Multiple studies have reported a link between reduced hippocampal volume and chronic post-traumatic stress disorder (PTSD) in some individuals. Other research suggests that a smaller hippocampus may be a pre-existing risk factor that increases the likelihood of developing or retaining PTSD symptoms after trauma.
A recent study indicates that, on average, people with larger hippocampal volumes are more likely to recover from PTSD than those with smaller hippocampi. Although further research is required to confirm the finding and clarify cause and effect, investigators are encouraged by the possibility that treatments which increase hippocampal volume could improve recovery for some patients. One therapeutic avenue under investigation is a short course of antidepressant medication: prior work reported that six months of antidepressant treatment led to measurable increases in hippocampal volume in a subset of people with PTSD.
Hippocampal volume and resilience in posttraumatic stress disorder
The hippocampus is a brain structure crucial for forming memories and evaluating contextual information about the environment—abilities that help determine whether a situation is safe or threatening. Because of these roles, the hippocampus has been a central focus in efforts to understand PTSD.
Early magnetic resonance imaging (MRI) studies observed decreased hippocampal volume in some individuals with chronic PTSD. Those results suggested that prolonged stress might produce atrophy in the hippocampus, a conclusion supported by animal research showing stress-related changes most prominently in the CA3 subregion of the hippocampus. Some human imaging work has likewise pointed to volume reductions localized to areas of the hippocampus that are particularly vulnerable to stress.
However, more recent studies have complicated that interpretation. For example, research including identical twins found that the non-traumatized twins of people with PTSD also had smaller hippocampal volumes. That pattern raises the possibility that a smaller hippocampus can be a pre-existing vulnerability that increases the risk of developing persistent PTSD after exposure to trauma. Mechanistically, a smaller hippocampus could reduce the ability to accurately interpret contextual cues or to regulate stress responses, including inhibitory control over hypothalamic circuits that modulate cortisol release.
The new study, published in Biological Psychiatry, examined hippocampal volume using structural MRI in Gulf War veterans who had recovered from PTSD, compared to veterans with chronic PTSD and control participants who had never had PTSD. The authors, led by Brigitte Apfel, reported that veterans who had recovered from PTSD had hippocampal volumes that were, on average, larger than those with chronic PTSD and comparable to veterans who had never had PTSD.
“These results need to be interpreted with caution because we did not measure brain changes over time,” Dr. Apfel noted. “However, the finding suggests that hippocampal damage associated with PTSD may be reversible once symptoms remit. If confirmed, this would shift part of the conceptual framework for PTSD treatment toward brain restoration as well as symptom reduction.”
Interpreting these findings still leaves open the question of directionality: does a larger hippocampus support recovery, or does recovery allow the hippocampus to return toward normal size? The study’s outcome is consistent with the hypothesis that a relatively small hippocampus is a risk factor for persistent PTSD, since individuals with larger hippocampi appeared more likely to recover. This view aligns with genetic and imaging research linking certain gene variants associated with emotional resilience to larger hippocampal volume. Alternatively, early-life adversity or other environmental exposures could produce smaller hippocampi and simultaneously reduce resilience, creating a complex interplay between biology and experience.
An important clinical question remains whether increases in hippocampal volume that accompany some treatments actually mediate clinical recovery. A previous study found that six months of antidepressant therapy increased hippocampal volume in people with PTSD, suggesting that pharmacological interventions might contribute to structural recovery. If treatment-induced hippocampal growth is causally related to improved symptoms, then targeting hippocampal health could become an explicit goal of PTSD therapy.
“It may be time to view hippocampal volume both as a modulator of stress resilience and as a target for the harmful effects of stress and the restorative effects of treatment,” commented Dr. John Krystal, Editor of Biological Psychiatry. “This more nuanced perspective could help explain how stress can create a ‘feed forward’ cycle that worsens responses to later stressors, while effective treatments might progressively build resilience.”
The study reinforces the need for longitudinal imaging studies that track hippocampal volume before and after trauma, and across treatment, to determine whether volume changes predict or follow clinical recovery. Such work will be essential to guide interventions aimed at promoting structural and functional brain recovery in people with PTSD.
Contact: Chris J. Pfister
Source: Elsevier