.8% (M ¼ 7.4%). In comparison, where AF personnel all had a mental disorder (n ¼ 19), a mean prevalence of 34.9% was reported. Two studies reported AjD with depressed mood (Giotakos & Konstantakopoulos, 2002) and AjD with mixed disturbance of emotion and conduct (George et al., 2019) as the most common AjD subtypes.
For studies where the sample consisted of veterans with a mental disorder (n ¼ 8), AjD prevalence ranged from 4.3 to 34% (M ¼ 12.8%).

Risk factors and protective factors of adjustment disorder
Sixteen studies discussed risk and protective factors (n ¼ 16). Most of these studies (n ¼ 13) used an AF personnel sample, with four studies using veteran samples. It should be noted that there was a lack of homogeneity between studies of the risk factors investigated. Childhood adversity (n ¼ 2) and personality traits (n ¼ 2) were examined as pre-enlistment risk factors associated with AjD in later life. Specifically, increased parental abuse (Giotakos & Konstantakopoulos, 2002) were conducted in Taiwan where AF personnel were conscripts. Conscripted personnel have limited comparability to the UK AF which is voluntary 1 as military experiences and length of time served can differ. Haskell et al. (2011) reported being female increased the risk of developing AjD (unadjusted OR: 1.19, 95% CI: 1.10-1.28) (adjusted OR: 1.24, 95% CI: 1.14-1.34). Larson et al. (2011) found that Army personnel were significantly more at risk of developing AjD, and Jones et al. (2010) identified AjD as the most common mental disorder amongst deployed Army personnel.
Protective AjD factors were discussed (n ¼ 4) including a higher level of education and increased IQ 2 (Gale et al., 2010), and AF personnel not deployed (Eick-Cost et al., 2017). Shrestha et al. (2018) found that increased optimism, coping, adaptability, positive affect, spirituality, and life meaning, minimal catastrophic thinking, and reduced loneliness significantly reduced the likelihood of AjD or reduced the severity of AjD symptoms (Shrestha et al., 2018). Lindstrom et al. (2006) found a negative association between combat support roles and adjustment disorder for female AF personnel compared to other military roles. However, the study does not clearly state which regiments were categorised as combat support roles and other occupational roles.

Adjustment disorder symptoms
Eight studies discussed symptoms of AjD. Two of the eight studies reported that most AF personnel experience moderate AjD symptoms (Doruk et al., 2008;Peterson et al., 2018). Other symptoms for AjD described by studies included dysphoria (Baker & Miller, 1991), non-replicative nightmares 3 (Freese et al., 2018) and somatic complaints (Ristkari et al., 2006). One study reported a significant increase in harm avoidance, and reduced self-directedness, cooperativeness, self-transcendence compared to healthy controls (Na et al., 2012). Oh et al. (2018) found AF personnel with AjD had significantly higher scores on the Beck Depression Inventory (BDI), The State-Trait Anxiety Inventory (STAI-T/S), Symptom Checklist-90 (SCL-90-R), and the Stress-Response Inventory (SRI) when compared to healthy controls. 4 The study also found significant increased physical measures of stress (e.g., skin temperature), in comparison to healthy controls when performing a stress task. Giotakos and Konstantakopoulos (2002) found from the SCL-90-R symptom checklist that somatization, obsessive compulsive, interpersonal sensitivity, depression, anxiety, hostility, phobic, anxiety and paranoid ideation all significantly were increased for AjD patients compared to healthy controls.

Comorbidities
Twenty-eight studies commented on AjD comorbidities. Of the veteran studies (n ¼ 10), most commented on AjD comorbid with PTSD. When compared to other disorders, AjD had lower PTSD comorbidity (5.6%) when compared AjD risk factors: For women, adjustment disorder was most common for veteran Native Americans or Alaska Native (12.3%) and African Americans (12.6%).
For men, adjustment disorder was most common for veteran African Americans (11.7%).

Good
Krishnan et al. Good to depression (12.7%) and anxiety (14.9%) (Smith et al., 2016). Similar results were found in two other studies (Cohen et al., 2009;O'Donovan et al., 2015). Hefner and Rosenheck (2019) reported a significantly higher risk for veterans to be diagnosed with AjD, if diagnosed with PTSD and two other mental disorders compared to PTSD alone (RR: 2.27). Babson et al. (2018) found that significantly more insomniac case participants had an AjD compared to participants that did not report insomnia. For AF personnel, Walter et al. (2018) reported that AjD was the second most common mental disorder comorbid with PTSD (37%) with most common being depression (49%). Similar findings were found for AF personnel where comorbid PTSD and AjD were reported more frequently than AjD alone (Kozari c-Kova ci c & Kocijan-Hercigonja, 2001) and personnel were significantly more likely to receive an AjD diagnosis if also diagnosed with PTSD.
Substance misuse was often identified as comorbid with AjD (n ¼ 6); however, no studies reported significant associations. Ho and Rosenheck (2018) did report an increased risk of being diagnosed with AjD for veterans if diagnosed with both cancer and a substance use disorder compared to just cancer alone (RR: 3.16). Doruk et al. (2008) found similar results, reporting that 33.8% of AF personnel with AjD reported a history of substance misuse compared to controls (4.3%) (p < 0.0001). Doruk et al. (2008) also reported more significant life events were experienced by AjD personnel with a history of substance misuse. Links between substance misuse, AjD and significant life events may be suggestive of harmful behaviours used as a coping mechanism for a precursor stressor.
Suicidal ideation and suicide attempts were discussed in relation to AjD (n ¼ 9) in AF personnel only. Seven studies reported a significant increase of suicidal ideation or attempts for AjD AF personnel compared to healthy controls, with one study reporting a risk ratio of 4.70 (95% CI: 3.50-6.20) (Bachynski et al., 2012). The remaining two studies found that suicide attempts were significantly associated with AjD (George et al., 2019;Kochanski-Ruscio et al., 2014). Kochanski-Ruscio et al. (2014) study found over half of personnel with AjD (57%) had attempted suicide at least once and 48% had multiple attempts. Na et al. (2013) found that significantly more personnel with AjD who had a history of suicidal ideation scored for alexithymia (an inability to identify one's emotions) compared to personnel with AjD but without a history of suicide attempts and healthy controls.

Effect of adjustment disorder on military attrition
Six studies examined AjD and personnel attrition in the military, with most studies (n ¼ 5) reporting higher rates of discharge or recommendation for separation from Service for AF personnel with AjD when compared to other mental disorders (Cigrang et al., 1998;Englert et al., 2003;Hansen-Schwartz et al., 2005;Jones et al., 2009;Niebuhr et al., 2006). Studies investigating early career personnel (i.e., basic trainees and conscripts) had higher discharge because of AjD (Cigrang et al., 1998;Englert et al., 2003;Hansen-Schwartz et al., 2005;Niebuhr et al., 2006). However, Britt et al. (2018) found that AF personnel diagnosed with AjD were significantly more likely to be given a waiver to still be able to join the military compared to any other mental disorders.

Adjustment disorder treatment
Over half of the studies discussing help-seeking for treatment and treatment of AjD (n ¼ 20) used veteran samples (n ¼ 13). AjD had the strongest significant association with mental health outpatient care (Maguen et al., 2012) and an increased likelihood of help-seeking through veteran programmes when compared to other disorders (Etingen et al., 2019;Hoff & Rosenheck, 2000). Smith et al. (2016) found that veterans received a diagnosis more frequently and quicker if AjD was comorbid with PTSD. Gerson et al. (2004) reported the typical AjD duration for veterans as three months from stressor to diagnosis, and of patients receiving care at medical units, AjD was the most frequently diagnosed mental disorder after discharge.
In terms of treatment pathways, psychotherapy or counselling were the most frequently reported treatment options for AjD (Funderburk et al., 2011;Schmitz et al., 2012), behavioural modification training (e.g. substance abuse class, coping skills training) or a combination of psychotherapy and psychotropic medication (Schmitz et al., 2012;Smith et al., 2016). No studies investigated the effectiveness of AjD treatment in a AF population.

Adjustment disorder prevalence estimates
From the review, the overall AjD prevalence estimate for AjD in both AF personnel and veterans is comparable, whilst there are notable differences in AjD prevalence estimates for those with a recognised mental disorder (AF personnel studies 35.7 vs. 12.8% for veterans).
Increased prevalence estimates of AjD amongst diagnosed AF personnel compared to veterans may suggest "stressors" that precursor AjD are more common in and specific to military life. Since experiences of military stressors are collectively shared amongst AF personnel (e.g., deployments), and stressors may be more persistent and ongoing (e.g., continuous deployment posts), meaning symptoms of AjD are slower to be relieved and more accepted as the "norm". Although, most studies included in the review were conducted in the US and relevance to the UK AF population, and specifically the stressors experienced, are limited as military experiences can differ between the UK and the US (Sundin et al., 2014).
However, this does not explain why differences in AjD prevalence estimates between AF personnel and veterans are only apparent amongst those with mental disorders, and not within the wider community. It could be argued that higher AjD estimates are reported for AF personnel with mental disorders as AjD is often described as a less "severe" disorder compared to other mental disorders. For instance, the DSM-5 diagnostic criterion outlines that an AjD diagnosis can only be given once all other potential mental disorders are screened out (American Psychiatric Association, 2013). Perhaps AjD diagnoses may not be as accurate as first thought. An AjD diagnosis may act as a "last case" diagnosis to offer, which in turn, increases prevalence. This is further supported by the results reported by Britt et al. (2018) where personnel were more likely to receive a waiver for AjD compared to any other mental disorder, suggesting personnel are less likely to be discharged and retained in the military more easily. However, the restructuring of the diagnostic criterion for AjD in the new ICD-11 which will bring AjD more in line with other disorders such as depression and PTSD, may counteract this argument if prevalence estimates remain relatively similar.

Predictive factors of adjustment disorder
Pre-enlistment risk factors for AF personnel with AjD were discussed including personality traits and experiences of childhood adversity, specifically parental abuse, and neglect (Chen et al, 2011;For-wey et al., 2002;Giotakos & Konstantakopoulos, 2002). Childhood adversity is often associated with a multitude of mental disorders in AF personnel (Montgomery et al., 2013) including PTSD (Ozer et al., 2003), psychosis (R€ ossler et al., 2014), and anxiety (Sareen et al., 2013). Applewhite et al. (2016) found that 85% of AF personnel help-seeking for a mental health issue reported at least one account of childhood adversity, with 40% reporting four or more accounts. However, links between childhood adversity and AjD require further exploration to unpick which features (e.g., abuse, neglect) put someone significantly more at risk later in adulthood. McLaughlin et al. (2010) reported an increased risk for depression among individuals who had three or more accounts of childhood adversity and experienced a stressful life event in the past year (27.3%) compared to those without childhood adversity (14.8%). However, this study did not include explorations of association between AjD, childhood adversity and stressful life events, considering the importance of a stressful event in the AjD diagnostic criterion.
Despite the paucity of studies reporting military risk factors compared to non-military risk factors, the findings cannot be disregarded. Several studies highlighted that basic trainees or conscripts with AjD were often recommended for separation of duty or discharged (Cigrang et al., 1998;Englert et al., 2003;Hansen-Schwartz et al., 2005;Niebuhr et al., 2006). This suggests a potential association between new recruits, enlisting and AjD. However, research exploring new recruits and AjD is scarce and is recommended to explore which experiences specifically increase the risk of AjD for new recruits. The breadth of the review topic meant studies which investigated predictive factors of AjD contained a multitude of methodological practices, sample sizes and aims. As a result, a meta-analysis approach could not be employed which would provide a more scientific comparison of predictive factors for AjD between studies.

Suicidal ideation
Significant associations with suicidal ideation were discussed. Suicidal ideation is often described in general population studies as a common symptom for AjD (Carta et al., 2009;Fielden, 2012;Gradus et al., 2010). There is a surprising lack of inclusion of AjD in research which examines mental disorders and suicidal ideation in the general population and AF population, although some studies suggest that suicidal ideation can be as common in AjD as it is in depression (Polyakova et al., 1998). Despite suicidal ideation reoccurring in literature included in this review, current diagnostic manuals (ICD-10 and DSM-5) do not identify suicidal ideation as a symptom of AjD. It is clear the association between suicidal ideation and AjD requires serious consideration and research focus to emphasise suicidal risk as part of the AjD diagnostic criterion for clinical practice. Including suicidal risk in the AjD diagnostic description will not only help clinicians identify those at greater risk but also the correct treatment pathway. However, this could have significant implications on personnel attrition and increased military discharge for AjD personnel with suicidal ideation. Occupational outcomes such as this may be perceived as negative, but AjD patients would have the opportunity to recover away from military duties, resulting in a quicker recovery if the AjD precursor stressor is specific to military experience. It is recommended that future research lends itself to understanding the link between AjD and suicidal ideation and the impacts this may have on treatment options, recovery, and career outcomes.

Treatment
Treatment for AjD has previously been highlighted as an under-researched area for the general population (Carta et al., 2009) and even more so for AF population as highlighted in this review. Psychotherapy and self-help tools are the most frequently given treatment for AjD (O'Donnell et al., 2018(O'Donnell et al., , 2019. Across all studies in the current review, no specific treatment stood out as most used and this reflects the lack of standardised clinical practice for AjD. For example, in the UK there is currently no guidance given by NICE to advise clinicians on AjD treatment. Research is needed to develop an evidence-based treatment framework for AjD and to contribute to the development of NICE guidelines. Little is known about the effectiveness of AjD treatment and prognosis for AF populations. The reduction in evidence-based AjD treatment research could be explained due to the disorder's relatively "short" symptom duration typically lasting up to six months (World Health Organization (WHO), 1993). Capturing participants when experiencing AjD symptoms provides a relatively short window, increasing research difficulty. Forming evidence-based guidelines for AjD treatment will help inform military healthcare policies to guarantee that patients are provided with the most effective care. This will benefit the patient as well as the military as an institution by preventing money and resources being wasted on AjD treatment for which the evidence on effectiveness is lacking. By guaranteeing the same standards of care and attention are provided to AjD as they are to other mental disorders, AjD awareness may increase and encourage help-seeking for AF personnel who experience AjD symptoms. Furthermore, without research into AjD prognosis, the implications of being diagnosed with AjD on clinical and career outcomes are still unknown. Future explorative research should follow AF personnel with AjD overtime to understand what implications AjD may have on career progression, recovery, and future mental disorder occurrence.

Conclusion
The systematic review aimed to summarise existing AjD research for AF populations, to outline clinical or career implications that AjD may have, and to identify areas needed for future exploration. Overall, AjD presented as a common mental disorder for AF populations across nations, and specifically for active AF personnel. Mixed risk and protective factors for AjD were identified, however, further investigations into military-specific risk factors is warranted as this research area was limited. Similarly, little research attention was afforded to exploring the implications AjD may have on clinical or career outcomes and the effectiveness of treatment, nevertheless the review findings suggest associations between AjD and increased military attrition which should be explored further.
Notes 1. Conscripts are often of a younger age demographic, and this is reflected in the participant samples for both studies as the mean age was 22 years, compared to the average age of UK military personnel being 31 years (Ministry of Defence, 2020). 2. Gale et al. (2010) reported that for each one-point decrease on the nine-point IQ scale, there was an increased risk of hospital admission for AjD (HR: 1.75, CI: 1.70-1.80). 3. Freese et al. (2018) describe non-replicative nightmares, for the specific purpose of their study, as a nightmare that is not an exact replica of a traumatic event the patient has experienced but the nightmare is symbolic of a traumatic event and has associations with their diagnosis. 4. According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) and the International Classification of Diseases (ICD-10), adjustment disorder often mirrors symptoms of other mental disorders such as depression and anxiety and symptoms for adjustment disorder include depressed mood, anxiety, and worry.

Disclosure statement
The views expressed are those of the author(s) and not necessarily those of the NIHR, Public Health England or the Department of Health and Social Care. No potential conflict of interest was reported by the author(s).

Funding
This work was supported by The Colt Foundation under [Grant RE17245]. This review is funded by The Colt Foundation. The views expressed are those of the author(s) and not necessarily those of The