Phantom pain is a perception that an individual experiences relating to a limb or an organ that is not physically part of the body, either because it was removed or was never there in the first place. However, phantom limb sensations can also occur following nerve avulsion or spinal cord injury. Sensations are recorded most frequently following the amputation of an arm or a leg, but may also occur following the removal of a breast, tooth, or an internal organ. Phantom limb pain is the feeling of pain in an absent limb or a portion of a limb. The pain sensation varies from individual to individual. Phantom limb sensation is any sensory phenomenon (except pain) which is felt at an absent limb or a portion of the limb. It has been known that at least 80% of amputees experience phantom sensations at some time of their lives. Some experience some level of this phantom pain and feeling in the missing limb for the rest of their lives. The term "phantom limb" was first coined by American neurologist Silas Weir Mitchell in 1871. Mitchell described that "thousands of spirit limbs were haunting as many good soldiers, every now and then tormenting them". 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".
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. 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. 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
Under ordinary circumstances, the genetically determined circuitry in the brain remains largely stable throughout life. For much of the twentieth century, it was believed that no new neural circuits could be formed in the adult mammalian brain, but experiments from the 1980s onward cast this into doubt. For instance, functional MRI studies in amputees have shown that almost all patients have experienced motor cortical remapping. 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.