Gray-matter alterations in pain processing brain structures in patients with somatoform pain disorder – a voxel-based morphometric study

Objective: Somatoform pain disorder is defined as persistent and chronic pain at one or more sites in which psychological factors are thought to play a major role. This disease is characterized by severe pain without somatic pathologies which sufficiently explain the painful condition. We investigated whether subtle structural gray matter changes occur in this patient group which was thought to have no underlying structural pathology so far.

Methods: 14 right handed women (mean age 51yrs) fulfilling DSM-IV criteria for somatoform pain disorder were recruited from an interdisciplinary pain clinic. The screening for somatoform symptoms (SOMS) was completed with the SOMS-2 questionnaire. We only included patients with high affective pain scores as measured by the pain perception scale (PPS). Patients fulfilling diagnostic criteria of fibromyalgia were excluded from the study. The spectrum of other axis-I psychiatric disorders was assessed with a structural clinical interview (SCID-I) and with the Beck depression inventory (BDI). 25 healthy age-matched women (mean age 52yrs) were investigated as control group. Magnetic resonance imaging was performed with a 1,5 Tesla Siemens MR scanner and voxel-based morphometry (VBM) was performed with SPM2. In the VBM analysis we included age and BDI score as nuisance variable (covariate of no interest) to minimize the possible confounding effect due to age or depression comorbidity.

Results: In the patient group, we found gray matter to be significantly decreased (p<0,05 FDR corrected) in the anterior and posterior cingulate, the anterior insular and prefrontal cortex. These are brain regions that are known to be involved in pain processing and more specifically also in pain inhibition.

Conclusions: Widespread structural changes in gray matter density were found in brain regions which are thought to be key players in cortical pain inhibition, thereby interacting with subcortical regions such as the periaquaeductal gray matter. We therefore suggest that a malfunction of pain inhibiting circuitries due to structural pathology is one important factor in the pathophysiology of somatoform pain disorder.