Understanding Phantom Limb Pain and Its Relationship with the Nervous System
Abstract
​
Phantom limb pain (PLP) is a common phenomenon that many amputees go through
post-amputation. They perceive pain and great discomfort in the area where a limb no longer
exists. Though the mechanisms for this condition stay poorly understood, it is clear that the
nervous system and its different parts play a huge role in its presence. In this paper, the
neurological, psychological, and cognitive factors that affect PLP will be explored. Concepts
such as cortical reorganization, neuroplasticity, and the body schema development help in
understanding the inner workings of PLP. How different psychological factors affect the pain felt
with PLP will also be explored. Various treatment options, such as mirror therapy, have been
proposed for dealing with this condition. Many of these show great promise and actual results in
reducing the prevalence of PLP. And while these treatments for PLP provide relief and
satisfaction for amputees, more research is essential to understand the mechanisms of PLP more
in depth.
Introduction
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Every year, around 30,000 to 40,000 amputations are performed in the United States.
These procedures can occur due to a wide variety of reasons such as extreme trauma, tumors, or
infection. Pain after the amputation of a limb is a very common sensation to feel and is also
known as phantom limb pain (PLP). This condition is clinically defined as the perception of pain
or discomfort in a limb that’s no longer there (1). Despite having a limb removed, many
amputees suffer from feeling as if their limb was still present, some will even experience extreme
discomfort or burning around the affected area. As common as this phenomenon is, its detailed
mechanisms are really not that well understood. However, it is a fact that the nervous system
plays a key role in the perception of pain after amputation.
​
The nervous system is a complex circuit of neural networks that have specific functions
about communicating and controlling certain bodily responses, it has two main parts. The central
nervous system (CNS) includes the brain and spinal cord, it’s where all sensory information is
received and interpreted. The peripheral nervous system (PNS) contains nerves branching off
from the spinal cord that transmit prescription information to all parts of the body (2). The
nervous system is able to send signals between the brain and the rest of the body. In this way, its
activity controls the ability to move, see, think, and feel things. When a limb is amputated, the
body goes through a significant alteration in its sensory pathways and experiences many changes
that attempt to stabilize the absence of the limb.
​
PLP is a phenomenon that in its core is not very well understood but by studying the
different parts of the nervous system and the roles that they fulfill, it is possible to have a better
understanding of such an impacting condition. This research is vital and dedicated to improving
the quality of life for those who experience PLP.
Discussion
​
The perception of all sensations, including pain, is mediated by the nervous system
(brain, spinal cord, and peripheral nerve branches). When a limb is amputated, the sensory
information that it would send off to the brain is instantly lost. However, with the way that the
brain works, it might continue to receive and interpret the signals from the nerves that were part
of the amputated limb. This is essentially what creates the feeling of the limb still being present
even though there are no more signals of sensation. This can be attributed to the map of the body
that the brain has, or also “body schema”. This concept is understood to be a template of the
whole body in the brain and any change to it, such as amputation, results in the perception of a
phantom limb. This proposed mechanism of PLP was originally developed and proposed by
Head and Holmes in 1912 and was further expanded by Ronald Melzack in 1989. Melzack
introduced the concept of the “neuromatrix and neurosignature” hypothesis. The neuromatrix
consists of a network of neurons in the brain that integrate many inputs from limbic and
somatosensory areas. The neurosignature refers to the patterns of brain activity that are
constantly updated based on awareness and perception of the body. The lack of inputs from the
absent limb to the neuromatrix causes an abnormal neurosignature—creating the feeling of PLP
(3).
Another proposed mechanism of PLP, and also possibly the most cited, is cortical
reorganization. During this reorganization in the brain, the areas representing the limb that has
been amputated are taken over by other zones in the primary somatosensory and motor cortex.
This is a similar concept to neuroplasticity, which is another mechanism heavily thought to
contribute to PLP. This term refers to the ability of the brain to reorganize certain areas and form
new neuron connections as a response to the amputation (4). This process explains why the
afferent nociceptive stimulation of the neurons around the severed area produce the sensation in
the absent limb. Misfiring of those neurons is possible during reorganization, which also results
in the perception of pain in the area.
PLP can also be heavily affected by psychological and cognitive factors in the way that
the pain presents itself or even the intensity of the condition. PLP is much like other chronic pain
syndromes in that episodes are very much influenced by different psychological factors such as
depression, anxiety, or stress (5). The emotional and psychological distress endured by amputees
can greatly contribute to the intensity of PLP, with a lot of individuals experiencing an increase
in painful sensation or discomfort at times of stress. Amputation of a limb is an impactful
experience for an individual and it comes with a lot of psychological changes as well as physical
ones. Moreover, the experience of PLP can be attributed to cognitive factors, such as memory.
Several studies and theories suggest that somatosensory memories for pain have a crucial role in
PLP. Melzack suggested that the pain prior to amputation is encoded and memorized and can
later on become triggered, leading to pain around the residual limb area (6). The effect that all
these psychological and cognitive factors have on PLP only reinforce the idea that the root of the
pain sensation revolves around the nervous system, especially the brain.
One of the most well-known and efficient ways to cope with PLP is the mirror therapy
for amputees. As seen on Figure 1, this treatment consists of placing a mid vertical sagittal
mirror on a table in front of the individual. They would then place their left arm, for example,
alongside the mirror and a reflection of his own arm would be optically superimposed on the
location of the phantom limb, the right arm. The goal of this therapy is to make the individual see
and feel the illusion that the phantom limb has been restored. They can move around while
looking in the mirror and receive visual feedback that their absent limb is obeying their
movements (4). In a study evaluating the efficacy of this treatment, fifteen subjects were asked to
go to this therapy for four weeks. After all tests were completed, it was concluded that the
subjects in the mirror therapy had a significant decrease in pain. Additionally, there was a
decrease in the daily time experiencing PLP (7). This visual feedback helps the individual to
“rewire” the brain by providing input that can greatly reduce the intensity of PLP.
In addition to mirror therapy, various treatments are being developed and explored to
further help those suffering from PLP. The most commonly administered pharmacological
treatments are gabapentin and pregabalin. These are used for PLP because they both affect the
nervous system in reducing the frequency and intensity of neuropathic pain, which is caused by a
dysfunction in the nervous system (8)(9). They do this by targeting nerve injuries and stabilizing
the nerve activity. By binding to calcium channels in nerve cells, these treatments reduce the
release of neurotransmitters that keep contributing to the painful sensation. Settling the nerves
that are sending way too many signals is a way to reduce the PLP (8).
Conclusion
​
All points considered, PLP remains a distressing condition for many amputees and its
explanation lies in the findings regarding the nervous system. As established, PLP takes place
because of the brain’s continuing perception of the limb that has been amputated. This is because
of the nervous system’s attempt to readjust to the loss of any input from that part of the body.
These mechanisms all fall under the discussed cortical reorganization and neuroplasticity in the
brain. On the other hand, psychological factors also greatly affect the intensity and persistence of
PLP. New treatments and coping strategies are under development to improve the quality of life
of amputees who suffer from this condition. They aim to target the brain and its neural pathways
in hopes to stabilize the changes from the nervous system. As PLP remains a complex condition
with not very well understood mechanisms, it is of great importance to continue research on this
topic and work towards more treatment options for those who need it.
References
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StatPearls [Internet]. StatPearls Publishing.
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system?. Eunice Kennedy Shriver National Institute of Child Health and Human
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approaches. Pain research and treatment, 2011.
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Figures
Figure 1. Mirror Therapy