| Phantom limb | |
|---|---|
| Classification and external resources | |
| ICD-10 | G54.6-G54.7 |
| ICD-9 | 353.6 |
| DiseasesDB | 29431 |
| MeSH | D010591 |
A phantom limb is the sensation that an amputated or missing limb (even an organ, like the appendix) is still attached to the body and is moving appropriately with other body parts.[1][2][3] 2 out of 3 combat veterans report this feeling. Approximately 60 to 80% of individuals with an amputation experience phantom sensations in their amputated limb, and the majority of the sensations are painful.[4] Phantom sensations may also occur after the removal of body parts other than the limbs, e.g. after amputation of the breast, extraction of a tooth (phantom tooth pain) or removal of an eye (phantom eye syndrome). The missing limb often feels shorter and may feel as if it is in a distorted and painful position. Occasionally, the pain can be made worse by stress, anxiety, and weather changes. Phantom limb pain is usually intermittent. The frequency and intensity of attacks usually decline with time.[5]
A slightly different sensation known as phantom pains can also occur in people who are born without limbs and people who are paralyzed.[6] Phantom pains occur when nerves that would normally innervate the missing limb cause pain. It is often described as a burning or similarly strange sensation and can be extremely agonizing for some people, but the exact sensation differs widely for individuals. Other induced sensations include warmth, cold, itching, squeezing, tightness, and tingling.[3][7]
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Although not all phantom limbs are painful, patients will sometimes feel as if they are gesturing, feel itches, twitch, or even try to pick things up. For example, Ramachandran and Blakeslee describe that some people's representations of their limbs do not actually match what they should be, for example, one patient reported that her phantom arm was about "6 inches too short".[7]
Some people with phantom limbs find that the limb will gesticulate as they talk. (But whether they feel the weight of the phantom limb while gesticulating is unclear). Given the way that the hands and arms are represented on the motor cortex and language centers, this is not surprising. Some people find that their phantom limb feels and behaves as though it were still there; others find that it begins to take on a life of its own, and does not obey their commands.
I placed a coffee cup in front of John and asked him to grab it [with his phantom limb]. Just as he said he was reaching out, I yanked the cup away.
"Ow!" he yelled. "Don't do that!"
"What's the matter?"
"Don't do that", he repeated. "I had just got my fingers around the cup handle when you pulled it. That really hurts!"
Hold on a minute. I wrench a real cup from phantom fingers and the person yells, ouch! The fingers were illusory, but the pain was real - indeed, so intense that I dared not repeat the experiment.— Ramachandran, Phantoms in the Brain, p. 43.[7]
Until recently, the dominant theory for cause of phantom limbs was irritation in the severed nerve endings (called "neuromas"). When a limb is amputated, many severed nerve endings are terminated at the residual limb. These nerve endings can become inflamed, and were thought to send anomalous signals to the brain. These signals, being functionally nonsense, were thought to be interpreted by the brain as pain. Treatments based on this theory were generally failures. In extreme cases, surgeons would perform a second amputation, shortening the stump, with the hope of removing the inflamed nerve endings and causing temporary relief from the phantom pain. But instead, the patients' phantom pains increased, and many were left with the sensation of both the original phantom limb, as well as a new phantom stump, with a pain all its own.[7] In some cases, surgeons even cut the sensory nerves leading into the spinal cord or in extreme cases, even removed the part of the thalamus that receives sensory signals from the body.
By the late 1980s, Ronald Melzack had recognized that the peripheral neuroma account could not be correct, and suggested that changes in a "neuromatrix" was responsible for phantom limb sensations, but explicitly rejected the idea that these changes occurred in primary somatosensory areas.[8] In 1991, Tim Pons and colleagues at the National Institutes of Health (NIH) showed that the primary somatosensory cortex undergoes substantial reorganization after the loss of sensory input.[9] Hearing about these results, Vilayanur S. Ramachandran theorized that phantom limb sensations could be due to this reorganization in the somatosensory cortex, which is located in the postcentral gyrus, and which receives input from the limbs and body.[3][7] Ramachandran and colleagues illustrated this theory by showing that stroking different parts of the face led to perceptions of being touched on different parts of the missing limb.[10]
Ramachandran argued that the perception of being touched in different parts of the phantom limb was the perceptual correlate of cortical reorganization in the brain. However, research published in 1995 by Flor et al. demonstrated that pain (rather than referred sensations) was the perceptual correlate of cortical reorganization.[11] In 1996 Knecht et al. published an analysis of Ramanchandran's theory that concluded that there was no topographic relationship between referred sensations and cortical reorganization in the primary cortical areas[12] Recent research by Flor et al. suggests that non-painful referred sensations are correlated with a wide neural network outside the primary cortical areas.[13]
Because phantom limb pain (PLP) is a complex phenomenon that produces a wide variety of symptoms researchers have advanced a number of theories during the past twenty years. It is important to note that many neurological researchers now distinguish between PLP and phantom limb sensations such as movement and position.[14] V.S. Ramachandran, who brought the subject of phantom limbs to a broad audience with his book Phantoms In The Brain, has advanced a number of theories to explain phantom limb pain. In particular Ramachandran and Hirstein[3] have linked phantom limb pain to both cortical and peripheral mechanisms with a five source model that included 1) residual limb neuromas, 2) cortical remapping, 3) corollary discharge, 4) body image, and 5) somatic memories.[14]
In 2009 Lorimer Moseley and Peter Brugger carried out a remarkable experiment in which they encouraged seven arm amputees to use visual imagery to contort their phantom limbs into impossible configurations. Four of the seven subjects succeeded in performing impossible movements of the phantom limb. This experiment suggests that the subjects had modified the neural representation of their phantom limbs and generated the motor commands needed to execute impossible movements in the absence of feedback from the body.[15] The authors stated that:"In fact, this finding extends our understanding of the brain's plasticity because it is evidence that profound changes in the mental representation of the body can be induced purely by internal brain mechanisms--the brain truly does change itself."
Theories that attempt to account for phantom limb pain fall into two broad classes: those that point toward peripheral nervous system mechanisms and those that point toward central nervous system mechanisms. The most commonly cited explanation for phantom limb pain is cortical reorganization.[14] Pain researchers such as Herta Flor and Lorimer Mosely have developed theories of phantom limb pain that rely on the idea of cortical reorganization.[16][17] Recently a group of researchers headed by Jack Tsao, MD. at Walter Reed Hospital has advanced a theory based on the concept of "proprioceptive memory." This theory argues that the brain retains a memory of specific limb positions and that after amputation there is a conflict between the visual system, which literally sees that the limb is missing, and the memory system which remembers the limb as a functioning part of the body.[14]
In 2006, Herta Flor suggested that phantom limb pain involves a form of unconscious learning similar to motor reflexes and perception skills. Flor theorized that preventing the formation of painful memories might be the most effective approach to preventing phantom limb pain. She conducted an experiment in which patients who were scheduled to undergo amputations were given memantine, a drug that blocks the activity of NMDA receptors, which are important to many types of learning and memory. Thirteen patients took memantine for four weeks after their amputations. These patients were then studied over the course of a year. The results of the this study indicated that changes in the brain were reduced and that the incidence and intensity of phantom limb pain was reduced.[18]
Some treatments include drugs such as antidepressants. Spinal cord stimulation (SCS) can be effective treatment for phantom pain. An electrical stimulator is implanted under the skin, and an electrode is placed next to the spinal cord. The nerve pathways in the spinal cord are stimulated by an electric current. This interferes with the impulses travelling towards the brain and lessens the pain felt in the phantom limb.[2] Instead, amputees feel a tingling sensation in the phantom limb.
Vibration therapy, acupuncture, hypnosis, and biofeedback may all be used to treat phantom pain but are often of little help. The pain can sometimes be helped by keeping busy and focusing attention on something else. Massaging the stump can sometimes help. For planned amputation, phantom pain can be reduced by preoperative pain management, effective control of pain by analgesic or neuroleptic is required. The brain seems to implant the sensations from the preoperative state.
One particularly novel treatment for phantom limb pain is the mirror box developed by Vilayanur Ramachandran and colleagues.[19][20] Through the use of artificial visual feedback it becomes possible for the patient to "move" the phantom limb, and to unclench it from potentially painful positions. Repeated training in some subjects has led to long-term improvement, and in one exceptional case, even to the complete elimination of the phantom limb between the hand and the shoulder (so that the phantom hand was dangling from the shoulder).
The success of the mirror method inspired a team of researchers at the University of Manchester in England to experiment a technology of "immersive virtual reality" to combat the discomfort caused by phantom limb syndrome.[21][22] The researchers reported that phantom limb pain can be relieved by attaching the sufferer's real limb to an interface that allows them to see two limbs moving in a computer-generated simulation. This works on the same principle as the mirror box technique in that the somatosensory cortex is being 'tricked', except that the computer created illusion is thought to be stronger. Another virtual reality research was reported in 2009.[23]
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