Prisms for pain. Can visuo-motor rehabilitation strategies alleviate chronic pain?

Abstract

Background and aims: Prism adaptation (PA) is a non-invasive procedure in which participants perform a visuo-motor pointing task while wearing prism goggles inducing a lateral displacement of the visual field and a mismatch between the seen and felt position of the pointing hand. PA is thought to induce a reorganization of sensorimotor coordination, and has been used successfully to rehabilitate neglect following right-hemisphere lesions. Because studies have shown that complex regional pain syndrome (CRPS) is associated with neglect-like symptoms, it was proposed that PA could be used to alleviate pain in these patients.

Database: A search for peer-reviewed articles on neglect-like symptoms in CRPS and on the use of prisms in CRPS was conducted using the PubMed database.

Results: There is still no agreement as to whether CRPS patients really present neglect symptoms and, if they do, what it is that they neglect. Furthermore, there is insufficient data to determine whether PA exerts an effect on CRPS symptoms. Finally, it remains unknown whether neglect can be observed in other types of lateralized pain, or whether PA could be useful for these patients.

Conclusion: By highlighting open issues, our review provides guidelines for future studies on the use of prisms in pain. The assessment of neglect in patients with CRPS as well as other types of lateralized chronic pain should be characterized using a combination of neuropsychological methods assessing the multiple aspects of neglect in a more refined manner. In addition, further studies should investigate the mechanisms through which PA may modulate pain.

Figure 1

Differences between prism adaptation and passive exposure. A. Prism adaptation (PA). Participants are asked to point in front of them or to perform any other visuo-motor task towards a target while wearing prismatic goggles. During pointing, participants view the target but not the hand. Because of the optical shift produced by the goggles, the target appears shifted as compared to its real position. During the first trials, participants make pointing errors towards the direction of the optical shift (early phase exposure). After some practice, these errors attenuate (late phase exposure). After PA, participants no longer wear the goggles and make pointing errors in the opposite direction. This after effect of PA results from a realignment of visual and motor coordinates. B. Passive exposure. Here, participants do not perform a visuo-motor task. Instead, participants view their hands and goggles induce a conflict between the visually perceived position of the hand (represented with the solid contour) and its actual position (represented with a dashed contour). Importantly, in this case, the hand itself looks shifted.

Figure 2

Prism adaptation (PA) can restore the pathological shift in body midline that has been observed in CRPS patients. After PA, Sumitani et al. (Sumitani et al., 2007a) observed that the shift towards the affected side was displaced towards the healthy side. The authors interpreted the shift towards the pathological side as a tendency to over-represent information coming from the affected side. Therefore a shift in the opposite direction was considered as reflecting an improvement over the pathological over-representation of inputs.