Analgesic effects of multisensory illusions in osteoarthritis

Analgesic effects of multisensory illusions in osteoarthritis SIR, There is increasing evidence that drug-free illusion therapies can be beneficial for the amelioration of chronic pain, particularly so for conditions in which some of the pain is thought to have a cortical origin. For example, mirror therapy and size reduction illusions can reduce pain in complex regional pain syndrome type 1 (CRPS1) patients, the majority of whom have disturbed body representations with some reporting their hand as larger than in reality [1] and others describing parts of their hand as foreshortened [2]. If cortical misrepresentation of body parts contributes to pain, then manipulating the appearance of those body parts might be a useful tool in the reduction of pain. This letter describes an exploratory experiment using unique visuo-proprioceptive illusions that manipulated the perceived size of painful and non-painful parts of the hand in an attempt to modulate pain experienced in OA. Illusions were applied using a MIRAGE system [3] that presents real-time video capture of the actual hand from the same position and perspective as if viewing the real hand directly (Fig. 1a). Visual manipulations can be applied to the image such that they appear to happen to the real hand. The experimental manipulations were stretching (Fig. 1b) and shrinking (Fig. 1c): gently pulling or pushing on part of the hand while simultaneously elongating or shortening the image gives a strong illusion that the hand is being stretched or shortened (see supplementary material for video). Control illusions included stretching and shrinking a non-painful part of the hand and visually enlarging/reducing the entire hand. A total of 20 OA sufferers [2 males; mean (S.D.) age: 70.5 (6.5) years] with clinically diagnosed arthritic pain in the hands and/or fingers for >12 months were tested. Participants gave written consent in accordance with the Declaration of Helsinki. The study met with ethical approval from the University of Nottingham School of Psychology Ethics Committee (code: 20072010). None of the participants were pain free on the day of testing and none had used pain medication other than paracetamol. Participants verbally reported their pain levels on a 21-point numerical rating scale (NRS) with 0 indicating no pain at all and 20 indicating the most unbearable pain im-aginable. The scores were adjusted relative to a baseline condition of gently pulling or pushing the hand without visual manipulation. NRS ratings of pain for the painful part of the hand were …