What happens in phantom limb pain, exactly?

What is it?

Phantom limb pain (PLP) is one of neurology’s most peculiar phenomena. 60-80% of people with amputated limbs have sensations indicating that the limb is still there. Some feel like it’s “frozen” into an unnatural position, and some feel like they can move the limb normally: this can sometimes even include “grabbing” things with a missing hand.

The majority of these sensations are painful. This is obviously problematic: the pain can be excruciatingly real to the patient, but how can it be treated, if the limb doesn’t actually exist?

What causes it and what happens during it?

Scientist haven’t been able to form any one coherent theory of the cause of PLP, but the one that is perhaps closest to the truth is based on changes in the brain. In the outer layer of our brain, the cortex, are areas that are specialized in receiving sensory signals from different parts of our body. The size and location of the brain areas doesn’t follow any strict causal rules regarding the size and location of the corresponding areas on our bodies; highly sensitive areas like our fingers occupy a much larger area in the brain than does an area of the same size in our back, and in our brains the area for our hand is adjacent to the area of our cheek, for example.

When a limb is lost, it obviously stops sending any information to the corresponding brain area. This can cause the nearby areas to take over the now useless area. Signals from the cheek can, for example, now be transported not only to the cheek area but also to the hand area which it has taken over. The brain can then interpret touching the cheek as if also the hand was being touched.

It is not clear why this can cause such pain. Normally, when the brain tells a body part to move, it soon receives a signal which tells that the movement has indeed taken place. When the limb isn’t there this feedback signal doesn’t arrive. The brain responds by amplifying its command. It is thought that since the brain also remembers the pain that is usually associated with such a motor demand (imagine clenching your fist so hard that your fingernails tug into your palm) but doesn’t receive information telling that nothing harmful is actually done, it resorts to the memories and interprets these loud motor signals as pain.

How can it be treated?

An interesting and quite effective treatment is the use of a mirror box. The patient places his remaining hand, for example, inside a mirror system, so that the reflection makes it appear to them as if they had two hands left. When they move their healthy hand, they can see the phantom hand also “move”. The brain often responds to this by actually sensing these movements in the nonexistent hand. Since the pain is commonly caused by the feeling that the limb is in an unnatural position, the patient can by using the mirror box “straighten” it, which usually relieves the pain significantly and can even lead to the disappearance of the phantom sensations.

 

Sources: https://en.wikipedia.org/wiki/Phantom_limb, http://bja.oxfordjournals.org/content/87/1/107.full, https://medlineplus.gov/ency/patientinstructions/000050.htm, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3198614/, https://en.wikipedia.org/wiki/Cortical_remapping, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2494616/

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