[CNS processing of pain in functional somatic syndromes]

Abstract

Functional pain syndromes usually are characterized by a local or generalized increase in pain sensitivity, spontaneous ongoing pain, and a variety of other common symptoms. Classification or definition of a syndrome is usually somewhat arbitrarily based on the predominantly affected body region or a main symptom, resulting in significant overlap between conditions. Support for the involvement of the central nervous system (CNS) comes from the frequent comorbidity of affective disorders and symptomatology, disturbances in cognitive function, changes in neuroendocrine function, and dysregulation of the autonomic nervous system, although only subgroups of patients are affected. Findings of neuroimaging studies in functional pain syndromes typically reveal plausible correlates for the patients' subjective complaints and indicate a central augmentation in pain processing but also in other sensory systems including a postulated interoceptive system. The pathophysiology of this augmentation is not clear, and the importance of peripheral input remains unresolved, as well as the contribution of spinal and supraspinal mechanisms. Affective and cognitive factors clearly influence pain processing in these syndromes, but do not fully explain the observed augmentation. For FMS, chronic low back pain, and irritable bowel syndrome a dysfunction of the descending inhibitory systems is supported by these findings. However, longitudinal studies are needed to confirm the causality of the reported associations and to establish a primary role of the CNS in these syndromes. Current techniques like VBM and H-MRS have revealed potential yet highly variable structural abnormalities of the CNS in several of the syndromes, but clinical relevance and conclusions from these studies remain far from clear. New theoretical concepts should drive prospective and interdisciplinary research based on well-defined hypotheses to use the full potential of the current neuroimaging techniques.