The Phantom Menaceby Sara Adaes, PhD | September 16, 2014
After the amputation of a body part, patients frequently feel that the amputated area is still present. Sensation of the position and movement of the limb, as well as of heat, cold, itching, and even pain, are often described for a limb that is no longer present.
Pain in a limb that has been amputated is known as “phantom limb pain”. According to statistical data, it can occur in up to 80% of all amputees, although to varying extents. The incidence of phantom limb pain seems to increase with age. In congenital amputees, there are occasional reports of phantom limb pain arising later in life, but these are extremely rare. Phantom limb pain is also very rare in children who are amputated at a very young age. Older children, on the other hand, show an incidence of phantom limb pain that is much closer to that of adults.
Whether phantom limb pain diminishes over time after the amputation is unclear. There are reports indicating that long-term amputees maintain a similar degree of pain, while others indicate that there is a slight decline over the years. Even so, it is but a slight decrease; this type of pain is most likely to prevail for life.
Given its incidence and the degree of disability it causes, the neurobiological mechanisms of phantom limb pain are naturally a matter of great interest. Changes in both the amputation site and in the central nervous system are thought to contribute to the generation of pain.
Locally, one of the paramount mechanisms is the generation of spontaneous neuronal discharges at the site of amputation. When nerves are cut or injured, swelling, growth and sprouting of axons occurs as an attempt to reconnect the cut nerve. In the case of an amputation, nerves have nothing to reconnect to, which leads to the formation of a twisted mass of axons known as a neuroma. These disorganized nerve fibers in neuromas have an increased excitability that leads to exacerbated pain due to stimulation of those areas; these nerve fibers can actually even fire without any stimulation, leading to spontaneous pain. This abnormal discharge of neurons, originating from areas other than nerve endings, is known as ectopic activity.
Due to the massive neurochemical changes that an amputation induces, this ectopic activity can even arise in neuronal structures other than the cut terminal, such as the cell body of the neuron. This makes it harder to eliminate pain that could otherwise be treated with a local anesthetic, for example.
These changes in the peripheral nervous system lead to changes in the central nervous system. Neurons in the spinal cord also become more responsive and the mechanisms of pain modulation and inhibition become less effective, contributing to the increased sensation of pain.
One important question concerning phantom limb pain is why is this pain being perceived as arising from a body part that is no longer there. It is believed that this is a consequence of changes in the somatosensory cortex in the brain, the area where touch and pain are perceived. The primary sensory cortex allows the identification of the body part from which pain is arising by being organized as a “map” representing the body. After amputation, there may be a shift in the representational scheme of the somatosensory cortex. This means that, for example, what used to represent the elbow now erroneously represents the hand; consequently, pain that is originating from the stump located next to the elbow will be perceived as originating from the hand, the phantom limb.
Due to its characteristics, the pharmacological treatment of phantom limb pain widely overlaps with that used for neuropathic pain conditions. However, these are only moderately effective.
An alternative therapy that has shown promising results is mirror therapy. In this procedure, a visual reconstruction of the lost limb is produced using a mirrox box that allows the reflection of the intact limb to visually replace the missing limb. After a continued use of this approach, significant decreases in pain have been obtained. This therapy is thought to act by allowing the restitution of the correct representation of the missing limb in the brain, although this is still unclear.
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