An investigation of perceptual biases in complex regional pain syndrome

Clinical and Neurophysiological Effects of Progressive Movement Imagery Training for Pathological Pain

Movement limitation is a common characteristic of chronic pain such that pain prevents the very movement and activity that is most likely to promote recovery. This is particularly the case for pathological pain states such as complex regional pain syndrome (CRPS). One clinical approach to CRPS that has growing evidence of efficacy involves progressive movement imagery training. Graded Motor Imagery (GMI) targets clinical and neurophysiological effects through a stepwise progression through implicit and explicit movement imagery training, mirror therapy and then functional tasks. Here we review experiences from over 20 years of clinical and research experience with GMI. We situate GMI in terms of its historical underpinnings, the benefits and outstanding challenges of its implementation, its potential application beyond CRPS. We then review the neuropathological targets of GMI and current thought on its effects on neurophysiological biomarkers. Perspective This article provides an overview of our experiences with graded motor imagery training over the last 20 years focussing on the treatment of CRPS. It does both cover the theoretical underpinnings for this treatment approach, biomarkers which indicate potential changes driven by GMI, and experiences for achieving optimal treatment results.

Normal manual straight ahead pointing in Complex Regional Pain Syndrome

There is evidence to suggest that people with Complex Regional Pain Syndrome (CRPS) can have altered body representations and spatial cognition. One way of studying these cognitive functions is through manual straight ahead (MSA) pointing, in which participants are required to point straight ahead of their perceived body midline without visual feedback of the hand. We therefore compared endpoint errors from MSA pointing between people with CRPS (n = 17) and matched controls (n = 18), and examined the effect of the arm used (Side of Body; affected/non-dominant, non-affected/dominant). For all participants, pointing errors were biased towards the hand being used. We found moderate evidence of no difference between Groups on endpoint errors, and moderate evidence of no interaction with Side of Body. The differences in variability between Groups were non-significant/inconclusive. Correlational analyses showed no evidence of a relationship between MSA endpoint errors and clinical parameters (e.g. CRPS severity, duration, pain) or questionnaire measures (e.g. body representation, "neglect-like symptoms", upper limb disability). This study is consistent with earlier findings of no difference between people with CRPS and controls on MSA endpoint errors, and is the first to provide statistical evidence of similar performance of these two groups. Our results do not support a relationship between clinical or self-reported measures (e.g. "neglect-like symptoms") and any directional biases in MSA. Our findings may have implications for understanding neurocognitive changes in CRPS.

Where is my arm? Investigating the link between complex regional pain syndrome and poor localisation of the affected limb

Background

Anecdotally, people living with Complex Regional Pain Syndrome (CRPS) often report difficulties in localising their own affected limb when it is out of view. Experimental attempts to investigate this report have used explicit tasks and yielded varied results.

Methods

Here we used a limb localisation task that interrogates implicit mechanisms because we first induce a compelling illusion called the Disappearing Hand Trick (DHT). In the DHT, participants judge their hands to be close together when, in fact, they are far apart. Sixteen volunteers with unilateral upper limb CRPS (mean age 39 ± 12 years, four males), 15 volunteers with non-CRPS persistent hand pain (‘pain controls’; mean age 58 ± 13 years, two males) and 29 pain-free volunteers (‘pain-free controls’; mean age 36 ± 19 years, 10 males) performed a hand-localisation task after each of three conditions: the DHT illusion and two control conditions in which no illusion was performed. The conditions were repeated twice (one for each hand). We hypothesised that (1) participants with CRPS would perform worse at hand self-localisation than both the control samples; (2) participants with non-CRPS persistent hand pain would perform worse than pain-free controls; (3) participants in both persistent pain groups would perform worse with their affected hand than with their unaffected hand.

Results

Our first two hypotheses were not supported. Our third hypothesis was supported —when visually and proprioceptively encoded positions of the hands were incongruent (i.e. after the DHT), relocalisation performance was worse with the affected hand than it was with the unaffected hand. The similar results in hand localisation in the control and pain groups might suggest that, when implicit processes are required, people with CRPS’ ability to localise their limb is preserved.

Measuring the sensitivity of tactile temporal order judgments in sighted and blind participants using the adaptive psi method

Spatial locations of somatosensory stimuli are coded according to somatotopic (anatomical distribution of the sensory receptors on the skin surface) and spatiotopic (position of the body parts in external space) reference frames. This was mostly evidenced by means of temporal order judgment (TOJ) tasks in which participants discriminate the temporal order of two tactile stimuli, one applied on each hand. Because crossing the hands generates a conflict between anatomical and spatial responses, TOJ performance is decreased in such posture, except for congenitally blind people, suggesting a role of visual experience in somatosensory perception. In previous TOJ studies, stimuli were generally presented using the method of constant stimuli-that is, the repetition of a predefined sample of stimulus-onset asynchronies (SOA) separating the two stimuli. This method has the disadvantage that a large number of trials is needed to obtain reliable data when aiming at dissociating performances of groups characterized by different cognitive abilities. Indeed, each SOA among a large variety of different SOAs should be presented the same number of times irrespective of the participant's performance. This study aimed to replicate previous tactile TOJ data in sighted and blind participants with the adaptive psi method in order to validate a novel method that adapts the presented SOA according to the participant's performance. This allows to precisely estimate the temporal sensitivity of each participant while the presented stimuli are adapted to the participant's individual discrimination threshold. We successfully replicated previous findings in both sighted and blind participants, corroborating previous data using a more suitable psychophysical tool.

Characterising sensorimotor adaptation in Complex Regional Pain Syndrome

It has been suggested that sensorimotor conflict contributes to the maintenance of some pathological pain conditions, implying that there are problems with the adaptation processes that normally resolve such conflict. We tested whether sensorimotor adaptation is impaired in people with Complex Regional Pain Syndrome (CRPS) by characterising their adaption to lateral prismatic shifts in vision. People with unilateral upper-limb CRPS Type I (n = 17), and pain-free individuals (n = 18; matched for age, sex, and handedness) completed prism adaptation with their affected/non-dominant and non-affected/dominant arms. We examined 1) the rate at which participants compensated for the optical shift during prism exposure (i.e., strategic recalibration), 2) endpoint errors made directly after prism adaptation (sensorimotor realignment) and the retention of these errors, and 3) kinematic markers associated with strategic control. Direct comparisons between people with CRPS and controls revealed no evidence of any differences in strategic recalibration, including no evidence for differences in a kinematic marker associated with trial-by-trial changes in movement plans during prism exposure. All participants made significant endpoint errors after prism adaptation exposure, indicative of sensorimotor realignment. Overall, the magnitude of this realignment did not differ between people with CRPS and pain-free controls. However, when endpoint errors were considered separately for each hand, people with CRPS made greater errors (indicating more rather than less realignment) when using their affected hand than their non-affected hand. No such difference was seen in controls. Taken together, these findings provide no evidence of impaired strategic control or sensorimotor realignment in people with CRPS. In contrast, they provide some indication that there could be a greater propensity for sensorimotor realignment in the CRPS-affected arm, consistent with more flexible representations of the body and peripersonal space. Our study challenges an implicit assumption of the theory that sensorimotor conflict might underlie some pathological pain conditions.

Ignoring space around a painful limb? No evidence for a body-related visuospatial attention bias in complex regional pain syndrome

BACKGROUND

Complex Regional Pain Syndrome (CRPS) is a disorder of severe chronic pain in one or more limb(s). People with CRPS report unusual perceptions of the painful limb suggesting altered body representations, as well as difficulty attending to their affected limb (i.e., a 'neglect-like' attention bias). Altered body representations and attention in CRPS might be related, however, existing evidence is unclear. We hypothesized that if there were a body-related visuospatial attention bias in CRPS, then any attention bias away from the affected side should be larger for or limited to circumstances when the (impaired) body representation is involved in the task versus when this is not the case.

METHODS

We included 40 people with CRPS, 40 with other limb pain conditions, and 40 pain-free controls. In half of the people with pain, their upper limb was affected, in the other half their lower limb. We administered computerized tasks of spatial attention, including free viewing of images, shape cancellation, temporal order judgement, and dot-probe. The degree to which different versions of each task involved body representation was manipulated by one or more of the following: (1) presenting stimuli nearer versus further away from the body, (2) using body related versus neutral stimuli, and (3) inducing mental rotation of body parts versus no mental rotation. In addition to perceptual judgements, eye movements were recorded as a sensitive index of spatial attention. Bayesian repeated measures analyses were performed.

RESULTS

We found no evidence for a (body-related) visuospatial attention bias in upper limb CRPS. Secondary analyses suggested the presence of a body-related visuospatial attention bias away from the affected side in some participants with lower limb CRPS.

DISCUSSION

Our results add to growing evidence that there might be no general visuospatial attention bias away from the affected side in CRPS.