Typically, people with CRPS suffer distorted body representations. This often includes a perception that an affected limb is larger than its actual physical size. Feelings of missing body parts or that the limb has nothing to do with them are also common.
There has long been a belief that this distorted body representation plays a significant role in both the cause and maintenance of CRPS; hence, for example, the long-established use of mirror-box therapy for the condition. However, there has remained considerable uncertainty as to what problems, if any, exist in the unconscious sensory processing of CRPS sufferers and how this might relate to their symptoms.
We must not forget that distorted body representations are not limited to people suffering CRPS or other defined medical conditions. Even the healthiest of us experience them. If you have ever had a local anaesthetic at the dentist you will almost certainly have experienced the sensation of areas of your face feeling swollen. However, while your lip, cheek or eyelid may appear a little droopy, they are not actually swollen.
In fact, studies have been published on the effect of local anaesthesia on our perception of body size and some very strange results have been noted. One study demonstrated that a local anaesthetic in the thumb resulted in an increase in a subject’s perception of the size of both their anaesthetised thumb and their unanaesthetised lips!
Distorted body representations are also common following immobilisation of a limb, but in most cases they are reversed swiftly once normal movement is regained.
So what is going on?
While our thumbs and lips are not adjacent on our bodies, they are adjacent in the part of our brain which processes sensory information from around our body. This is known as the somatosensory cortex. The effects noted above demonstrate that what our senses tell us is not just shaped by stored information, but also by our ability to continually update our experiences. This occurs through the constant flow of sensory signals reaching the brain from those nerves responsible for our senses; our peripheral nerves.
Now, for the first time, researchers at the University of Bath have compared people with CRPS and ‘healthy’ controls in their ability to update their body representation. As expected, they found that those with CRPS were less stable in their ability to update their body representation, explaining why they have such distorted representations of their affected limbs.
However, crucially, it was not just differences between the two groups which stood out. Taking the CRPS group in isolation, the researchers found their ability to update their body representation seemed to differ between the affected and non-affected sides of their body.
This body-side-specific finding is very important as it seems to support the idea that the underlying problem may lay in central processes in the brain which are specific to the affected side of the body, rather than those processes responsible solely for the CRPS-affected limb.
This was not a large study, involving just 18 people in each group. Nevertheless, the results may have helped considerably to further the understanding of both the underlying cause and mechanisms of CRPS.