Phantom limb is the sensation that an amputated or missing limb is still attached to the body, and these sensations are often painful. Approximately 60-80% of patients with missing limbs experience phantom limb pain (Nikolajsen and Jensen, 2001), with the pain being mostly intermittent but constant in some patients (Kooijman et al., 2000). Patients described the pain as throbbing, stabbing, shooting and burning. Phantom pain is usually felt in the distal parts of limbs (toes and feet for lower leg amputees and fingers and hands for upper limb amputees; Jensen et al., 1985).
What factors contribute to phantom limb pain?
Retrospective studies have found that pre-amputation pain is a risk factor for post-operative pain (e.g. Riddoch, 1941; Houghton et al., 1994), but other studies have shown that this is only a risk factor for pain three months after amputation, but not six months (Nikolajsen et al., 1997).
Other research has demonstrated that post-amputation pain mimics the type and location of pre-amputation pain (Katz and Melzack, 1990). A study by Hill et al. (1996) detailed a case of a patient who had recurring infections in her leg for two years before it was amputated. The patient’s more severe pain pre-amputation was from twice-daily cleaning and packing of a drainage site on her leg, and post-amputation the patient reported feeling phantom pain where the draining site was, although her leg was no longer there. This pain continued for several years following the amputation.
Other research has demonstrated that other factors such as stress, anxiety and weather changes affect occurrences of phantom pain (Nikolajsen and Jensen, 2006). Additionally, some patients reported a decrease in pain after wearing a prosthesis (Weiss et al., 1999).
So what’s happening in the brain in phantom limb patients?
Pioneering research by Wall et al. (1977) looked at single neurons from the dorsal column nuclei (a pair or nuclei, clusters of neurons, in the sensory pathways in the brain stem, at the base of the brain) and found they showed significant changes in receptive field size following limb amputation. They suggested that this change is a result of unmasking synapses (the space between neurons through which chemicals travel) that are normally silent.
Further research has suggested that in the somatosensory cortex, the area of the brain that processes sensory information, there may be changes in patients following amputation. Merzenich et al. (1984) amputated the middle finger of monkeys and found that within two months, the area of cortex corresponding to the use of the finger they removed started responding to touch stimuli to the other middle finger, which was still attached. This suggests that the area is taken over by sensory input from the adjacent finger.
Recent advances in neuroimaging have also made it easier to study the brains of patients following amputation. Using magnetic neuroimaging, neuroscientists have been able to obtain detailed somatosensory maps of the face, hands and other body parts. Yang et al. (1993) found that following amputation, remapping occurs. Using neuroimaging they observed the somatosensory cortex in upper limb amputee patients and found that sensory input from the face and upper arm invaded the hand territory.
So how do we treat phantom limb pain?
The drug treatments of choice for phantom limb pain are tricyclic antidepressants, such as doxepin (drugs used primarily to treat depression, but have been found to help with chronic pain) and sodium channel blockers (drugs which impair the conduction of sodium in the brain).
Research has shown non-medical interventions such as hypnosis, transcranial electrical nerve stimulation (TENs), and acupuncture can also alleviate phantom limb pain (Siegel, 1979; Lundeberg, 1985; Katz and Melzack, 1991).
An interesting treatment, which has had mixed results but is simple and inexpensive, is mirror therapy. The patient places there intact limb into a box of mirrors so it appears to be the amputated limb, allowing them to “move” the phantom limb. A case report by Kim and Kim (2012) details the experience of a 30-year-old male who received an above-elbow amputation eight months after fracturing the ulna and radius in his left forearm. The patient said that he could still feel his removed arm, and experienced cramping and electric-like pain every few minutes. After the patient reported no improvement in symptoms following drug therapy and spinal cord stimulation, he completed four 15-minute mirror therapy sessions a week. After one week, the patient reported the phantoms sensation in his missing limb had changed, and after four weeks he reported the cramping pain had almost gone and his limb felt “normal” again.
Rizzolatti (2006) used mirror neuron theory to explain the fundamentals of mirror therapy. Mirror neurons were first discovered in the premotor cortex in monkeys’ brains, but were later discovered to be in human brains too. A mirror neuron fires when someone performs an action or watches someone else perform the same action. The neuron mirrors the behaviour of others, as if the person themself is committing the action. This occurs so a person cannot just imitate the behaviour of others, but also experience the same sensations and emotional state of the other person. Rizzolatti stated that during mirror therapy, a patient is able to feel the same sensations in their missing limb as their intact limb by observing the mirror image. This decreases pain as it resolves conflict between the motor intention, proprioception and visual system.
Phantom limb pain occurs in the majority of amputee patients, with the type of pain varying between patients but sometimes mimicking pre-amputation pain. Research has demonstrated that in phantom limb patients remapping of the somatosensory cortex occurs as sensory input from other body parts invades the area of the cortex related to the missing limb. Treatment for phantom limb pain includes drug therapy and non-medical interventions such as hypnosis, acupuncture and mirror therapy. More research into this area is needed to develop further treatment for patients with phantom limb pain.
References:
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