Phantom pain

The term “phantom limb” was first coined by American military surgeon Silas Weir Mitchell in 1871. Mitchell described that “thousands of spirit limbs were haunting as many good soldiers, every now and then tormenting them” (Bittar et al, 2005). However, in 1551, French military surgeon Ambroise Paré recorded the first documentation of phantom limb pain when he reported that, “For the patients, long after the amputation is made, say that they still feel pain in the amputated part” (Bittar et al, 2005).

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Epidemiology

Phantom limb pain and phantom limb sensations are linked, but must be differentiated from one another. While phantom limb sensations are experienced by those with congenial limb deficiency, spinal cord injury, and amputation, phantom limb pain occurs almost exclusively as a result of amputation (Kooijman et al, 2000). Almost immediately following the amputation of a limb, 90-98% of patients report experiencing a phantom sensation. Nearly 75% of individuals experience the phantom as soon as anesthesia wears off, and the remaining 25% of patients experience phantoms within a few days or weeks (Ramachandran and Herstein, 1998). Of those experiencing innocuous sensations, a majority of patients also report distinct painful sensations.

The prevalence of phantom limb pain differs based on the location of the amputation. The prevalence of phantom pain in upper limb amputees is nearly 82%, while the prevalence of pain in lower limb amputees is only 54% (Kooijman et al, 2000). Age and gender have not been shown to affect the onset or duration of phantom limb pain. Although it has not been fully explored, one investigation of lower limb amputation observed that as stump length decreased, there was a greater incidence of moderate and severe phantom pain (Bittar et al, 2005).

Neurological Basis and Mechanisms for Phantom Limb Pain

The neurological basis and mechanisms for phantom limb pain are all derived from experimental theories and observations. Little is known about the true mechanism causing phantom pains, and many theories highly overlap. Historically, phantom pains were thought to originate from neuromas located at the stump tip. Traumatic neuromas, or non-tumor nerve injuries, often arise from surgeries and result from the abnormal growth of injured nerve fibers. Although stump neuromas contribute to phantom pains, they are not the sole cause. This is because patients with congenital limb deficiency can sometimes, although rare, experience phantom pains. This suggests that there is a central representation of the limb responsible for painful sensations (Ramachandran and Herstein, 1998). Currently, theories are based on altered neurological pathways and cortical reorganization. Although they are highly intertwined, mechanisms are often separated into peripheral, spinal, and central mechanisms.

Peripheral Mechanisms

Neuromas formed from injured nerve endings at the stump site are able to fire abnormal action potentials, and were historically thought to be the main cause of phantom limb pain. Although neuromas are able to contribute to phantom pain, pain is not completely eliminated when peripheral nerves are treated with conduction blocking agents (Ramachandran and Herstein, 1998). Physical stimulation of neuromas can increase C fiber activity, thus increasing phantom pain, but pain still persists once the neuromas have ceased firing action potentials. The peripheral nervous system is thought to have at most a modulation effect on phantom limb pain (Bitter et al, 2005)

Spinal Mechanisms

In addition to peripheral mechanisms, spinal mechanisms are thought to have an influencing role in phantom pains. Peripheral nerve injury can lead to the degeneration of C fibers in the dorsal horn of the spinal cord, and terminating A fibers may subsequently branch into the same lamina (Bittar et al, 2005). If this occurs, A fiber inputs could be reported as noxious stimuli. Substance P, involved in the transmission of pain signals, is usually expressed by Aδ and C fibers, but following peripheral nerve damage, substance P is expressed by Aβ fibers (Bittar et al, 2005). This leads to hyperexcitability of the spinal cord, which usually occurs only in the presence of noxious stimuli. Because patients with complete spinal cord injury have experienced phantom pains, there must be an underlying central mechanism responsible for the generation of phantom pains.

Central Mechanisms and Cortical Remapping

Under ordinary circumstances, the genetically determined circuitry in the brain remains largely stable throughout life. It was thought, until about 30 years ago, that no new neural circuits could be formed in the adult mammalian brain (Ramachandran and Hirstein, 1998). Recently, functional MRI studies in amputees have shown that almost all patients have experienced motor cortical remapping (Cruz et al, 2003). The majority of motor reorganization has occurred as a downward shift of the hand area of the cortex onto the area of face representation, especially the lips. Sometimes there is a side shift of the hand motor cortex to the ipsilateral cortex (Cruz et al, 2003). In patients with phantom limb pain, the reorganization was great enough to cause a change in cortical lip representation into the hand areas only during lip movements (Cruz et al, 2003). It has also been found that there is a high correlation between the magnitude of phantom limb pain and the extent to which the shift of the cortical representation of the mouth into the hand area in motor and somatosensory cortical reorganization has occurred (Karl et al, 2001). Additionally, as phantom pains in upper extremity amputees increased, there was a higher degree of medial shift of the facial motor representation (Karl et al, 2001). There are Multiple theories that try to explain how cortical remapping occurs in amputees, but none have been supported to a great extent.

The Neuromatrix

The neuromatrix theory proposes that there is an extensive network connecting the thalamus and the cortex, and the cortex and the limbic system (Bittar et al, 2005). It is a theory that extends beyond body schema theory and incorporates the conscious awareness of oneself. This theory proposes that conscious awareness and the perception of self are generated in the brain via patterns of input that can be modified by different perceptual inputs (Glummarra et al, 2007). The network is genetically predetermined, and is modified throughout one’s lifetime by various sensory inputs to create a neurosignature. It is the neurosignature of a specific body part that determines how it is consciously perceived (Bittar et al, 2005). The input systems contributing to the neurosignature are primarily the somatosensory, limbic, and thalamocortical systems. The neuromatrix theory aims to explain how certain activities associated with pain lead to the conscious perception of phantom pain. The persistence of the neurosignature, even after limb amputation, may be the cause of phantom sensations and pain. Phantom pain may arise from abnormal reorganization in the neuromatrix to a pre-existing pain state (Melzack, 1992).

Opposition to the neuromatrix theory exists largely because it fails to explain why relief from phantom sensations rarely eliminates phantom pains. It also does not address how sensations can spontaneously end and how some amputees do not experience phantom sensations at all (Bittar et al, 2005). In addition, a major limitation of the neuromatrix theory is that it too broadly accounts for various aspects of phantom limb perception. It is also likely that it is too difficult to be tested empirically, especially when testing painless phantom sensations (Glummarra et al, 2007).

Treatment Options

There are many different treatment options for phantom limb pain that are actively being researched. Most treatments do not take into account the mechanisms underlying phantom pains, and are therefore ineffective. However, there are a few treatment options that have been shown to alleviate pain in some patients, but these treatment options usually have a success rate less than 30% (Bittar et al, 2005). It is important to note that this rate of success does not exceed the placebo effect. It is also important to note that because the degree of cortical reorganization is proportional to phantom limb pains, any perturbations to the amputated regions may increase pain perception (Bittar et al, 2005).

Non Surgical Techniques

Mirror Box Therapy

Mirror box therapy allows for illusions of movement and touch in a phantom limb by inducing somatosensory and motor pathway coupling between the phantom and real limb (Glummarra et al, 2007). Many patients experience pain as a result of a clenched phantom limb, and because phantom limbs are not under voluntary control, unclenching becomes impossible (Ramachandran and Rogers-Ramachandran, 1996). The theory behind the mirror box treatment is that the brain has become accustomed to the fact that a phantom limb is paralyzed because there is no feedback from the phantom back to the brain to inform it otherwise. Ramachandran and Rogers-Ramachandran believed that if the brain received visual feedback that the limb had moved, then the phantom limb would become unparalyzed (Ramachandran and Rogers-Ramachandran, 1996).

To create the visual feedback, mirror boxes are constructed to create an illusion of a second limb. The mirror box is constructed so that it has a vertical mirror placed in the center, and the lid remains off. The intact limb is placed on one side of the mirror, and in the patient’s sight, while the amputated limb is placed on the other side, out of sight. The patient sees an intact second limb through the mirror and sends motor commands to both limbs to make symmetric movements. The movement gives the brain positive feedback that the phantom has moved, and it becomes unparalyzed (Ramachandran and Rogers-Ramachandran, 1996).

In a study of ten patients with upper phantom limb paralysis, nine patients were able to move the phantom limb, and eight of the patients able to move the phantom limb had their pain alleviated (Ramachandran and Rogers-Ramachandran, 1996). Since Ramachandran and Ramachandran’s pioneer study, there have been multiple additional studies to support the mirror box findings for patients with upper limb phantom pain. MacLachlan, McDonald, and Walcoch presented the first case of mirror box treatment for lower limb phantoms in 2004. The patient, Alan, experienced a painful crossing of his toes in the morning, and the pain worsened as the day progressed. After three weeks of mirror box treatment twice a day, Alan no longer felt any painful sensations from crossed toes (MacLachlan, McDonald, and Walcoch, 2004).

Pharmacological Treatment

Pharmacological techniques are often continued in conjunction with other treatment options. Doses or pain medications needed often drop substantially when combined with other techniques, but rarely are discontinued completely. Tricyclic antidepressants, such as amitriptyline, and sodium channel blockers, mainly carbamazepine, are often used to relieve chronic pain, and recently have been used in an attempt to reduce phantom pains. Pain relief may also be achieved through use of opioids, ketamine, calcitonin, and lidocaine (Bittar et al, 2005).

Coping Strategies

Physical and sociopsychological factors are thought to have been involved with the presence of phantom limb pain for nearly 20 years (Richardson et al, 2007). Emotional states such as anxiety and depression are thought to predispose, trigger and maintain painful sensations in amputees. In a study involving 59 patients with lower limb amputation, pre-amputation coping strategies were found to be associated with the presence of phantom limb pain, but did not affect the intensity or duration of the pain. Most often, passive coping strategies prior to amputation correlated to an increase in the presence of phantom pain (Richardson et al, 2007). Passive coping strategies include praying and hoping, and catastrophizing, while active coping strategies include diverting attention, ignoring sensations, reinterpreting the pain sensation, coping self-statements, and increased behavioral activities. Active coping strategies were not shown to correlate with the presence of phantom limb pain (Richardson et al, 2007).

Surgical Techniques

Deep-Brain Stimulation

Deep brain stimulation is a surgical technique used to alleviate patients from phantom limb pain. Prior to surgery, Patients undergo functional brain imaging techniques such as PET scans and functional MRI to determine an appropriate trajectory of where pain is originating. Surgery is then carried out under local anesthetic, because patient feedback during the operation is needed. In the study conducted by Bittar et al, a radiofrequency electrode with four contact points was placed on the brain. Once the electrode was in place, the contact locations were altered slightly according to where the patient felt the greatest relief from pain. Once the location of maximal relief was determined, the electrode was implanted and secured to the skull. After the primary surgery, a secondary surgery under general anesthesia was conducted. A subcutaneous pulse generator was implanted into a pectoral pocket below the clavicle to stimulate the electrode (Bittar et al, 2005). It was found that all three patients studied had gained satisfactory pain relief from the deep brain stimulation. Pain had not been completely eliminated, but the intensity had been reduced by over 50% and the burning component had completely vanished (Bittar et al, 2005).

See also

• Phantom eye syndrome
• Phantom limb

References

• Bittar, Richard G.; Otero, Sofia & Carter, Helen et al. (May, 2005), " ", Journal of Clinical Neuroscience 12 (4): 399-404

• Cruz, Vitor Tedim; Nunes, Belina & Reis, Ana Mafalda et al. (2005), " ", NeuroRehabilitation 18 (1): 299-305

• Glummarra, Melita J.; Gibson, Stephen J. & Georgiou-Karistianis, Nellie et al. (April, 2007), " ", Brain Research Reviews 54 (1): 219-232

• Karl, Anke; Birbaumer, Niels & Lutzenberger, Werner et al. (May, 2001), " ", The Journal of Neuroscience 21 (10): 3609-3618

• Kooijman, Carolien M.; Dijkstra, Pieter U. & Geertzen, Jan H. B. et al. (July, 2000), " ", Pain 87 (1): 33-41

• MacLachlan, Malcolm; McDonald, Dympna & Waloch, Justine (2004), " ", Disability and Rehabilitation 26 (14/15): 901-904

• Melzack, R (1992), " ", Scientific American 266 (4): 120-126

• Ramachandran, V. S. & Hirstein, William (2008), " ", Brain 121 (1): 1603-1630

• Ramachandran, V. S. & Rogers-Ramachandran, D. (April, 1996), " ", Proceedings of the Royal Society of London B-Biological Sciences 263 (1369): 377-386

• Richardson, Cliff; Glenn, Sheila & Horgan, Maureen et al. (October, 2007), " ", The Journal of Pain 8 (10): 793-801