Phantom Limbs
Introduction:
The phantom limb phenomenon is the sensation of an amputated or congenitally missing limb. About 90% of amputees will experience phantom sensations with about 75%- 80% of cases the feeling in the missing limb will be excrutiatingly painful (Melzek). If the sensations aren't painful patients will report experiencing annoying or unpleasant sensations. The onset of these feelings can be immediate or take weeks to months for symptoms to occur. The feeling of a phantom limb can be persistent and always there, or only when one consciously thinks about it (Corporeal Awareness).

This condition was once thought to be a psychiatric illness and has been reported since the middle ages (Living with Ghostly limbs). More recently, research has taken off on the subject and scientists have been learning a lot more about the condition. One of the earliest explanations for the phantom limb syndrome was that it was a psychological disorder. This idea has since been proved inaccurate and now is associated with several structures in the nervous system. There are changes seen in symptomatic patients mainly at the level of the cortex, but there is also evidence of peripheral and central mechanisms that can explain the presence of phantom sensations or pain.


homunculus_lateral_to_medial.jpg


The somatosensory system is comprised of pathways and structures in the brain that receive sensory information from throughout the body. Because of this systems association with sensory input and output it is an important part in understanding the phantom limb phenomenon. The somatosensory cortex has a representation of the body called a body map. This body map has a plasticity that plays a major role in explaining the feeling of a limb that isn't actually there (The perception of phantom limbs). A more detailed description of this system is available if you follow the link:
http://kin450-neurophysiology.wikispaces.com/Phantom+Limb






http://occultcorpus.com

Mechanisms:

Peripheral Theories
Theories have been proposed to explain phantom limb sensations at the peripheral level, however continued research has proven these ideas to be incomplete. Even though they cannot account entirely for the ghostly feelings, they may contribute to the sensations. The peripheral theories have been around the longest and has to do with the remaining nerves in the stump of the amputated limb. The ends of these nerves grow into nodules called neuromas which continue to generate impulses. The impulses sent by the nodules flow up through the spinal cord and parts of the thalamus and eventually reach the somatosensory cortex where the impulses could be processed incorrectly and create the illusion of a phantom (phantom limbs).
After a limb is amputated and the nerves are severed deafferenation follows which interrupts the afferent projections to the spinal cord. This is where the neuromas will form. The neuromas are hyperexcitable and send out spontaneous discharges because there is an increased accumulation of molecules in the neuroma. The presence of additional molecules will enhance the expression of sodium channels which is responsible for the excess discharges from the cells of the neuroma.
This is mainly an explanation for the source of stump pain, but has also been proposed as a reason for presence of a phantom limb. Scientists now agree that this cannot be a total explanation for phantom sensations because of the presence of the same feelings of a phantom limb and stump pain in congenital absence of limbs. There is also the reason that surgical removal of the neuromas don't relieve the symptoms of pain in most patients. Some reported alleviated pain after the removal, but it was common that the pain would return (Brain Sense).

Central Neural Theories
Anther theory suggests that phantoms arise from excessive spontaneous firing of neurons at the spinal cord level. The sensory input that the spinal cord neurons were receiving is no longer available after amputation. This could cause the excessive firing of the neurons that received information from the now missing limb. The output from these cells is transmitted to the cortex just as if the spinal neurons had received external stimulation. This theory has also shown to be insufficient in completely describing phantom pain. Parapalegics with a complete break to the spinal cord sometimes feel pain in their legs and groin even though the break occured farther up (Phantom Limbs).
Another central theory of phantom limbs explains that the axons of the amputated peripheral nerve form connections with neurons in the receptive field of the spinal cord. Some neurons that are not associated with pain tranmission will extend into the area of the spinal cord that does process pain inputs which will increase the neuronal activity in these areas due to the sprouting of neurons and expansion of the neuronal receptive field. This brings about a change in the firing pattern of the central nociceptive or pain sensitive neurons which results in spinal disinhibition. The now disinhibited pain tracts are firing its input will reach the supra spinal centers. The lack of afferent input and the changes at the level of the spinal cord have been proposed to result in the generation of phantom limb pain (Brain Sense).

Supra Spinal Theories
More recently, work has been done to show phantom limb sensations arise higher still in the central nervous system in the brain. Coritical reorganization has been seen after amputation of a limb. Cortical areas representing the amputated extremity are take over by neighboring representational zones in both the primary somatosensory cortex and the motor cortex. The extent of cortical reorganization has been found to be directly related to the degree of pain and size of the deafferenated region. The somatosensory cortex involvement is related to a more intense phantom limb experience. Imaging studies have been conducted to support this theory and see what is happening at the level of the cortex (Brain Sense).
The work of Melzek has proven to be influential in how people are now understanding the phantom limb pathology. He suggests that phantom limbs are a result of a neuromatrix in the brain. This neuromatrix is described a a network of neurons that generate a characteristic patten of impulses indicating that the body is intact and distinctly ones own. This could explain the feeling of a phantom limb if the system operated without the actual sensory input. There are 3 different pathways that makeup the neuromatrix. There are three pathways that are involved in making up this grouping of cells to become the neuromatrix. First there is the basic sensory pathway through the thalamus to the somatosensory cortex. The pathway leading through the reticular formation of the brianstem to the limbic system is part of this grouping. This pathway is important for the experience of emotion and motivation. The last of the main pathways are the cortical systems that are important to recognition of self and evaluation of sensory signals. The parietal lobe is a major part of this system for the reason that it is essential for the sense of self (Melzek).
The way the neuromatrix is proposed to function is that it receives sensory information from the periphery and it passes through each of the pathways and are processed by each one and converted into an integrated output. This output is sent to other areas of the brain and transformed into conscious perception. It is believed that the neuromatrix is pre-wired or genetic as opposed to wired by experience. This is thought to be because of the presence of a phantom in patients who were born with out a limb (Melzek).


Treatments:

Unfortunately there is no cure for phantom limb sensations, however there are multiple ways doctors can attempt to relive the discomfort in their patients suffering from phantom pain. These include drug and non-drug therapies, surgery, deep brain stimulation and more creative procedures like mirror box therapy. The outcome of these treatments is patients specific and often takes time to try several approaches and pick the therapy that works the best. It is not guaranteed that any of the treatments will be effective and only about half of people with persistent long term phantom pain are lucky enough to respond to any treatment (Melzek).

Drug Therapies:
Aceteminophen, non-steroidal anti- inflammatories, opioids are all pain relievers that have been used to treat symptoms of phantom pain. Anti-depressants are also taken and may reduce symptoms by altering the neurotransmitters in the brain.

Non-Drug Therapies:
Massage, hypnosis, biofeedback, acupuncture and meditation can be effective in controlling the pain of a phantom limb.

Surgery:
There are several surgical procedures that have granted some sufferers relief. One such treatment is injecting painkilling agents into the epidural space of the spine. Sympathectomies are carried out to destry a cluster of nerve cell bodies in the spinal cord that are believed to be the cause of pain. Cutting into or blocking nerve roots in the spine called rhyzotomies are performed and also corodotomies which is a cervical severing of the pain conducting fibers of the spinal cord. All these operations are performed in hopes of solving the phantom limb problem, however it is unsure whether the surgery will be a success.

Deep Brain Stimulation:
This is another approach that has attempted to solve the phantom pain. This treatment involves implanting a battery operated neurostimulator that delivers impulses to specific areas of the brain. The goal of this procedure is to block the abnormal signals from the nerves that trigger the perception of phantom pain.

Mirror Box Therapy:

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This is a promising approach to relieving phantom pain. The concept is to trick the brain into seeing the phantom as real. Suppose the left hand has been amputated if one watches the right hand in a mirror and perceives it as the left amputated hand. This concept is believed to work because it allows the amputee to gain control over the brain in producing the sensations of a false limb.








http://endthepainproject.org/images/mirrorbox2.jpg

Conclusion:

Phantom limbs and the pain that come with them have been recognized for centuries. Still no one is able to exactly explain the mechanisms and causes of this disorder. Phantom limbs raise questions about fundamental assumptions in the scientific community. One such assumption is that sensations one feels are produced only by stimuli. The occurance of phantom limbs could suggest that sensory inputs merely modulate experience of the body, but don't generate it (Melzek). with the continuation of phantom limb research it is possible to further our understanding of the brains complexity of the sensory systems and be able to help the people that are affected by phantom limbs.


QUIZ (answers after references)

1. The cause of phantom limb pain is understood in great detail.
True or False?

2. The most recent theories for phantom limb pain are the peripheral mechanisms.
True or False?

3. All of the following are treatments for phantom limb pain except?
a. rhyzotomy
b. monitered diet
c. meditation
d. pain medication

4. What percentage of amputees will experience phantom limb pain?
a. 50%
b. 65%
c. 35%
d. 90%

5. What is the basic idea behind the theory for peripheral source of phantom limb pain?

6. Give a brief explanation of the neuromatrix.


References

Brynie, Faith. Brain Sense The science of the Senses and How We Process the World Around Us. New York : AMACOM, Print.

Giummarra, M. J., Georgiou-Karistianis, N., Nicholls, M. R., Gibson, S. J., Chou, M., & Bradshaw, J. L. (2010). Corporeal awareness and proprioceptive sense of the phantom. British Journal Of Psychology, 101(4), 791-808. doi:10.1348/000712610X492558

Melzack, R. (2006). Phantom LIMBS. Scientific American Special Edition, 16(3), 52-59.

Nicolelis, M. (2007). Living with Ghostly Limbs. Scientific American Mind, 18(6), 52-59.

Ramachandran, V. S., & Hirstein, W. (1998). The perception of phantom limbs.


Answers to quiz questions

1. False
2. False
3. b.
4. d.
5. The idea behind the peripheral theory for phantom limb pain is the formation of neuromas where the nerves were severed during amputation. The cells in the neuromas are hyperexcitable and send excessive signals to the brain suggesting to it that there is something out at the periphery that isn't actually there.
6. The neuromatrix is a hypothesized grouping of cells in the cortex that has been proposed to explain phantom limbs. It is a network of different pathways that take in the sensory information and compacts the information to send it to other areas of the brain. The neuromatrix is said to be genetically wired so some sensory information may be in your brain not by experience but by genetics.