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Abnormality in the Self-monitoring Mechanism in Patients With Fibromyalgia and Somatoform Pain Disorder

From the Department of Anesthesiology, Pain Clinic (M.K., N.R.-M., A.G.), the Department of Biometrics (L.H.), and the Department of Clinical Psychiatry and Psychotherapy (M.B., T.P.), Hannover Medical School, Hannover, Germany.

Address correspondence and reprint requests to Priv.-Doz. Dr. Matthias Karst, Hannover Medical School, Department of Anesthesiology, Pain Clinic, D-30625 Hannover, Germany. E-mail: karst.matthias{at}mh-hannover.de

Background: Auditory hallucinations and passivity experiences are associated with an abnormality in the self-monitoring mechanism that normally allows us to distinguish self-produced from externally produced sensations. It is unclear if chronic central pain disorders such as fibromyalgia and somatoform pain disorders also involve a defect of the self-monitoring mechanism.

Methods: Responses to tactile stimulation were assessed in four groups of subjects (N = 40): patients with fibromyalgia, patients with somatoform pain disorder, patients with schizophrenia with auditory hallucinations and/or passivity experiences, and normal control subjects. The subjects were asked to rate the perception of a tactile sensation on their left and right hands. The tactile stimulation was either self-produced by movement of the subject's right or left hand or externally produced by the experimenter.

Results: Normal control subjects experienced self-produced stimuli as less intense than identical, externally produced tactile stimuli. In contrast, patients with fibromyalgia, patients with somatoform pain disorder, and patients with schizophrenia with auditory hallucinations and/or passivity experiences gave the same perceptual ratings for tactile stimuli produced by themselves as those produced by the experimenter (intergroup difference, p = .043; 95% confidence interval [CI], 0.16–0.68). Post hoc tests revealed that this significance was mainly caused by the fibromyalgia (p = .046; 95% CI, –1.66––0.13) and the somatoform pain disorder group (p = .033; 95% CI, –1.71––0.06).

Conclusions: We conclude that central pain disorders such as fibromyalgia and somatoform pain disorders interfere with the correct functioning of the self-monitoring mechanism that normally allows us to distinguish self-produced from externally produced tactile stimuli.

Key Words: fibromyalgia • somatoform pain disorder • schizophrenia • forward model • tickling

Abbreviations: FM = fibromyalgia; SPD = somatoform pain disorder; HP = hallucinations and/or passivity; NS = normal subjects; ACR = American College of Rheumatology; ICD = International Classification of Diseases; BPRS = Brief Psychiatric Rating Scale; SOMS = Screening for Somatoform Symptoms; DSM = Diagnostic and Statistical Manual of Mental Disorders; STAI = State-Trait-Anxiety Inventory; VAS = visual analog scale; SPS = self-produced tactile stimulus; EPS = externally produced tactile stimulus; ANOVA = one-way analysis of variance.