Treatment of Phantom Limb Pain through VR and AR guided Phantom Limb Execution
Phantom Limb Pain (PLP) is a chronic condition that can affect amputees, possibly as a consequence of brain restructuring to confront the newly missing of a limb. PLP can severely limit the quality of life of people who suffer from it. The synptoms manifest as a constant pain generated from a body part that isn't actually there. It is still unclear how exactly this happens but a study suggests that this is due to a "phenomenon of the CNS that is related to plastic changes at several levels of the neuraxis and especially the cortex". The developed treatments (there are over 60) currently in use to tackle this condition have been mostly unsuccessful; "examples include mirror therapy, various types of medications, acupuncture, and implantable nerve stimulators" (source) . The most used are "Phantom Limb Imagery" (PMI) and "Mirror Therapy", they are often used in combination. PMI consists in training the patient to think about (imagine) movement of the phantom limb while keeping the muscles of the stump completely relaxed. The idea is to create a mental separation between the non existing limb (and its movements) and the actual muscles still present in the stump that was connected to it. Mirror therapy follows on the same trend by aiding the imagination of the movement through a mirrored image of the other limb moving. So for example if the patient is missing the left hand, he/ she will move the right hand in front of a mirror to better envision the phantom movements of the missing left hand.
Graphic representation of the "Graded Motor Imagery", a therapy that includes PMI and Mirror Therapy. Source.
The PME approach proposed by Lendaro et al. is somewhat similar to the PMI therapy but moves in the opposite direction. While PMI forces the patient to mentally separate the stump muscles from the phantom limb movements, PME promotes the connection between the two, teaching the patient how to correctly engage the muscles in the stump to reflect the imagined phantom movements. At the moment, there has been a single, small trial conducted on 14 patients, which has yielded promising results. On average, the patients experienced a reduction of pain (during both night and day) of about 50% (source)
The trial was conducted on 14 patients with upper limb amputation and who suffered from a cronic phantom pain condition (i.e. had already tried at least one clinical approach to relieve the pain, with no success). The procedure consisted in 2 sessions per week , lasting 2 hours each, for a total of 12 sessions. During the therapy sessions, the patients learned to control the virtual limb by correctly engaging the muscles in the stump. The patient worked with three different setups during the therapy:
The first setup consisted of an augmented reality application, implemented with a simple webcam and a large screen. The patient could see themselves in the screen with the difference their mirrored image had all of her limbs! This works by attaching "surface electrodes over the stump [to] record synergistic muscle activation during motor volition of the phantom limb (phantom motor execution). Myoelectric pattern recognition is used to decode motor volition and voluntarily control the virtual limb" (source). The more muslces remained og the missing limb, the easier and more effective this movement analysis would be. This is possible because movements of the extremities of our arms / legs, often originate from muscles that are way back, closer to the body. The limb in the AR space is correctly overlayed on the patient's image thanks to a QR marker positioned on the stump and tracked by the webcam.
The Second setup was very similar to the first, but here the experience was more immersive as it was implemented in Virtual Reality. Tehe patient would be completely surrounded by a virtual world, and only see their virtual self, complete of all limbs! The paper is not very explicative of how exactly the application looked like and what the patient could / could not do in this world. For example, it would be interesting it the he / she could pick up objects with the virtual representation of the missing limb.
Finally, the third implementation aimed at engaging the user in a fun and rewarding activity while continuing to promote the correct muscle engagements. This approach is referred to as "serious gaming". The proposed game was a simple car racing one, where the patients could control the behaviour of the car with the virtual limb movements, previously learned in tthe other two setups. The fact that a game is envolved, alleviates the stress of the patient, who is distracted by the fun of the activity and it allows to create a deeper connection witht the missimg limb as the limb movements are not the primary objective of the patient's thoughts but still the result is the same muscles being activated.
Here is an explicative video demonstration:
Advantages and Limitations of PME
The primary advantage of Phantom Motor Execution is that (apparently) there are no disadvantages! It is a non intrusive, non medicinal therapy that should not (and has not so far) bring any kind of drawback. This does not mean that it always works. For example, one limitation in the trial has been that subjects with severe PLP were not admitted to the trial, meaning that the therapy is only meant for treating weaker manifestations of the disorder. Furthermore, it did't completely free patients from pain: the average reduction (measured through questionaires) has been of about 50% among the 14 patients.
This being said, the therapy is very promising, in primis because, up to this day, there aren't many valid alternatives to treat PLP. Another promising aspect is that the patients progressively felt better with every session; this suggests that the benefits of the therapy can improve with more sessions and are not limited to the current results.
Reading about this widely unknown issue and how it is being resolver through AR and VR has been very interesting. I had read other applications of these tachnologies to medical purposes but they had never really convinced me and often seemd poorly implemented and tested. The authors of this new therapyinstead took a very serious approach and reached relevant results. They have raised my hopes for future engagement of VR and AR in the medical realm.