The principal finding of this study is a smaller left hippocampal volume (by 17 %) in early onset adolescent depression as compared to age- and sex-matched healthy controls. In past studies that reported positive findings regarding decreased hippocampal volumes in depressed patients, a decrease of similar magnitude was noted (8 – 19 %) [4
]. Similar to another study, the smaller hippocampal volume was more prominent in males than females [11
]. Akin to the trend noted by MacMillan and colleagues [24
], the difference was also more substantial on the left than the right. Hippocampal size did not correlate with age in either group. However, hippocampal size was correlated negatively with age of onset in the depressed subjects. This is consistent with Steffens and others' [8
] finding of smaller hippocampal size associated with older age of onset. Of special interest is that we noted a positive correlation with duration of illness and hippocampal size. This may explain why younger adult studies of depression (as opposed to middle aged and elderly samples) have not shown a decrease in hippocampal size [25
], as the difference may be lost during early adulthood. However, this should be considered speculative and the correlation should be replicated. As these patients were all early onset cases, we cannot speculate about the effects of number of episodes on hippocampal volume.
It is a commonly held hypothesis that cell loss related to depression in the hippocampus involves glucocorticoids. Glucocorticoid hypersecretion can lead to cell death in the hippocampus and may be responsible for the cell loss noted in this region in a number of neuropsychiatric disorders [26
]. Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis is a relatively consistent finding in adults with MDD, with hypercortisolemia being prominent [27
]. This is state-dependent and tends to resolve with recovery. Hence, the relationship between the number of days depressed and atrophy in the hippocampus noted by Sheline et al.
] is logical, as excessive glucocorticoids can lead to increased cell death [26
]. In adolescents with depression, the evidence of cortisol abnormalities is much less compelling. One study failed to note any differences in the summary cortisol measures [28
], while two studies noted an increase in cortisol after sleep onset in depressed adolescents [29
]. After sleep onset, cortisol is usually quiescent. It has been hypothesized that it reflects more of a variation in the diurnal variation of cortisol than a robust increase in overall cortisol activity [29
]. Mineralcorticoid (MR) receptors appear to be operative at low cortisol concentrations and may offer tonic inhibition to the HPA axis during the nadir of the circadian rhythm [31
]. As the diurnal activity of the HPA axis is disrupted in MDD youth, particularly when cortisol activity is at its lowest [29
], it may be indicative of altered MR function, with MRs being found in the amygdala and hippocampus. Classic cortisol hypersecretion is thought to be rare in depressed children and adolescents [33
]. This is also consistent with our lack of a negative correlation between duration of illness and left hippocampal volume in depression. However, the reasons behind the positive correlation are unclear and require replication.
As positive correlations have previously been noted between hippocampal volume and anxiety in depressed subjects and controls [24
], the fact that we did not control for this may represent a limitation of this study. No subject had a comorbid anxiety disorder, however, and unlike the depressed subjects with anxious symptoms used in the study by Rusch et al.
], our sample demonstrated differences in volume when comparing diagnostic groups. It is also worth noting that Vythilingam and colleagues [34
] did not find any differences in hippocampal volume in panic disorder, and hence anxiety may not have as profound an effect on hippocampal volume as one would assume.
To our knowledge, this is the first published report regarding hippocampal volume in youth with early onset depression compared to healthy controls. These conclusions should be considered preliminary, considering the small sample sizes used. Future directions include the examination of high-risk for depression subjects, larger sample sizes and longitudinal studies of illness course. The use of convergent measures such as tests of working memory along with the volumetric data would also be of value. Also, the evaluation of related limbic structures, such as the amygdala, using MRI is of keen interest.