Background. Posttraumatic stress symptoms (PTSS) are commonly experienced by people with chronic pain. Although PTSS and chronic pain are associated with similar effects on brain morphology, the present study aimed to clarify the relationship between chronic pain and PTSS on the brain. Methods. Fifty-two people with chronic pain and 38 pain-free healthy controls (HC) underwent T1-weighted magnetic resonance imaging. Severity of PTSS was measured using the civilian version of the posttraumatic stress disorder checklist (PCL-C). A series of multiple linear regressions determined the main effects of group, PTSS severity (PCL-C total score and symptom-specific scores) and their interaction on grey matter volume of selected regions-of-interest. Results. The interaction term was significantly associated with variations in grey matter volume in the left and right putamen, the left middle cingulate cortex (MCC) and the right posterior insula. Results showed significantly smaller left and right putamen when reporting higher PTSS levels, and significantly larger left MCC and right posterior insula at lower PTSS levels in people with chronic pain compared to HCs. In addition, increasing PTSS severity was significantly associated with larger left and right putamen in HCs, and significantly associated with smaller left MCC and right posterior insula, in people with chronic pain. Conclusions. Severity of PTSS moderated chronic pain-related grey matter alterations. More severe PTSS, especially avoidance, was associated with smaller left MCC, a core region of the pain matrix. The MCC is strongly linked with the somatosensory network, and critical for empathy, especially toward pain-related stimuli.

The authors have declared no competing interest.

This work was supported by a project grant from the National Health and Medical Research Council of Australia (ID1084240) and a Rebecca Cooper Fellowship from the Rebecca L. Cooper Medical Research Foundation awarded to S.M.G. N.N.-N. was supported by the Australian Government Research Training Program Scholarship (administered by the University of New South Wales) and a supplementary scholarship, and PhD Pearl Award administered by Neuroscience Research Australia.

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Human Research Ethics committees of the University of New South Wales (HC15206), the University of Sydney (HREC06287) and Northern Sydney Local Health District (1102-066M) gave ethical approval for this work.

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