Chronic pain disrupts the reward circuitry in multiple sclerosis.

Pain commonly affects multiple sclerosis (MS) patients, and has the potential to become chronic and burden an already damaged central nervous system. Imaging studies are providing insights into brain restructuring associated with chronic pain and different chronic pain conditions seem to evoke distinct plasticity patterns. Our objective was to study the structural and functional brain changes of chronic neuropathic pain of MS. Employing structural and resting functional magnetic resonance imaging we compared MS patients with chronic central pain with MS patients without pain matched with respect to age, gender, subtype and duration of MS and disability. Mean duration of pain was 7.6 years. Comparing the pain and no-pain groups, brain functional default-mode network differences were found. There was decreased coactivation in the caudate nucleus and nucleus accumbens bilaterally. Also, for the relapsing-remitting subgroup of patients, grey matter thickness changes predominated in the pain group in the mesial region of the temporal lobes, caudate, putamen, thalami and the fronto-parietal cortex; in the group without pain, changes predominated in the frontopolar and orbitofrontal cortices and in the occipital areas. A dysfunction in the reward system in chronic pain of MS was found, particularly in the brain areas involved in its motivational aspects, as such probably reflecting the maladaptive physiology of chronic pain, and possibly the signature of pain in MS, in a disease where reward impairment seems to be already one of its features.