Seahorses are beautiful creatures. The biologists convince us that seahorses are fish, even if they don’t look anything like fish. They also tell us, intriguingly, that seahorses are monogamous and the males do the childbearing.
But why is a neurologist talking about seahorses. It’s all in the name. The Latin name for seahorse is hippocampus , derived from hippos for horse, and kampos for sea monster. Where biologists saw fish, the ancients saw monsters. And you really can’t blame them…take a closer look
It is no mystery why neuroanatomists name this important part of the brain after the seahorse, the resemblance is eerily striking.
Neurologists are passionate about the hippocampus for various reasons. In people with memory complaints, for example, hippocampal atrophy may predict the development of Alzheimer’s disease . A shrunken hippocampus is also seen in some forms of epilepsy. Neurologists therefore endlessly harangue their neuroradiology colleagues to look closely at their patients’ brain MRI scans, and to tell them that the hippocampus is shrunken…even if it’s just a little bit smaller. Unfortunately for the neuroradiologists, the MRI scans do not come colour-coded as in the illustrative scan below.
This blog post is however about major depression, and not about epilepsy or dementia. Depression, that bad feeling we all feel every now and then is frustrating, but major depression is devastating. And we now know that it is accompanied by major alterations in the structure of the brain. And, yes, the changes are in the hippocampus. I got interested in this subject when I came across a piece in Neurology News reporting that people with depression have a smaller hippocampus.
The association of depression with hippocampal atrophy is however an old one. Proceedings of the National Academy of Science (PNAS) reviewed the relationship in an editorial from 2011 titled Depression, antidepressants, and the shrinking hippocampus. The author addressed the unresolved puzzle…which of the two came first. Reminiscent of the chicken and egg scenario, it is not clear if the hippocampal atrophy causes depression, or vice versa. To add to the puzzle, the paper conjectured the possibility of a third, unknown agent, causing both the depression and the small hippocampus.
This question was the focus of a meta-analysis published in Molecular Psychiatry this year. It reviewed the brain imaging data of 15 studies, involving about 1700 people with major depression. Titled Subcortical brain alterations in major depressive disorder, the authors confirmed the link between depression and hippocampal atrophy, and also showed that the shrinkage is worse in those who developed depression at an early age, and in those who have had frequent episodes of depression.
Does depression lead to hippocampal atrophy? The meta-analyses hinted so, but there were too many caveats for the authors to arrive at a definitive conclusion. They admit that more needs to be done to unravel depression….leaving the mystery of the shrinking seahorses to continue to another day.