Vasogenic edema versus neuroplasticity as neural correlates of hippocampal volume increase following electroconvulsive therapy.

Background: Volume increases of the hippocampus after electroconvulsive therapy (ECT) are a robust finding, pointing into the direction of neurogenesis. However, such volumetric increases could also be explained by edema and/or neuroplastic changes (such as angiogenesis).

Objectives: If edema explains the volume increase of the hippocampus we hypothesize it would lead to increased mean diffusivity (MD). If neuroplastic would explain the volume increase, it would lead to decreased MD. To investigate angiogenesis as explanation we studied the perfusion fraction f and the pseudodiffusion component D∗ obtained from intravoxel incoherent motion (IVIM) data, and relative perfusion changes obtained from arterial spin labelling (ASL) data.

Methods: Using ultra-high field (7 tesla) MRI we acquired IVIM and ASL data. We compared MD, f, D∗ and ASL values for both hippocampi in 21 patients (before and after 10 ECT sessions) and 8 healthy controls (without ECT) in a linear mixed model adjusting for age and gender.

Results: We found a significant decrease in MD (which was absent in the healthy controls) in the left and right hippocampus (t = -3.98, p < 0.001). In addition, a decrease in f (t = -4.61, p < 0.001, but not in controls) and no differences in D∗ or ASL perfusion values (both p > 0.05) were found.

Conclusions: The decrease in MD in perfusion fraction f suggest that formation of edema nor angiogenesis are responsible for the ECT-induced volume increases in the hippocampus. Also, it supports the hypothesis that hippocampal volume increases might be due to neuroplastic changes.