The burden of common mental disorders (CMDs) in the working population is high, not only because of the high prevalence of sickness absence due to CMDs, but also because of the high risk of recurrent sickness absence due to CMDs. The RD of sickness absence due to CMDs was 84.5 per 1,000 person-years. Recurrences occurred within 8–11 months (95% CI 6–14 months), depending on the initial diagnosis. In 90% of employees who had a recurrence, the recurrence occurred within 3 years.
The question is whether the results are transferable to other working conditions. The volume and length of follow-up period are as such that the relationships found are likely to be consistent. In this population, a large variation exists in mentally and physically strenuous jobs, which also gives an indication for reproducibility in other populations. We found clear relationships with age, gender and salary scale, and it is plausible that this pattern will also be found in other populations. On the other hand, the size of the relationships will presumably vary between companies. The fact that we found different recurrence densities in Telecom versus Post companies supports this hypothesis. It is recommended to reproduce this study in companies with different working conditions.
Determinants of recurrent sickness absence due to CMDs
The number of previous episodes and subclinical residual symptoms appears to be the most important predictors of recurrence of major depressive disorder (MDD). Gender, civil status and socioeconomic status seem not related to the recurrence of MDD (Burcusa and Iacono
2007; Hardeveld et al.
2010). We investigated the risk of recurrent sickness absence due to CMDs (same or another mental disorder) by gender, age, marital status and salary scale.
Sickness absence due to CMDs occurred more often in women, and this has been reported earlier (Bijl et al.
2002; Hensing and Wahlstrom
2004). Mueller et al. (
1999) reported that women had a higher recurrence of a major depressive disorder than men. It is interesting to note that this gender difference seems to disappear after an initial episode of sickness absence due to CMDs. This finding might be biased by the longer episodes of sickness absence found in women than in men (Blank et al.
2008), but this merits further investigation.
In men, depressive symptoms were related to higher recurrence of sickness absence due to CMDs than distress symptoms and adjustment disorders. Men are found to have shorter episodes of sickness absence due to depressive symptoms than women (Koopmans et al.
2008a). Possibly, men with depressive symptoms take less time than needed to recuperate before they start working again, which makes them more vulnerable to repeated episodes of sickness absence due to CMDs.
The RD of sickness absence due to CMDs decreased with age. This is in line with the finding that the incidence of sickness absence due to CMDs in the general population in the Netherlands is higher in employees aged 18–45 than in older employees (Bijl et al.
2002; Spijker et al.
2002). Younger employees might be less able to cope with stressful life events, compared to older employees (Diehl et al.
1996). However, Nieuwenhuijsen et al. (
2006) reported a negative association between recovery from mental disorders in employees over 50 years of age. Another explanation might be that younger employees have a lower threshold for sickness absence (Cant et al.
2001). The decrease in RD of sickness absence due to CMDs with age might be also due to differential loss to follow-up, because of early retirement or a disability pension for older employees. Another reason might be a longer duration of sickness absence due to CMDs or other causes in older employees, as several studies have found a longer duration of sickness absence in older employees (Allebeck and Mastekaasa
2004; Duijts et al.
2007). Also a healthy worker effect might explain the age difference, because employees who have suffered from CMDs are more at risk for disability or termination of employment (Koopmans et al.
2008b).
Married women had a higher risk of recurrence than single women, but this difference was not observed in men. Married women might be more vulnerable for CMDs because they combine their work with household and care tasks (Griffin et al.
2002). Mueller et al. (
1999) reported that “never married” was a significant predictor of recurrence of an episode of major depression. Lack of a relationship or social support might be a risk factor for the development of depression, and it is possible that social relationships and social support are more important for women than for men. For women, but not for men, dissatisfaction with private life and low social support from colleagues were predictors of long-lasting episodes of sickness absence due to depression (Godin et al.
2009). The lower rate of recurrence of sickness absence due to CMDs in unmarried women could be caused by the longer duration of absence in this group. However, the median duration of sickness absence due to CMDs was the same for married women as for unmarried women (67 days).
Men and women with a lower salary scale had a higher risk of recurrence of sickness absence due to CMDs than those with a higher salary scale. Salary scales reflect social status, and there is evidence of a socioeconomic gradient in CMDs, with a higher risk in the lowest socioeconomic status group (Muntaner et al.
2004). Our results add that in employees of low socioeconomic status, the risk of recurrent sickness absence due to CMDs is also higher than in employees of high socioeconomic status.
A strength of the present study is that we investigated medically certified diagnoses instead of self-reports from the employees, as in the Norwegian HUNT-study for example Mykletun et al. (
2006). However, we had no data on comorbidity, and we did not know whether the diagnoses changed over time. An employee can only be registered with one diagnosis for each episode of sickness absence. This is a common shortcoming in studies of sickness absence registers (Wahlstrom and Alexanderson
2004). Moreover, the validity of psychiatric diagnoses is a subject of ongoing debate. Employees with depressive or anxiety disorders often present somatoform complaints (Escobar et al.
1987; De Waal et al.
2004). As somatization (the presentation of physical symptoms instead of depressive symptoms or anxiety) is insufficiently recognized in primary care (Ormel et al.
1994), we expect that sickness absence due to CMDs in our sample underestimates the actual incidence of CMDs. Sickness certification by the occupational physicians was based either on the clinical diagnosis obtained from the treating physician (general practitioner or psychiatrist), or determined according to the occupational health guidelines (Van der Klink and van Dijk
2003). Our results may also be biased when occupational physicians were more aware of mental symptoms in a recurrent sickness absence due to CMDs.
It should be noted that the RD person-years are over-estimated, because we used the time from the start of the first episode of sickness absence due to CMD instead of the recovery date, whereas someone who is on sick leave is actually not at risk for recurrent sickness absence. The reason for this is that the start of a sickness absence episode is more reliable, because episodes of sickness absence can end due to several reasons: not only return to work, but also leaving employment, the end of the company’s contract with the occupational health service, and changes in the labour-contract.
Overestimation of the person-years at risk may have resulted in an underestimation of the risk of recurrence. The risk of recurrence may also have been underestimated because of the high turnover in the study population, as employees who were absent due to sickness are more likely to resign or to be discharged than those who have never reported sick. Furthermore, the risk of recurrent sickness absence due to depressive symptoms and anxiety may have been underestimated due to the longer duration of sickness absence.
Practical implications
In accordance with the Dutch guidelines (Van der Klink et al.
2007), we advise relapse prevention consultations for a period of 3 years after return to work. This could provide extra opportunities and time for treatment (e.g. cognitive behavioral treatment) and preventive actions (e.g. the reduction of stressors at the workplace or in private life). Occupational physicians should be trained not only to diagnose CMDs, but also to recognize factors associated with a recurrent course (Blier et al.
2007). Van der Klink et al. (
2003) reported that it is possible to influence the recurrence rate of sickness absence due to adjustment disorders. They found that the risk of recurrent sickness absence due to adjustment disorders was 20% lower in the graded activity intervention group than in the “care as usual” group.
Moreover, it would be interesting to develop a screening strategy for distress, depressive and anxiety symptoms and at-work performance deficits. This would make it possible to detect mental problems in an early subclinical stage and to intervene before they develop into disorders that result in sickness absence (Lerner and Henke
2008). Moreover, we recommend that more longitudinal studies should be carried out to investigate sickness absence due to CMDs, focusing on long-term sickness absence as well as recurrences and multiple episodes of sickness absence.