Phantom limb pain is a phenomena that can be described as pain felt in the area where an arm or a leg has been amputated. Each part of our brain corresponds to specific areas or specific tasks in our bodies, and so many parts of our body can be controlled by the brain. Phantom limb pain happens because once those connections from the body to the areas of the brain end, with the limb being gone, severed nerve endings continue to send pain signals to that area (thinking that the limb is there). By this definition, many attribute PLP to only people who were born with a limb that later on in life ends up being amputated. This way, the brain has enough time to make its physical connection. Contradictorily enough though, studies have found that it's also possible for congenital amputees to experience PLP.
Congenital limb deficiencies, or congenital limb amputations, are birth defects which can be defined by the "aplasia or hypoplasia of bones of the limbs" (Melzack, et al.). This means that a fetus' limbs don't develop normally/enough (hypoplasia) or even at all (aplasia) as it forms in the uterus. A study done by Melzack, et al., in which they studied the possibility of phantom limp pain with patients of CLD and with amputees from before they were 6 years old, says that "phantom limbs are experienced by at least 20% of the congenitally limb-deficient subjects", which is about 20% more than what anyone would expect. Pain though, seems to not be experienced by everyone, so even though they feel the phantom limb, only 20% feel the pain that comes along with it.
In the Dumanian et al., study, in which they introduced Targeted Muscle Reinnervation (TMR), patients with neuromas or phantom limb pain were randomized and worked on. TMR is done by identifying the nerves that controlled the now-missing limb, and connecting it to another muscle so that those now control a different area of the body. We know that once nerves are severed, they are essentially rendered useless and can end up creating a painful neuroma- which gives sense as to why phantom limb pain can happen. So how exactly does it happen for people who've never even had that limb. A hypothesis can be that although the limb was never developed, the connection between the nerves and the brain already happened, and so the only issue was that there wasn't enough cell tissue to create the limb. I find a particular example from the Mezlack et al., study to be extremely interesting: a girl with congenital limb absence of her left arm below the elbow had a horse riding accident at the age of 16, in which her artificial arm fell off and so afterwards, she felt a phantom limb which she'd never felt before. It seems that the nerves that controlled her left forearm had been developed and once she hit the area where they were supposed to extend, her brain mistook it for a presence and now it believes she's always had her complete arm. This confirms the idea that although the limb is missing the nerves are always there. It's hard not to wonder whether or not TMR would work on people with congenital limb deficiency, as they could possibly have nerves that already control missing limbs and that could be rearranged to control a different muscle and reroute any pain felt by the patient.
References:
Melzack, R. (1997). Phantom limbs in people with congenital limb deficiency or amputation in early childhood. Brain, 120(9), 1603–1620. https://doi.org/10.1093/brain/120.9.1603
Phantom limb pain and chronic pain. Phantom Limb Pain and Chronic Pain | Michigan Medicine. (n.d.). Retrieved December 16, 2021, from https://www.uofmhealth.org/health-library/ty6877
Wilcox, W. R., Coulter, C. P., & Schmitz, M. L. (2015). Congenital limb deficiency disorders. Clinics in Perinatology, 42(2), 281–300. https://doi.org/10.1016/j.clp.2015.02.004
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. Annals of Surgery, 270(2), 238–246. https://doi.org/10.1097/sla.0000000000003088
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