The brain’s neural pathways are like a city’s infrastructure. Once the routes and support structures are firmly in place, it is difficult to remove them to construct a new route. This helps explain amputees’ reports of phantom limbs and the painful sensations they radiate. How much of the pain is real and how much is psychological has yet to be determined, but treatments address both sources.

The phantom limb was first documented by Dr. S. Weir Mitchell after observations with Civil War amputees.1 It is a fascinating enigma that has appeared in literature: Captain Ahab’s missing leg in Herman Melville’s Moby Dick, Captain Hook’s lost hand in J. M. Barrie’s Peter Pan, and Long John Silver’s absent leg in Robert Louis Stevenson’s Treasure Island. Why does the brain yearn for the absent limb so much that fantasizes emerge? The answer may reside in ascending sensory pathways from the peripheral nervous system. Once established, the brain finds it difficult to change expected input from these neural pathways.

During infancy, the brain examines the body to understand itself spatially and topologically, building upon this image from the senses throughout life. Interestingly, those who undergo amputations in infancy experience neither the sensation nor the pain of phantom limbs because the missing limbs had not been there long enough to establish a solid pathway.2 However, for those that retain their limbs, the development of the senses in early childhood is faster than at any other point. Changing body image at an advanced age is too drastic and demanding for the brain. One contributing factor is the elderly’s diminished brain size. On average, the brain loses 5-10% of its weight between the ages of 20 and 90, with a higher proportion lost with increasing age.3 In addition, the grooves on the surface widen while the swellings and depressions become smaller. Deep grooves in the brain indicate increased surface area for synapses, the connecting space between neurons, to form. Moreover, the formation of neurofibriallary tangles, decayed portions of the dendrites receiving the sensory information from other neurons, impede information transmission.3 Finally, abnormally hard clusters of damaged or dying neurons, known as “senile plaques,” emerge and accumulate. Neurons are not replaced when they die, so as one gets older, one literally has less to work with. Thus, with decreased plasticity, the body image becomes fixed with one’s brain regressing to the stage formed in earlier years.

This pathway, however, is not indestructible because amputees report that phantom limb sensations decrease with time. Due to the plasticity of the brain, the brain takes time to “rewire” itself by abolishing old connections in favor of new, useful connections elsewhere. For example, after an amputation, patients often describe the entire appendages, with the most awareness at the distal (end) portions of the limb (i.e. fingers and toes compared with the forearms and calves, respectively).4,5 This is because distal anatomical structures contain the greatest amount of sensory nerves and command a larger portion of the somatosensory cortex. In time, however, the phantom limb perception shrinks until it disappears into the stump.6-8

These concepts are visualized by the sensory homunculus (Figure 1), where the size of the appendage reflects the sensitivity and thus concentration of neurons there. Thus, infants use their hands, lips, and tongue frequency in order to shape and understand their world. Since more neurons are dedicated to these extremities, it takes longer to rewire the corresponding pathways. Instead, the brain completely rewires the proximal portions of the limb so that the phantom sensation in the length of the appendage seems to shrink faster than the distal portions.

When subjects encounter identical stimuli, the sensation experienced is usually comparable between them. For example, when we touch a pot on a lit stove, we feel burning and not tickling. With amputees, this precedence doesn’t hold. Each amputee’s phantom limb is unique: it can feel authentic and present but fake, painful, or painless. There is little to suggest that patients are lying about the pain, yet it is well known that the brain frequently tricks the body.

Psychological pain can also manifest itself as physical pain. Amputee patients who feared an inability to recover were hostile to and jealous of other members of society, consequently experiencing pain in the phantom limb with these heightened emotions. However, once these patients underwent therapy and obtained a positive attitude, the pain faded.2

Traditional approaches to alleviating pain, such as injection of nerve blocks, myoelectric prosthesis, and cordotomy, have been more procedural.9,10 A nerve block is an injection of a local anesthetic to stop transmission of a message along the nerve so that the brain never receives the pain signal from the stump. A myoelectric prosthesis is an artificial limb, which uses electronic sensors to translate muscle and nerve activity into the intended movement. While the brain is manipulated into replacing the phantom limb with an artifical one, prosthetics often do not alleviate phantom pain. One theory for this is that since visual sensory information contradicts tactile sensory information, the brain refuses to be tricked. Cordotomy is the most invasive of the procedures listed because it requires a neurosurgeon to disable certain rising tracts in the spinal cord. Thus, it is only employed in severe cancer- or trauma-related cases. Despite the variety of approaches, the results are slightly effective at best.9

Recently, researchers have turned to mind-body therapies to relieve chronic phantom pain, yielding tentatively successful results. A review by Dr. Vera Moura of the Department of Physical Medicine and Rehabilitation on Integrative Medicine at University of North Carolina Hospitals tied together studies that used hypnosis, guided imagery, and biofeedback (such as visual mirror exercises).11 These non-invasive mind-body alternatives consider the psychological aspect of pain. Hypnosis has been found to reduce postsurgical pain, so researchers attempted to transfer its effects to amputees.12 In several studies, arm amputees varying in sex and age saw a reduction in pain frequency and intensity after attending hypnosis sessions.13-15 These studies indicate that mind can truly triumph over matter, but caution must be taken because trial sizes were small and hypnosis is a murky field. Therefore, more research is necessary before any definite conclusions can be made.

Guided imagery is another mind-body approach that extends beyond the typical denotation of our senses, and it utilizes more neural pathways than normal to create a memorable, mental image. This treatment combines interactions between patient and therapist and patient and body image.9 In Zuckweiler’s experiment, 14 patients with diverse backgrounds had 5 to 15 imagery sessions, during which they attempted to reprogram their minds to accept the new body form.16 Patients were taught Zuckweiler Image Imprinting (ZIP), which involves taking an object and storing it as a mental image. They were then asked to compare their phantom limb pain to the object in their mental image and switch the sensations associated with the two objects. Over time, as the phantom sensation decreased by using different mental images, the discrepancy between the new body image without the limb and old body image with the limb was reconciled. Zuckweiler’s study showed successful pain intensity reduction within only six months. ZIP forces patients’ minds to accept their new bodies. Since his method encompasses visual, auditory, and kinesthetic learning, customized treatment allows patients to comprehend and create new connections.

The final mind-body approach is biofeedback, of which there are two popular kinds. Thermal biofeedback teaches patients to increase the peripheral skin temperature at the stump.17 This seems unlikely, since body temperature is an autonomic function along with vital processes such as heart rate and breathing. In some instances, however, an individual can have partial, conscious control. Although the hypothalamus is responsible for standard body temperature of 37.0°C (98.6°F), it is possible for consciousness to affect peripheral skin temperature. Successful patients begin to link skin temperature with pain.18 Physiologically, the regulation of one function often results in the coupling of the response to a stimulus. For example, thermal biofeedback was coupled with breathing relaxation techniques, which caused the temperature of the stump to increase and relax, decreasing the pain and thus increasing the patient’s ability to contend with remaining pain. It is unknown, however, if thermal biofeedback is an effective treatment for all phantom pain; like most areas of science and medicine, more research is needed.

The second biofeedback type, visual mirror feedback (VMF), uses a box with mirrors to fool the brain. A rectangular box with no top and two holes for each arm (or leg) is set in front of a patient. In the middle of the box is a one-sided mirror septum facing the limb that is intact (Figure 2). Patients are thus presented with the illusion that both appendages are whole. Dr. Ramachandran, the inventor of this technique, conducted a study in which 10 amputee patients were treated with VMF in six sessions of 5 to 15 minutes a day for several weeks.19 Every patient had a positive reaction that included reduced pain, pain intensity, mobility restriction, and spasms. Once again, there was a conscious effort to train the brain, so patients were able to redirect unpleasant sensations. This therapy is almost opposite to ZIP since the patient is picturing the limb as whole to alleviate the pain rather than ignoring it. VMF treatment is one of the most common due to its success amongst many different amputees.

A theory behind mind-body approaches’ emerging successes is the conscious effort patients put forth to overcome pain. In previously mentioned traditional procedural methods, patients passively receive a certain treatment and hope to obtain a positive result. In some cases, there are even negative side effects; for example, a nerve block may lead to rashes, itching, and an abnormal rise in blood sugar. Invasive procedural approaches like the cordotomy can only be attempted once. Mind-body approaches can be practiced, optimized over time, and are much safer than procedural methods.

Understanding of phantom pain has progressed significantly since its initial documentation during the Civil War. Traditional procedural methods to treat it have been developed, but recently, the psychological aspect of pain and sensation has been addressed in mind-body methods. Unfortunately, neither approach has achieved complete success, partially because of the individualistic nature of phantom limbs and the associated pain. The neurological explanations behind both phenomena are relatively unknown, but it is agreed the ghostly perceptions are a mixture of psychological and real sensations. Perhaps the most effective treatments are those that address both.

References

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