Anyone living with chronic pain knows that it amounts to much more than an unpleasant bodily sensation. Fuzzy thinking, faulty memory, anxiety and depression often accompany long-term pain, suggesting that the condition is more of a whole-brain disorder than simply pain signaling gone haywire. New research from Northwestern University reveals a possible cause: an impaired hippocampus, a region critical for learning, memory and emotional processing.

Using anatomical brain scans, the researchers found that people suffering from chronic back pain or complex regional pain syndrome had a smaller hippocampus than healthy people. They then studied mice for further clues about how this region contributes to chronic pain's cognitive side effects. As reported April 25 in the Journal of Neuroscience, mice in chronic pain had trouble with a test of emotional learning, and they displayed greater anxietylike behaviors than normal mice. In the hippocampus, electrical and biochemical signaling was disrupted. Perhaps most striking was the mice's failure to produce new neurons in the hippocampus—one of the few brain areas where adult mice and humans can grow new neurons.

Lead researcher A. Vania Apkarian suspects that the hippocampal size difference seen in humans might reflect the lack of neuron growth and other problems seen in the mice. Without new neurons forming, memory and emotional processes would also become impaired. The work underscores the importance of treating “the suffering we associate with chronic pain” as a brain-based disorder, Apkarian says, in addition to trying to target its perceived source in the body.