R E V I E W Conversion disorder : towards a neurobiological understanding Samuel B Har vey1 Biba R Stanton2 Anthony S David1 1Department of Psychological Medicine, 2Department of Neurology, Institute of Psychiatry, King’s College London, De Crespigny Park, London, UK Correspondence: Samuel B Harvey Section of Neuropsychiatry, P068, Department of Psychological Medicine, Institute of Psychiatry, Kings College London, De Crespigny Park, Denmark Hill, London SE5 8AF, United Kingdom Tel +44 20 7848 0138 Fax +44 20 7848 0572 Email s.harvey@iop.kcl.ac.uk Abstract: Conversion disorders are a common cause of neurological disability, but the diagnosis remains controversial and the mechanism by which psychological stress can result in physical symptoms “unconsciously” is poorly understood. This review summarises research examining conversion disorder from a neurobiological perspective. Early observations suggesting a role for hemispheric specialization have not been replicated consistently. Patients with sensory conversion symptoms have normal evoked responses in primary and secondary somatosensory cortex but a reduction in the P300 potential, which is thought to reflect a lack of conscious processing of sensory stimuli. The emergence of functional imaging has provided the greatest opportunity for understanding the neural basis of conversion symptoms. Studies have been limited by small patient numbers and failure to control for confounding variables. The evidence available would suggest a broad hypothesis that frontal cortical and limbic activation associated with emotional stress may act via inhibitory basal ganglia–thalamocortical circuits to produce a deficit of conscious sensory or motor processing. The conceptual difficulties that have limited progress in this area are discussed. A better neuropsychiatric understanding of the mechanisms of conversion symptoms may improve our understanding of normal attention and volition and reduce the controversy surrounding this diagnosis. Keywords: conversion disorder, hysteria, neurophysiology, functional imaging, SPECT, fMRI Introduction The concept of “hysteria” or conversion disorder has always been controversial (Halligan and David 1999), but there is no doubt that medically unexplained neurological symptoms are common and cause considerable disability. About one third of patients in neurology outpatient clinics have symptoms thought to be “non- organic” in nature (Carson et al 2003; Snijders et al 2004) and these symptoms have a poor prognosis for recovery (Carson et al 2003; Stone et al 2003). Despite early concerns that many patients labeled with hysteria or conversion disorder are later found to have an organic explanation for their symptoms (Slater 1965), more recent studies suggest that the incidence of misdiagnosis is now low and that the stability of a diagnosis of conversion disorder in well-investigated patients is high (Couprie et al 1995; Crimlisk et al 1998). Conversion disorder is defined by DSM-IV as a deficit of sensory or motor function that cannot be explained by a medical condition and where psychological factors are judged to be associated with the deficit because symptoms are preceded by conflicts or other stressors (APA 1994). It is differentiated from factitious disorder by the fact that the symptom or deficit is not intentionally produced or feigned. In ICD-10, conversion symptoms are classified as dissociative disorders (eg, dissociative motor disorder), with similar diagnostic criteria (WHO 1992). Neuropsychiatric Disease and Treatment 2006:2(1) 13–20 © 2006 Dove Medical Press Limited. All rights reserved 13