Hippocampal volume and depression: insights from epilepsy surgery

Background: Major depression is common after epilepsy surgery. It has previously been suggested that surgical removal of limbic system structures such as the hippocampus may contribute to this comorbidity. Recent magnetic resonance imaging studies have found smaller hippocampal volumes in depressed patients in comparison with controls. Aims: The current study examined whether preoperative hippocampal volumes were associated with depression experienced after epilepsy surgery. Patients undergoing mesial (n = 26) and non-mesial (n = 16) temporal lobe resections were assessed preoperatively, and for 1 year postoperatively. Assessment included a clinical interview and the Beck Depression Inventory. Hippocampal volumes were measured on the preoperative T1-weighted magnetic resonance imaging scans of the patients and 41 neurologically normal controls. Results: A similar proportion of mesial and non-mesial temporal patients had a preoperative history of major depression. Postoperatively, 42% of mesial and 19% of non-mesial temporal patients were depressed. There was no relationship between hippocampal volume and preoperative depression in either group. Depression after surgery was associated with significantly smaller hippocampal volumes contralateral to the resection in the mesial temporal group (p = 0.005). This effect was seen in mesial temporal patients who developed de novo depression (p = 0.006). Hippocampal volume was unrelated to postoperative depression in the non-mesial group. Conclusion: This study highlights the role of neurobiological factors in the development of postoperative depression. These initial findings have implications for understanding depression following epilepsy surgery as well as the pathogenesis of depression more generally.