As the folks at Pure Pedantry point out, the discovery that stress precedes volume reductions in the hippocampus in PTSD is a significant insight and settles a long-running debate: Do stress and depression shrink the hippocampus (a brain area vital to learning, memory, and navigation), or does a small hippocampus make you vulnerable to stress and depresison?
We've known for a while (courtesy of research by Yvette Sheline and others) that people who've been repeatedliy or severely depressed have smaller hippocampi. But it wasn't clear which was chicken and which egg. This new study shows that the stress causes the shrinkage.
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One thing that Yvette Sheline's study does not say is whether the depressed people with smaller hippocampi had been treated or not. They were reported as being "repeatedly and severely depressed" so one might assume they have been medicated; it's important to consider that there is at least a possibility the shrinkage is treatment related or exacerbated. It is important not to fall into a trap of thinking these morphological changes in the brain are always disease based; they could be iatrogenic.
About the question of whether the shrinkage occurs first. Yes.
And about the question of whether shrinkage is the result of medication(s). No. Or rather, it can be that way too.
This is one of the many questions which are impenetrably hard, when we don't know the answer, and teeth-grittingly simple when we do.