Phantom limb

A phantom limb is the sensation that an amputated or missing limb is still attached to the body and is moving appropriately with other body parts. Approximately 50 to 80% of amputees experience these phantom sensations in their amputated limb, and the majority of these people report that the sensations are painful. Phantom sensations and phantom pain, a type of referred pain, may also occur after the removal of body parts other than the limbs, e.g. after amputation of the breast, extraction of a tooth (phantom tooth pain) or removal of an eye (phantom eye syndrome).

Phantom pains can also occur in people who are born without limbs and people who are paralyzed. Phantom pains occur when the missing limb causes discomfort. Other induced sensations include warmth, cold, itching, squeezing and burning. The missing limb often feels shorter and may feel as if it is in a distorted and painful position. Occasionally, the pain can be made worse by stress, anxiety and weather changes.

Clinical description
Although not all phantom limbs are painful, patients will sometimes feel as if they are gesturing, feel itches, twitch or even try to pick things up. For example, Ramachandran and Blakeslee describe that some people's representations of their limbs don't actually match what they should be, for example, one patient reported that her phantom arm was about "6 inches too short".

Some people with phantom limbs find that the limb will gesticulate as they talk. (But whether they feel weight of phantom limb while gesticulating is unclear). Given the way that the hands and arms are represented on the motor cortex and language centers, this is not surprising. Some people find that their phantom limb feels and behaves as though it is still there, others find that it begins to take on a life of its own, and doesn't obey their commands.

Neurological basis
Until recently, the dominant theory for cause of phantom limbs was irritation in the severed nerve endings (called "neuromas"). When a limb is amputated, many severed nerve endings are terminated at the remaining stump. These nerve endings can become inflamed, and were thought to send anomalous signals to the brain. These signals, being functionally nonsense, were thought to be interpreted by the brain as pain.

Treatments based on this theory were generally failures. In extreme cases, surgeons would perform a second amputation, shortening the stump, with the hope of removing the inflamed nerve endings and causing temporary relief from the phantom pain. But instead, the patients' phantom pains increased, and many were left with the sensation of both the original phantom limb, as well as a new phantom stump, with a pain all its own. In some cases, surgeons even cut the sensory nerves leading into the spinal cord or in extreme cases, even removed the part of the thalamus that receives sensory signals from the body.

In the early 1990s, Tim Pons, at the National Institutes of Health (NIH), showed that the brain can reorganize if sensory input is cut off. Hearing about these results,V. S. Ramachandran realized that phantom limb sensations could be due to "crosswiring" in the somatosensory cortex, which is located in the postcentral gyrus, and which receives input from the limbs and body. Input from the left side of the body goes to the right hemisphere and vice versa. The input from extremities comes into the somatosensory cortex in an ordered way, the representation of which is referred to as the somatosensory homonculus. Input from the hand is located next to the input from the arm, input from the foot is located next to input from the hand, and so on. One oddity is input from the face is located next to input from the hand.

Ramachandran reasoned that if someone were to lose their right hand in an accident, they may then have the feelings of a phantom limb because the input that normally would go from their hand to the left somatosensory cortex would be stopped. The areas in the somatosensory cortex that are near to the ones of the hand (the arm and face) will take over (or "remap") this cortical region that no longer has input. Ramachandran and colleagues first demonstrated this remapping by showing that stroking different parts of the face led to perceptions of being touched on different parts of the missing limb. Through magnetoencephalography (MEG), which permits visualization of activity in the human brain, Ramachandran verified the reorganization in the somatosensory cortex.

Treatment
Some treatments include drugs such as antidepressants. Spinal cord stimulation (SCS) can be effective treatment for phantom pain. An electrical stimulator is implanted under the skin, and an electrode is placed next to the spinal cord. The nerve pathways in the spinal cord are stimulated by an electric current. This interferes with the impulses travelling towards the brain and lessens the pain felt in the phantom limb. Instead, amputees feel a tingling sensation in the phantom limb.

Vibration therapy, acupuncture, hypnosis and biofeedback may all be used to treat phantom pain but are often of little help. The pain can sometimes be helped by keeping busy and focusing attention on something else. Massaging the stump can sometimes help.

For planned amputation, phantom pain can be reduced by preoperative pain management, effective control of pain by analgesic or neuroleptic is required. The brain seems to implant the sensations from the preoperative state.

One particularly novel treatment for phantom limb pain is the mirror box developed by Ramachandran and colleagues. Through the use of artificial visual feedback it becomes possible for the patient to "move" the phantom limb, and to unclench it from potentially painful positions. Repeated training in some subjects has led to long-term improvement, and in one exceptional case, even to the complete elimination of the phantom limb between the hand and the shoulder (so that the phantom hand was dangling from the shoulder).

More recently, virtual reality has been used to combat the discomfort caused by phantom limb syndrome,. Scientists from the University of Manchester have shown that phantom limb pain can be relieved by attaching the sufferer's real limb to an interface that allows them to see two limbs moving in a computer-generated simulation. This works on a similar principle to the mirror box technique in that the somatosensory cortex is being 'tricked' except that the illusion is stronger.