These data replicate and extend earlier findings validating preschool depression from an independent study sample (Luby et al., 2002
; Luby et al., 2003a
). The rates of co-morbidity found are similar to those reported in depressed school age children (Angold and Costello, 1993
). Findings of impairment in functioning specifically associated with depression in multiple domains and contexts, rated by both parents and teachers, underscores the clinical significance of this early onset syndrome. The fact that impairment was detected using several independent measures emphasizes the robust nature of this finding. Further, the finding of functional impairment without significant delays in basic development on the Vineland demonstrates that depressed and impaired preschoolers are not yet developmentally delayed, suggesting a window of opportunity for early intervention.
Odds ratios derived using MLR addressed the specific risk of each depressive symptom for MDD versus other disorders. Findings demonstrated that all symptoms of depression may serve as clinical markers in the general population, as their occurrence was associated with a four to twenty-eight times greater likelihood of being depressed compared to being healthy. Key symptoms that differentiated depressed from disruptive preschoolers were sleep problems, guilt, weight changes, anhedonia and diminished cognitive abilities. Key symptoms that differentiated depressed from anxious preschoolers were guilt, diminished cognitive abilities, psychomotor agitation and weight changes. Of particular note, extreme fatigue and guilt were highly specific to the depressed group when anxious and disruptive co-morbidities were controlled and therefore may be useful in clinical settings as pathognomonic markers of depression (see ). As expected, the symptom of irritability was not useful to differentiate depression from other disorders. However, unexpected was the finding that sadness was also a relatively non-specific marker of depression compared to other disorders. In addition, anhedonia, while uncommon in anxiety disorders, did not emerge as a marker that could differentiate depression from anxiety.
A melancholic depressed sub-group, characterized by anhedonia, displayed the highest depression severity. This emerged within a statistically significant hierarchy of depression severity when depressed sub-groups were also compared to other disorders. This replicates earlier findings from a smaller independent sample (Luby et al., 2004a
). The significant differences in depression severity between the depressed and anxiety groups further suggest that this symptom constellation represents a unique mood disorder and not simply more general internalizing phenomena. This stands in contrast to the hypothesis that young children would display more undifferentiated internalizing disorders rather that discrete DSM-IV disorders (Achenbach, 1995
; Cole, 1997
; Keenan and Wakschlag, 2002
Findings from this study conducted in a screened sample of preschool children from community sites replicate and extend earlier findings from an independent study sample validating a preschool depressive disorder. Further study in a representative, population-based sample is needed to derive prevalence rates for preschool depression. Early indications that the pre-valence of preschool depression (2%) is comparable to that found in older children have been provided from a representative community sample (Egger and Angold, 2006
). Reliance on diagnosis and ratings of impairment based on caregiver report is a limitation of the data presented. Investigations of performance based and/or objective observational measures of functioning are now needed. Another limitation was that the PAPA was not designed to assess BP-II or more subtle symptoms of bipolarity, thus we cannot rule out the possibility that additional subjects in the sample may have also had mixed depression or more subtle bipolar spectrum symptoms (Akiskal, 1995
; Disalver et al., 2005
These findings provide further validation for depression in preschoolers. The clinical significance of this early onset disorder is underscored by the finding of impairment in functioning associated with depression in numerous domains. Further, the finding of symptoms and impairments evident across contexts in a young child has been shown to support clinical significance based on longitudinal data in disruptive disorders (Campbell, 2002
). The symptoms of guilt and extreme fatigue emerged as highly specific markers of depression strongly differentiating depressed preschoolers from those with anxiety and disruptive disorders and thus potentially useful as clinical markers. Replication of these findings at independent research sites is a critical next scientific step.
The identification of depression during the preschool period, even as early as 3 years of age could have important public health significance beyond its obvious implications for relieving the suffering of preschoolers and their families. The earliest possible identification and intervention in mental disorders during this period of rapid developmental and neurobiological change may represent a window of opportunity for more effective treatment. While this remains an empirically unexplored issue in the area of depression, the unique efficacy of earlier intervention has been established in other early onset psychiatric disorders (Boggs et al., 2004
; Dawson et al., 2000
; Eyberg et al., 2001
; Faja and Dawson, 2006
; Hood and Eyberg, 2003
; Webster-Stratton and Reid, 2003
). The possibility that earlier intervention could change the trajectory of this chronic and relapsing disorder remains an exciting possibility. Validation of preschool depression and the demonstration of significant associated impairment now provide the necessary evidence for future public attention to this early onset disorder and related testing of early intervention strategies.