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Why amputees feel phantom limb pain

We’ve all heard tales of how amputees still feel pain in their missing limb, but why does this happen? Researchers from Osaka University in Japan in collaboration with the University of Cambridge, have suggested that it’s all down to a reorganisation of the wiring in the brain, and have identified a potential treatment for the problem.

The vast majority of amputees suffer from chronic pain following the removal of a limb. So far there’s no way to treat this pain. Now the research team have not only identified why amputees feel this pain, but they’ve discovered a potential method to treat the pain which uses artificial intelligence techniques rather than drugs.

By using a brain-machine interface, the research team were able to train ten amputees to control a robotic arm with the power of their brain. During this training, they found that if an individual attempted to control the robotic arm by associating the movement with their missing limb it made their pain worse. However, by training them to associate the movement of the robotic limb with their unaffected hand, they were able to decrease the pain.

From these results, the team were able to demonstrate that the patients who were experiencing chronic pain associated with either an amputation or some form of nerve injury, had ‘crossed wires’ in the part of the brain that’s responsible for movement and sensation. Furthermore, by fixing that disruption, the pain was able to be treated. This was also true of other forms of chronic pain, such as that caused by arthritis.

Currently there’s almost 5,000 amputations carried out every year in the UK alone. Of these, the vast majority of people who have had an arm or hand amputated feel as though their hand is still there.

Furthermore, between 50 and 80% of these amputees experience chronic pain in the ‘phantom’ hand, such as burning sensations. Unfortunately, conventional pain killers have no effect on this type of pain.

The researchers wanted to find a way to treat the pain, which didn’t rely on drugs. As phantom limb pain is believed to be caused by faulty ‘wiring’ of the part of the brain that’s responsible for processing sensory input and for executing movement, they decided to start by decoding the neural activity of an amputee’s brain. The team used a brain-machine interface to ascertain the mental action necessary in order for the patient to move their ‘phantom’ hand. They then used artificial intelligence techniques to convert the decoded movements into a robotic neuroprosthetic.

From this work, the researchers discovered that as the affected side of the brain became more adept at using the robotic arm, the pain in the phantom hand increased, leading them to suggest that although the movement aspect of the brain was functional, the person wasn’t getting any sensory feedback. By altering their technique so that the ‘wrong’ side of the brain was used, the patients saw a marked reduction in the amount and severity of the pain. By taking advantage of the brain’s ability to restructure and learn new things, they were able to control the pain associated with the missing limb.

While this technique is very promising, so far the effects have been temporary. However, the team believe that a treatment based on this technique could be a possibility within the next ten years.

The results of the study were reported in the online journal Nature Communications.

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