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Patients who have lost a limb often feel the continuing presence of the limb as a 'phantom'. Phantom sensations can be of touch and movement, but around 65 per cent of patients develop chronic pain, with sensations of stabbing, burning or cutting which are often severe. The cause of this 'phantom limb' pain remains unclear and, as with other chronic pain syndromes, there are few effective therapies.
One theory, says Dr Jonathan Cole, a clinical neurophysiologist at Poole Hospital and Senior Lecturer at the University of Southampton, is that a loss of sensory input to the brain causes pain to be elaborated within the brain itself in some way, although it is experienced in the phantom limb. But, he points out,"more recently there has been some evidence that motor outputs or commands may also have a role in the development of pain".
Anecdotal evidence suggests that pain can be reduced by activity in the brain involved in imagined movement of the phantom limb. "One patient who'd had his legs amputated below the knee said that that when he was in a swimming pool, if he imagined flipping his feet, the pain went away," says Dr Cole. "Another found that if he imagined playing a round of golf, his pain went away."
With Showcase support, Dr Cole is assembling a multidisciplinary team to see if this effect can be harnessed to therapeutic advantage. A German group led by Dr Herta Flor has made some interesting observations with mechanical prostheses, and Dr Cole has been further influenced by Dr Vilayanur Ramachandran, a neurologist at the University of California. "He asked patients who had lost an arm to put their remaining arm in a mirror box, so that it appeared to be the missing arm on the other side of their body, and move it." This technique reduced pain in some patients but, points out Dr Cole, its effectiveness may be limited because patients are activating the wrong side of their brain to move their phantom arm.
Dr Cole experienced a more sophisticated version of Ramachandran's mirror in a fully immersed virtual reality (VR) system at NASA's Johnson Space Center. "NASA had this wonderful robot with arms which move like people's arms. They give you a virtual reality headset, and fit your arms and hands into gloves with infrared sensors. Looking down, instead of your own arms, you see the robot's arms via cameras mounted in the robot's head. When you move your arms, the robot arms move in exactly the same way, only more slowly. Within a minute you feel completely embodied in the robot: what we see and move we become."
Aided by Professor Richard Wynne and Professor Jian Jun Zhang at the University of Bournemouth, and Dr Chris Miall, Dr Andrew Glennerster and Dr Peter Kyberd at the University of Oxford, Dr Cole is seeking to recreate this 'robotic' experience to produce pain relief for amputees. The project will use computer animation software developed by Professor Zhang and exploit the advanced motion detection equipment and sophisticated fully immersive VR environment at the University of Oxford.
Patients with phantom limb pain will have a motion detector placed on their stump. This will be linked to a VR headset in which is displayed a copy of their own existing limb mirror reversed to look like their absent one. Patients will learn to activate the virtual arm by combinations of movements at the stump, and as movement is learnt, they will feel more and more embodied in the virtual arm and pain should be relieved. Since the virtual limb will be controlled using the correct side of the brain, this technique is likely to create a stronger sense of attachment to the virtual arm - and be more potent in the relief of pain than the mirror box.
"We also hope to produce a much lower-tech system that people may be able to take away, and use in their homes," says Dr Cole. "It may even be that once we have given them the tools to enable them to activate their muscles and 'move' their virtual arm, they may be able to 'imagine' the movement, as the swimmer and golf player did, so the need for VR reinforcement may diminish."
"So far, it's a series of speculative ideas linked together, with an underpinning of hope," says Dr Cole. "If it turns out to be successful, we learn more about the mechanism of chronic phantom limb pain as well as having a method helpful in reducing chronic pain."
