Scientists, researchers, and physicians alike still have not been able to pinpoint the precise cause of phantom pain, a term coined by Silas Weir Mitchell in 1871. Phantom limb is used to reference a syndrome in which patients experience pain or other sensations originating from a nonexistent limb. There may also be discrepancies between the actual location of a limb and that which is perceived. A fundamental aspect of phantom limb syndrome is that the person experiencing the phenomenon is fully aware of the illusion. Consequently, the syndrome does not encompass delusional cases.
According to V.S. Ramachandran and William Hirstein, “[a]lmost everyone who has a limb amputated will experience a phantom limb” [1, 1608]. Yet, the scientific community has struggled to understand the phenomenon from a neurological level. It is known that the phenomenon derives from the brain and spinal cord, and is not a purely psychological condition. Areas of the brain formerly linked to nerves of the missing limb have been monitored using positron emission tomography and magnetic resonance imaging. During phantom pain experiences, activity has been observed in these areas, suggesting that mixed signals from the brain are a causal factor of the phenomenon. The areas of the brain and spinal cord which were once specifically allocated for accepting and processing inputs from the now amputated limb may adapt to the amputation erratically. Researchers believe that this instability can lead to feelings of pain which serve to warn the body of abnormality. Additionally, the brain may “rewire” the regions formerly dedicated to the missing limb to a different body part, causing the feeling of pain in the amputated limb when another area of the body is actually stimulated.
The lack of a discrete explanation for phantom limbs is most evident when examining cases involving congenital phantoms, occurring in individuals who are born without a limb or body part. These cases cannot be explained by the activity of nerves associated with amputated limbs as they never developed. A single, specific case study by Ramachandran and Hirstein examined a woman born without either of her arms who claimed to feel the sensation of phantom arms, could describe the relative lengths of her phantom arms, and could discuss the ways the swung when walking.
The congenital phenomenon is addressed to a greater extent in a study by Ronald Melzack et al. The study concludes that 19.7 percent of participants “with congenital limb deficiency met the criteria for the experience of a phantom limb” [2, 1606]. It is believed also that the phantom sensation experienced by congenital patients is due to stimulations of reafference signals targeted to the phantom limbs during gesticulation. As noted by Ramachandran and Hirstein, “what is remarkable … is that the neural circuitry generating these gesticulatory movements is ‘hardwired’ and has actually survived intact in the absence of any direct visual or kinaesthetic reinforcement” [1, 1606]. While studies like the ones performed by Melzack and Ramachandran and Hirstein offer insight on what is stimulating phantom sensations in congenital patients, they simultaneously propose more questions. Despite the similar symptoms experienced by congenital patients and amputees, the two subcategories of phantom limb syndrome require completely different explanations.
Nevertheless, one linking factor between all forms of phantom limb is psychology. While discrete neurons likely play heavily into experiences of phantom limb syndrome, there is still a psychological component associated with congenital, and to a certain degree, all, cases. In spite of disproving the common belief that “people who are congenitally limb-deficient or suffer a limb amputation at an early age do not experience phantom limbs,” researchers speculate that watching the behavior and movements of other people’s limbs likely aids in the development of phantom sensations for congenital patients [2, 1603].
Interestingly, studies have shown that phantom limb sensations are generally more intense when limb amputation occurs for a limb that causes pain, as opposed to when non-painful limb is surgically removed. Phantoms resulting from the former tend to be “more vivid, and persist longer”, potentially “due to the greater attention paid to the mutilated or painful limb before it is lost” (1, 1606). While this observation has great implications, it is worth noting that there are no observable differences in the intensity of phantom limb sensation felt by amputees who received amputation due to different reasons or of different body parts. There is also little discrepancy in phantom vividness, according to Sherman, in respect to age, sex, or socioeconomic status [3, 1983]. Prior to amputation, though, those experiencing pain through their limbs are conclusively more likely to develop and conserve strong memories of the pain.
The physical condition of the amputated area after recovery also affects not only the duration but the intensity of the phantom sensations. When pressure is applied to the stump of the removed limb, the phantom pain tends to intensify greatly. In some cases, great amounts of irritation, pressure, and contact on a stump area can serve to rekindle phantom sensations which had previously faded away.
Dr. Benjamin Philip, an instructor within the neuroscience and occupational therapy program at Washington University in St. Louis School of Medicine, notes that there are many implications to identifying a specific cause of phantom limb, as it “is a debilitating syndrome” and “very few treatments exist for the patients who have severe, sometimes life-long pain after amputation.” By pinpointing the cause of phantom pain, Dr. Philip suggests that “we could prevent these patients from developing it” in the first place. “Every patient is unique,” Dr. Philip states, “many patients have no phantom at all; others have pain only, others have sensation only, others feel part of a limb, or can move parts of it, in any combination you can imagine.” Consequently, researching phantom limb is difficult, due to the subjectivity on a patient to patient basis. Dr. Philip points out that “it’s difficult to learn much from individual patients” and that conclusions, such as Ramachandran’s, drawn from a specific case study of congenital limbs, “should be taken with a grain of salt, because they are based on rare cases that are unlike most amputees.” Instead, Dr. Philip provides insight on how research toward identifying a definite cause of phantom limb may be conducted in the future, offering that preemptive studies of patients at risk for amputation would need to be recorded before continuing to “follow their medical and rehabilitation progress along with regular MRI scans or other measurements of their brain’s functional organization” after amputation. With increasing awareness to the common phenomenon, research interests will likely also increase in phantom limb and search for not only its specific neurological causal factors, but also more intuitive treatment remedies for suffering amputees.