Background: Longitudinal, population-based, research is important if we are to better characterize the lifetime patterns and determinants of affective disorders. While studies of this type are becoming increasingly prevalent, there has been little discussion about the limitations of the methods commonly used.
Methods: Discussion paper including a brief review of key prospective population-based studies as the basis for a critical appraisal of current approaches.
Results: We identified a number of common methodological weaknesses that restrict the potential of longitudinal research to characterize the diversity, prognosis, and determinants of affective disorders over time. Most studies using comprehensive diagnostic instruments have either been of relatively brief duration, or have suffered from long periods between waves. Most etiologic research has focused on first onset diagnoses, although these may be relatively uncommon after early adulthood and the burden of mental disorders falls more heavily on individuals with recurring disorders. Analysis has tended to be based on changes in diagnostic status rather than anges in symptom levels, limiting study power. Diagnoses have generally been treated as homogeneous entities and few studies have explored whether diagnostic subtypes such as atypical depression vary in their etiology or prognosis. Little research has considered whether there are distinct trajectories of symptoms over time and most has focused on individual disorders such as depression, rather than considering the relationship over time between symptoms of different affective disorders. There has also been limited longitudinal research on factors in the physical or social environment that may influence the onset, recurrence or chronicity of symptoms.
Conclusion: Many important, and in some respects quite basic, questions remain about the trajectory of depression and anxiety disorders over the life course and the factors that influence their incidence, recurrence and prognosis. Innovative approaches that consider symptoms of all affective disorders, and how these change over time, has the potential to greatly increase our understanding of the heterogeneity of these important conditions and of the individual and environmental characteristics that influence their life course. Using longitudinal research to define sub classes of affective disorders may also be of great benefit for studies seeking to define the genetic determinants of susceptibility to these conditions.