How Manchester is laying phantom limb pain to rest
Departments Research impact and institutes Robotics and AI 13th November 2019
Almost every individual who undergoes an amputation experiences sensation in their missing limb. Just imagine it… part of your brain knows that your limb has been removed – whether medically or as the result of an accident – but at some other level, it still believes the limb exists.
Of the more than 80 per cent of individuals who undergo an amputation and continue to feel sensation in the limb, the vast majority will experience pain. It might feel like the limb is burning; that it’s being electrocuted; that it’s being crushed, or even that it is bent and twisted into an impossible position. The person may even continue to feel the sensation of whatever caused them to lose the limb, such as a crushing pressure.
It’s an unpleasant condition and one that is still little understood.
Cracking an age-old mystery
Research into the phenomenon first took place at the turn of the 19th century, when physician and novelist Silas Weir Mitchell suggested phantom limb syndrome was caused by irritation to the peripheral nervous system at the site of the amputation. To be honest, the theories haven’t moved on much from there.
A later theory put forward that the condition was caused by the brain reorganising itself when it failed to receive a stimulus from the missing body part. This confusion caused the nervous system to send pain signals – a typical response when it senses something is wrong.
However, phantom limb syndrome has also been noted in babies born without a limb. Even though they never had a chance to use it or feel sensation in it, the infants still react as though the limb were there.
This discovery prompted a new line of thinking – that the human brain is hardwired to assume the nervous system and body fits a particular mould, and therefore to expect a certain response from each area of that body. The confusion caused by a part of the body not being there is interpreted by the brain as pain.
Reflection therapy
While we may be some way off fully understanding the cause, there’s no questioning the existence of the condition.
“For the people who experience phantom limb pain it can be really debilitating,” explains Steve Pettifer, Professor of Computer Science at the Department of Computer Science. “This is partly because of the sensation of pain and also because of the strangeness of knowing that that pain is ‘not really there’ – the limb is not there, but the pain is very, very real.”
So, how do you treat a condition if you don’t know what causes it? Well Prof Pettifer potentially has the solution.
Through his work with a psychologist, Prof Pettifer came up with a way to reimagine the ‘mirror box’ technique.
While treating phantom limb pain has historically been almost as difficult as understanding what causes it, mirror box therapy is one method that’s been shown to have some success in alleviating the symptoms. Invented by neuroscientist VS Ramachandran, the therapy allows the recipient to feel a sense of relief from phantom limb pain by providing an image of the lost limb that they can move, scratch or unclench. The image is, in reality, a reflection of their remaining limb.
Prof Pettifer and his team at The University of Manchester have taken this one step further by developing a new virtual reality therapy for people experiencing sensations in a phantom limb. Much like the mirror box method, the therapy provides the recipient with an image of their missing limb, allowing them to experience alleviation from some of the sensations they have been feeling.
“The idea with the virtual reality set-up is that we give people back a virtual version of their missing limb,” he explains. “So if you’ve lost your left arm, when you’re in the virtual environment, you will see a left arm where you’d expect your missing left arm to be and you can move and control that by moving and gesturing with your right arm. What you see is your missing arm making those gestures.”
Among the people who have received this therapy, most have reported a reduction in phantom limb pain. This has been shown to last anywhere from a few hours to a few days.
The next step is to make the technology available within people’s homes. To date, amputees have been invited in to the University to try out the therapy, but as virtual reality technology has become more economical, it has opened up the opportunity for it to be used in the home. Prof Pettifer and his team hope this will allow them to reach more people and gain a valuable insight into its success – and to rule it out as being a placebo.
Steering clear of uncanny valley
Interestingly, the success of the therapy does not depend on the image of the limb being an exact match to the missing limb – or even close to realistic. “We have created a neutral representation of the limb that’s not obviously male or female, not obviously any particular skin colour or tone. This is to avoid the effect of ‘uncanny valley’,” says Prof Pettifer.
If you’ve watched almost any movie in the last 20 years, you’ll no doubt be familiar with the concept of uncanny valley. While CG has certainly come on leaps and bounds since the Stars Wars prequels, early Harry Potter, Twilight and The Polar Express (*shudder*), as the technology has improved, so too has the frequency of our trips to uncanny valley.
“Uncanny valley occurs as you get more and more realistic in your human representation but without quite achieving it – the effect can be more revolting than a crude cartoon,” says Prof Pettifer. “So, for example, we never question the appearance of The Simpsons. But as you become more and more realistic but never quite perfect, the brain perceives it as unpleasant as it doesn’t feel right.”
As a result, his team have intentionally stayed away from uncanny valley by keeping their imaging fairly crude and cartoon-like.
Interestingly, this is a decision that was also made by a team of our Mechanical Engineering postgrads when they created a 3D-printed bionic limb for amputees. The creation won the Digital Innovation Challenge at the 2018 4.0 Summit in Manchester.
The bionic limb looks very much like what it is – a robot hand (meaning the wearer can enjoy those Luke Skywalker vibes). “We didn’t want to replicate a human hand, but embrace the fact that it’s not,” said the team’s leader, Alex Agboola-Dobson, at the time.
Only time will tell if virtual reality proves to have a legitimate therapeutic effect on the phenomenon of phantom limb pain. “The long-term goal would be that you can have this as a therapy that people can use in the home. But we’re a long way from that at the moment,” says Prof Pettifer.
However, there may one day be a time when lost limbs can be biologically regenerated. Don’t miss Part Two, when we talk to Professor Sarah Cartmell about her research on bone and tissue regeneration.
Don’t miss part two by subscribing to The Hub on our homepage.
Words – Hayley Cox
Images – Prof Steve Pettifer, Laura Dahl, Eden, Janine and Jim, Johan Larsson