Loss of sensory attenuation in patients with functional (psychogenic) movement disorders

Abstract

Functional movement disorders require attention to manifest yet patients report the abnormal movement to be out of their control. In this study we explore the phenomenon of sensory attenuation, a measure of the sense of agency for movement, in this group of patients by using a force matching task. Fourteen patients and 14 healthy control subjects were presented with forces varying from 1 to 3 N on the index finger of their left hand. Participants were required to match these forces; either by pressing directly on their own finger or by operating a robot that pressed on their finger. As expected, we found that healthy control subjects consistently overestimated the force required when pressing directly on their own finger than when operating a robot. However, patients did not, indicating a significant loss of sensory attenuation in this group of patients. These data are important because they demonstrate that a fundamental component of normal voluntary movement is impaired in patients with functional movement disorders. The loss of sensory attenuation has been correlated with the loss of sense of agency, and may help to explain why patients report that they do not experience the abnormal movement as voluntary.

Functional Neuroanatomy and Neurophysiology of Functional Neurological Disorders (Conversion Disorder)

“…These findings are consistent with abnormal SMC activity during voluntary motor preparation in functional movement. 23 Pareés et al 142 also showed that patients with FND with movement symptoms have impaired sensory attenuation compared with healthy controls. Sensory attenuation describes the reduction of the intensity of sensation caused by movements that are self-generated compared with by others.…”

Sensory Attenuation Assessed by Sensory Evoked Potentials in Functional Movement Disorders

“…We have found that patients with FMD did not display attenuation of N20 and N30 amplitudes at the onset of movement compared to healthy controls. Suppression of SEPs around the onset of a voluntary movement is a well described physiological phenomenon [11], and is plausibly the electrophysiological correlate of the psychophysiological reduction in intensity of self-generated sensory stimuli probed by previously described tasks such as the force matching task [9]. Indeed, the attenuation of the intensity of sensations and their electrophysiological/functional imaging correlates when they are self-generated is a widely reported phenomenon across sensory domains [12,13].…”

“…Furthermore, the fact that we were able to show a significant group effect with a relatively small sample size suggests the effect sizes in question are relatively large. Second, as in our previous study on force matching [9],we did not include patients with functional tremor, the most common FMD, because tremor commonly involves upper limbs and this was an exclusion criterion for the study. Third, we have speculated that suppression of SEPs at the onset movement is the electrophysiological correlate of the phenomenon of SA assessed by a force-matching paradigm, but we did not directly compare the two phenomena in this study, and therefore this remains a speculative interpretation.…”

“…Recently, we have demonstrated abnormal SA in patients with FMD. We used a classic "force matching" paradigm where subjects are asked to match a force delivered to their finger either by pressing down directly on their finger or operating a robot to press down on their finger [9]. This requires a complex experimental set-up and we were interested in developing a protocol that could probe the same aspect of movement control but in a simpler way and thus could be more suitable as a potential biomarker for FMD for future clinical studies.…”

Functional neurological disorders: mechanisms and treatment

“…A particularly useful study showed that such signs not only have reasonable sensitivity and specificity, but also acceptable inter-rater reliability [17]. Additional studies have described new specific clinical and laboratory features, such as accelerometry for functional head tremor, or using reaction time to assess functional dystonia which may assist in positively identifying the diagnosis [18][19][20]. In keeping with this shift in thinking the most recent version of the diagnostic and statistical manual of mental disorders (DSM), DSM-5, moved from the DSM-IV definition based on a psychological aetiology (largely following Breuer and Freud's conversion hypothesis) to one based on the positive identification of these typical presenting neurological signs.…”