1 Centre for Neuroscience Studies, Queen's University, Kingston, Canada.
2 Centre for Neuroscience Studies, Queen's University, Kingston, Canada; Department of Medicine, Queen's University, Kingston, Canada; Department of Biomedical and Molecular Sciences, Queen's University, Kingston, Canada; Department of Psychology, Queen's University, Kingston, Canada.
3 Centre for Neuroscience Studies, Queen's University, Kingston, Canada; Department of Psychology, Queen's University, Kingston, Canada; Department of Psychiatry, Queen's University, Kingston, Canada.
4 Centre for Neuroscience Studies, Queen's University, Kingston, Canada; Department of Medicine, Queen's University, Kingston, Canada; Department of Psychiatry, Queen's University, Kingston, Canada.
5 Department of Psychology, Queen's University, Kingston, Canada; Department of Psychology, Concordia University, Montréal, Canada; CHU Sainte-Justine Hospital Research Centre, University of Montréal, Montréal, Canada; Department of Psychiatry, McGill University, Montréal, Canada. Electronic address: linda.booij@concordia.ca

Background: Subthreshold depression is highly prevalent in adolescence, but compared to major depressive disorder, the clinical impact is under researched. The aim of this review was to compare subthreshold depression and major depressive disorder in adolescents by reviewing available literature on epidemiology, risk factors, illness trajectories, brain anatomy and function, genetics, and treatment response.

Methods: We conducted a scoping review of papers on subthreshold depression and major depressive disorder in adolescence published in English. Studies in adults were included when research in adolescence was not available.

Results: We found that individuals with subthreshold depression were similar to individuals with major depressive disorder in several regards, including female/male ratio, onset, functional impairment, comorbidity, health care utilization, suicidal ideation, genetic predisposition, brain alterations, and treatment response. Further, subthreshold depression is about two times more common than major depressive disorder.

Limitations: The definition of subthreshold depression is highly variable across studies. Adolescent-specific data are limited in the areas of neurobiology and treatment.

Conclusions: The findings of the current review support the idea that subthreshold depression is of clinical importance and provides evidence for a spectrum, versus categorical model, for depressive symptomatology. Given the frequency of subthreshold depression escalating to major depressive disorder, a greater recognition and awareness of the significance of subthreshold depression in research, clinical practice and policy-making may facilitate the development and application of early prevention and intervention.