Many people who have lost limbs in accidents, to amputation and even those born without correctly formed limbs, experience strange sensations in the absent limb or appendage. These feelings are known as phantom sensations and are very common, with around 70% of amputees experiencing phantom limbs.
Although scientists do not know what causes phantom limbs definitively, the common consensus is that the sensations are formed by the reorganisation of somatosensory cortex in the brain. It is believed that once the nerves for the missing limb stop receiving stimuli, they are removed by the brain to give more room to functional neurones. This also explains why a person's hearing gets better if they lose their sight - by replacing the now redundant 'eye processing' neurones with those that deal with hearing, the brain can analyse sounds more efficiently. Phantom limbs are thought to occur when this reorganisation is maladaptive, or is not fully completed, so that the brain still receives phantom signals from neurones for limbs that are no longer present.
The symptoms of phantom limbs offer some support for this theory since the sensations that patients experience are often similar. Usually, patients describe the phantom limb as being shorter than the original was. The difference in size can be very pronounced, being as much as 6 inches shorter in some cases! Many patients also describe feeling itches and tingling sensations in their phantom limb, which are symptoms that can all be explained by the reduction in processing capacity for the limb in the brain.
Whereas this is all very interesting, the phantom sensations don't stop here for many patients and the majority of individuals who experience this phenomenon suffer from varying degrees of discomfort and pain. Phantom Limb Pain (PLP) is very common and, as with phantom limb syndrome, doctors don't really know why it occurs. There are 3 leading theories however, which all have strong support within the medical and scientific communities:
- The first is called maladaptive plasticity and is the same as the theory discussed above, suggesting that PLP is the result of maladaptive changes in the neuronal distribution of the somatosensory cortex following amputation.
- The second suggests that PLP is a result of the conflict between the signals from the missing limb's old neurones, which are telling the brain that the limb is there, with the information from the patient's eyes, which is telling them that the limb is not there. It is believed that the conflict between this information confuses the brain, leading to intense pain in the phantom limb.
- The last generally accepted explanation suggests that vivid memories of limb positions kick in after it has been amputated, and that these memories hold the limb in a certain position that the patient is unable to alter.
Without further understanding of the causes of PLP however, it is unlikely that scientists can develop a 'fix all' cure for patients due to the complexity of the condition. This is a real problem for many patients of PLP since the condition is most likely neurological, meaning that pain killers have no effect as a palliative and attempts to use drugs have repeatedly failed in the past.
A promising treatment for PLP does exist however, which has been successful in many cases. The treatment was developed by Vilayanur S. Ramachandran and his colleagues in the 1990's and involves the use of mirrors to trick a patient's senses into thinking that they are moving their phantom limb.
This trickery is accomplished using a mirror box, in which the patient places their healthy limb in one hole and their stump in the neighbouring one. The top of the box is covered over their amputated limb, which the patient then appears to see as being whole again by watching the reflection of their healthy limb. When ready, the patient is asked to perform 'mirror symmetric' movements in both limbs simultaneously. Their brain then interprets moving their phantom limb and appears to 'see' it moving so any conflicting signals disappear, allowing the limp to shift from uncomfortable, painful positions.
|Ramachandran's mirror box. Patients suffering from PLP put their healthy and phantom limbs into the box and, due to the reflection of the mirror, appear to have two healthy limbs again.|
A good example for mirror box therapy (based on an actual case), would be to imagine that one of your hands has been amputated. Following the operation you experience the sensation that your missing hand is constantly clenched, with the feeling being so strong that you are continuously in pain. In the therapy you are asked to clench and unclench both of your hands at the same time. By appearing to see your phantom hand move, your brain interprets that it is now unclenched and the pain disappears.
Regular sessions of mirror box treatment have been able to alleviate PLP in many patients until it eventually disappears by itself, which occurs in most cases given enough time. As with the causes of phantom limbs and its associated pain, scientists can only offer theories to how mirror therapy works and it is possible that we may never know definitively due to the complexity of the human brain. Developing our understanding of mirror therapy may help to resolve an exceedingly curious phenomenon that has long baffled scientists and medics alike.