Cortical and Peripheral Basis of Phantom Limb Pain

Phantom limb pain is a condition that causes amputees to experience pain in the area of a limb that is no longer present. Phantom limb pain affects millions of amputees inhibiting their quality of life. There is a lot of research being put into this field to help these amputees, however, treatment of this condition is very difficult because the neural basis of phantom limb pain is still not fully understood.  Dr. Dumanian et al. explain their innovative peripheral approach to helping curb the symptoms of phantom limb pain in their article “Targeted Muscle Reinnervation treats neuroma and phantom pain in major limb amputees”. While Makin and Flor, focus on the cortical reorganization of amputees postamputation in their article, “Brain (re)organization following amputation: Implications for phantom limb pain”. These two articles show the link between how the brain is reorganized due to a specific condition, as well as, a successful treatment to help reduce the symptomatic effects of the condition.

            Gregory Dumanian et al. focus on the peripheral factors that influence painful symptoms in phantom limb patients. Their work aims to treat the terminal-neuromas that cause phantom limb pain. They describe their new treatment, Targeted Muscle Reinnervation, which is designed to treat these terminal-neuromas, “The terminal neuroma is removed, and the newly freshened nerve is coapted to a newly divided nearby motor nerve…What distinguishes TMR from all other treatments of neuromas is that the fascicles of the mixed major and sensory nerves are channeled toward nerve receptor targets” (Dumanian et al.). Dumanian et al. studied 28 major limb amputees (amputations above the wrist or ankle), who had no previous neuroma treatments. Patients were randomly given either the standard neuroma treatment or the TMR treatment. Standard neuroma treatment consisted of excising a neuroma and then burying the nerve into a nearby muscle.  Subjects in both conditions reported their pain level according to the 11-point numerical rating scale and the patient-reported outcomes measurement information system assessment. The results of this study showed the TMR treatment was able to successfully decrease phantom limb pain in amputees. These results are revolutionary for amputees, as they could have a better prognosis postamputation. Dumanian et al. efficiently explain and solve the peripheral aspect of phantom limb pain, but this is only part of the picture.

            Makin and Flor on the other hand draw from different studies and focus on the reorganization of the primary somatosensory cortex (S1) and its influence on phantom limb pain. Makin and Flor discuss the tonotopic mapping of the primary somatosensory cortex, and alterations to the organization of the maps lead to perceptual and behavioral changes, some of these changes lead to phantom limb pain. A tonotopic map is a concept that states that specific parts of your body are encoded for in a specific region of the brain. They explain how our ability to understand phantom limb pain has significantly improved due to technological advancements in imaging that have led to great acuity in somatosensory tonotopic mapping. Makin and Flor elaborate on one theory that suggests that neurons in the deprived hemisphere (one missing the limb) shift in response to the amputation, “In particular, it has been proposed that the displaced facial inputs caused by the deprivation-triggered remapping prompt aberrant processing in the S1 hand area, which may be interpreted as phantom sensation or pain arising from the missing hand” (Makin and Flor). This shift in neuronal circuity causes the primary somatosensory cortex to exhibit maladaptive changes like phantom limb pain. These changes can be misinterpreted as pain by the somatosensory cortex, which leads to the sensation of pain. Patients with a great cortical shift of somatosensory neurons tended to experience more intense phantom limb pain (Makin and Flor). Makin and Flor go on to discuss possible treatment options for phantom limb pain such as TMS, tDCS, and mirror treatment. Mirror treatment was shown to be effective because it caused the neurons to shift to a more normal organization. It is important to emphasize that these theories are not fully tested, and it still cannot be determined if the changes in the brain are a cause or consequence of phantom limb pain (Makin and Flor).

It is essential to examine both the cortical and peripheral effects of phantom limb pain to better understand the mechanisms and pathways behind this condition. Dumanian et al. explain the peripheral effects and treatments while Makin and Flor explain the cortical effects and treatments. Understanding the effects of both could lead to breakthroughs in how phantom limb pain is treated. An integrative approach of these two aspects of phantom limb pain could help millions of amputees and help deepen our understanding of neural networks and their effects on perception.

References

Dumanian, Gregory A., et al. “Targeted Muscle Reinnervation Treats Neuroma and Phantom Pain in Major Limb Amputees.” Annals of Surgery, vol. 270, no. 2, 2019, pp. 238–246., https://doi.org/10.1097/sla.0000000000003088.

Makin, Tamar R., and Herta Flor. “Brain (Re)Organisation Following Amputation: Implications for Phantom Limb Pain.” NeuroImage, vol. 218, 2020, p. 116943., https://doi.org/10.1016/j.neuroimage.2020.116943.