Phantom pain is a painful 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.
Sensations are reported most frequently following the amputation of a limb, but may also occur following the removal of a breast, tongue, or internal organ. Phantom eye syndrome can occur after eye loss. The pain sensation and its duration and frequency varies from individual to individual.
Comparisons with related terms
Phantom pain should be distinguished from other conditions that may present similarly, such as phantom limb sensation and residual limb pain.
Phantom limb sensation Phantom limb sensation is any sensory phenomenon, except pain, which is felt at an absent limb or a portion of the limb. It is estimated that up to 80% of amputees experience phantom limb sensations at some time of their lives. Some experience some level of this phantom feeling in the missing limb for the rest of their lives.
Residual limb pain Residual limb pain, also referred to as stump pain, is a painful perception that originates from the residual limb, or stump, itself. It is typically a manifestation of an underlying source, such as surgical trauma, neuroma formation, infection, or an improperly fitted prosthetic device. Although these are different clinical conditions, individuals with phantom pain are more likely to concomitantly experience residual limb pain as well.
Signs and symptoms The symptomatic course of phantom pain is widely variable, but the onset often presents within the first week after amputation. The reported pain may be intermittent and lasting seconds to minutes, but can be continuous with acute exacerbations. The duration of symptoms varies among individuals, with some reporting decreased pain over time and others reporting a more stable or even increasing trajectory. Sensations may be described as shooting, stabbing, squeezing, throbbing, tingling, or burning, and sometimes feels as if the phantom part is being forced into an uncomfortable position.
While the sensation often affects the part of the limb farthest from the body, such as the fingers or toes, other body parts closer to the brain, such as the arm or leg, can still experience similar sensations. It is thought that phantom pain more commonly involves the part of the limb farthest from the body because of its larger cortical representation within the somatosensory cortex.
Triggers Overall, the sensations may be triggered by pressure on the remaining part of the limb, emotional stress, or changes in temperature.
Causes Individuals may experience phantom pain following surgical or traumatic amputation of a limb, removal of an organ, or in instances of congenital limb deficiency. It is most commonly observed after amputation, although less frequent cases have been reported following the removal of a breast, tongue, or eye. Phantom pain is seen more often in older adults as compared to individuals with congenital limb deficiency or amputation at an early age. It has also been reported that individuals with a prior history of chronic pain, anxiety, or depression are more likely to develop phantom pain than those without these risk factors.
Pathophysiology The neurological basis and mechanisms for phantom pain are all derived from experimental theories and observations. Little is known about the true mechanisms causing phantom pain, 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 may contribute to phantom pains, they are not the sole cause. The reason is because patients with congenital limb deficiency can sometimes also experience phantom pains. This finding suggests that there is also a central mechanism responsible for generating painful sensations.
Currently, theories are based on altered neurological pathways and maladaptive changes within the peripheral nervous system, spinal cord, and brain.
Peripheral mechanisms Neuromas formed from injured nerve endings at the stump site show increased sodium channel expression and are able to spontaneously fire abnormal action potentials. This increased activity of Aδ and C fibers, which are involved in pain and temperature sensation, can contribute to phantom pain. However, it has been noted that pain still persists once the neuromas have ceased firing action potentials or when peripheral nerves are treated with conduction blocking agents. The peripheral nervous system is therefore thought to have at most a modulation effect on phantom limb pain.
Spinal mechanisms
In addition to peripheral mechanisms, spinal mechanisms are thought to have an influencing role in the development of phantom pain. 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. These changes to the nerve fiber inputs may also lead to an expansion of the neuronal receptive fields, such that previously non-noxious stimuli are now interpreted as noxious. This process of hyperexcitability and receptive field changes is broadly referred to as central sensitization. It is also known that increased expression of glutamate and NMDA, coupled with decreased inhibition from GABAergic neurons, further contributes to the mechanism of central sensitization. However, because patients with complete spinal cord injury have experienced phantom pain, there must also be an underlying central mechanism within the brain.
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 cortical remapping. After amputation, cortical remapping is the process by which areas of the somatosensory and primary motor cortices representing the lost limb are invaded by nearby regions. This allows areas of the brain formerly receiving input from the lost limb to be stimulated from the nearby invading cortical regions.
Most of the studies using functional MRI to investigate cortical remapping in humans have been in upper limb amputees. Following the loss of an arm, the majority of motor reorganization occurred as a downward shift of the hand area of the cortex onto the area of face representation, especially the lips. In individuals 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. Additionally, as phantom pains in upper extremity amputees increased, there was a higher degree of medial shift of the facial motor representation. It has also been found that there is a high correlation between the severity of phantom limb pain and the extent to which cortical reorganization has occurred.
The neuromatrix The neuromatrix theory, initially coined by psychologist Ronald Melzack in the 1990s, proposes that there is an extensive network connecting the thalamus and the cortex, and the cortex and the limbic system. It extends beyond body schema theory and 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. 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. 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. After limb amputation, changes to the neuromatrix and neurosignature may be the cause of phantom pain localized to the lost limb. Phantom pain may also arise from abnormal reorganization in the neuromatrix to a pre-existing pain state.
Support for the neuromatrix theory is largely from studies where cordotomy, and therefore elimination of pain signals transmitted to the brain, fail to treat phantom pains. Opposition to the 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. In addition, a major limitation of the neuromatrix theory is that it is difficult to be tested empirically, especially when testing painless phantom sensations.
Diagnosis Phantom pain is a clinical diagnosis based on the signs and symptoms an individual describes. There are no specific laboratory studies or imaging findings that support its diagnosis. However, it is important for a doctor to perform a thorough physical examination to assess for other potential causes of pain. Evaluation of the residual limb should be done to inspect for signs of infection, bursa or pressure ulcer formation, or deep tissue injury. If an individual has a prosthesis, evaluation of fit and alignment should also be performed. A thorough neurological and musculoskeletal examination should be conducted, including assessment of strength, range of motion, and reflexes, to rule out any other central or peripheral causes for the pain.
Management Various methods have been used to treat phantom limb pain. There are currently no specific management guidelines or stepwise approaches to therapy. Instead, treatment involves a multimodal approach with a variety of available interventions. Doctors may prescribe medications, and some antidepressants or antiepileptics have been shown to have a beneficial effect on reducing phantom limb pain. Physical methods such as light massage, electrical stimulation, and hot and cold therapy have been used with variable results.
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 difficult to investigate. 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 of less than 30%. This rate of success does not exceed the placebo effect.
Medication