Epidemiology of Major Depressive Disorder

Major depressive disorder (MDD) is a serious health problem and will be the second leading cause of burden of disease worldwide by 2030. To be able to prevent MDD, insight into risk factors for the onset of MDD is of clear importance. On the other hand, if onset of MDD has occurred, one may argue that different course patterns of MDD can be identified and that it is essential to examine their relationship to symptoms and function over time. Insight into these course patterns could assist in preventive strategies and management of MDD. The first aim of this thesis was to examine the differential impact of risk factors for the onset of MDD across groups at risk and to investigate to what extent our findings accord with the vulnerability-stress model for MDD. The second aim was to examine the natural course and outcome of MDD. The first main topic of this thesis includes the influence of risk factors on the onset of MDD. The first noteworthy finding is that several risk factors can be identified that may help to predict onset of different types of MDE. These factors are relatively easily assessable and may assist in preventive strategies. The second noteworthy finding is that the majority of risk factors had a greater impact in women than in men on the risk of onset of MDD and were not restricted to a specific class of risk factors. These findings may partly account for the observed difference in incidence of MDD between men and women. Third, recent life events carry the largest risk of onset of MDD in mid-life. Understanding the different vulnerability to life events according to age may help to indicate groups at a particular risk and assist in preventive strategies. In the present thesis we showed that several vulnerability factors and stress factors not only work in an additive way but also interact, which accords with the interactive vulnerability variant of the vulnerability-stress model. The second main topic comprises the course and outcome of MDD. The first noteworthy finding is that we found that the majority of patients diagnosed with MDD have a chronic or intermittent course. Chronic courses are associated with higher levels of depressive symptoms and somatic symptoms and greater mental dysfunction at baseline. The second noteworthy finding is that depression and anxiety together or alone had a lower level of physical function at baseline than those with no diagnosis. Physical function may increase over time, but the rate of increase may not be different between the diagnostic groups. On the other hand, lower levels of physical function may lead to onset of depression and anxiety over time. Therefore, it is essential to prevent lower levels of physical function, and to prevent onset of both depression and anxiety. The third noteworthy finding is that a minority of patients with MDD is recognised in primary care. Those who are unrecognised have comparable outcome after 12 and 39 months as participants with recognised depression.