While certainly not the worst part of losing a limb, phantom limb pain is not to be scoffed at. Phantom limb pain (PLP) is a result of losing a limb, most commonly today through amputation, that causes pain sensations perceived to be coming from the limb that is no longer there. Even today, the exact mechanism for the pain is not well understood, however neurological factors pertaining to the peripheral nervous system as well as central nervous system seem to be involved as well as psychological factors. Despite the lack of exact knowledge of the mechanism of PLP, researchers have developed techniques that have been successful in treating PLP, namely targeted muscle reinnervation (TMR) and phantom motor execution (PME).
One of the more interesting discoveries in PLP research was done recently by Dumanian et al. in their attempt to help treat painful neuromas. A neuroma is a tumor or growth that occurs on a nerve that sends pain signals to the CNS. Amputation is one way in which a neuroma can develop, and that is what leads Dumanian et al. to find a treatment for phantom limb pain. Traditionally, a neuroma is treated by excising it and burying the fresh nerve within a muscle so that when a new neuroma develops, the muscle will cushion the neuroma and be less painful. The drawbacks to this approach are obvious, the pain often returns. Dumanian and his colleagues discovered an alternative method to the traditional approach called targeted muscle reinnervation, in which a neuroma is excised and then makes a connection to a motor nerve innervating a muscle, allowing the now mixed nerve to innervate the muscle. Interestingly, by doing this in amputees, the patients display a statistically significant reduction in PLP over time while the residual pain they were originally trying to treat for neuromas was decreased, but not significant. So, by treating neuromas, PLP is treated and leads to a reduction in the pain felt by a patient.
Another unique treatment method that is growing in popularity is the usage of virtual reality (VR), augmented reality (AR), and serious gaming (SG) to treat PLP. VR, AR, and SG all utilize some kind of device that a patient wears that allows them to be immersed within a virtual world. These devices facilitate what is called phantom motor execution, a method that involves manipulating the CNS and PNS in order to engage the muscles at the site of amputation when attempting to do movements with the amputated limb. When in VR, AR, or SG, the device is able to detect the intention to move the amputated limb and displays the movement virtually, so that to the patient it appears they are moving their own limb. Not only does it appear to the patient they are moving their own limb, but the devices are also able to provide feedback so the experience in VR, AR, or SG is as realistic as possible. Results show that phantom motor execution does decrease PLP, and this is huge news as virtual reality devices are becoming widely accessible.
To add to the growing body of literature on phantom motor execution, Lendaro et al. take a new spin on the method and analyze not only its efficacy in treating PLP, but also its application at home through a holistic approach to improve its usefulness. Now that virtual reality devices are widely available for personal use, phantom motor execution can be treated at home by patients themselves. Therefore, Lendaro and her team consisting of a biomedical engineer, a medical anthropologist, and a user interface designer examined patient use of virtual reality devices at their homes to come up with ways to optimize its treatment use. Unsurprisingly, they found that patients used the treatment most when they experienced PLP and decreased its use over time as pain went away. Patients additionally displayed resurgences in use if the pain came back. The authors greatest contribution to PLP treatment though has to be discovery of the importance of personal usage patterns. In order for the method to be successful, each patient has to use the device in accordance with their own needs. To facilitate usage, Lendaro et al. list a number of functions that can and should be incorporated into future VR devices such as matching the augmented reality limb to resemble the patients, the ability to set personal goals, and a display to show how much time a person has spent using the device. Taken together, patients will optimize their usage of the VR device leading to better pain outcomes as long as the device meets their personal needs.
Combined, targeted muscle reinnervation and phantom motor execution treat PLP in different ways. If used in tandem, the relief felt by amputee patients can be compounded since the mechanisms of action for the treatments differ. Therefore, Dumanian et al. and Lendaro et al.’s research compliment each other as they offer two different solutions to the same problem. In the near future, hopefully clinicians will be able to access their patient's individual needs to help maximize both treatment methods to improve their patient’s quality of life. For now, it is simply a triumph that amputee patients are able to achieve relief from their PLP through the work of great researchers such as Dumanian and Lendaro.
References
Dumanian, G. A., Potter, B. K., Mioton, L. M., Ko, J. H., Cheesborough, J. E., Souza, J. M., Ertl, W. J., Tintle, S. M., Nanos, G. P., Valerio, I. L., Kuiken, T. A., Apkarian, A. V., Porter, K., & Jordan, S. W. (2019). Targeted Muscle Reinnervation Treats Neuroma and Phantom Pain in Major Limb Amputees: A Randomized Clinical Trial. Annals of surgery, 270(2), 238–246. https://doi.org/10.1097/SLA.0000000000003088
Flor H. (2002). Phantom-limb pain: characteristics, causes, and treatment. The Lancet. Neurology, 1(3), 182–189. https://doi.org/10.1016/s1474-4422(02)00074-1
Lendaro, E., Middleton, A., Brown, S., & Ortiz-Catalan, M. (2020). Out of the Clinic, into the Home: The in-Home Use of Phantom Motor Execution Aided by Machine Learning and Augmented Reality for the Treatment of Phantom Limb Pain. Journal of pain research, 13, 195–209. https://doi.org/10.2147/JPR.S220160
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