Oxford University, England, researchers, along with a colleague from Oxford University Hospital’s Nuffield Orthopaedic Centre, Oxford, hope their identification of brain responses correlated with the level of phantom limb pain can aid the development of treatment approaches, as well as increase understanding of how the brain reorganizes and adapts to new situations. Their results were published March 5 in the journal Nature Communications.
“[I]t’s thought that around 80 percent of amputees experience some level of pain associated with the missing limb,” said first author Tamar Makin, PhD, a postdoctoral research fellow at Oxford University’s Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB). “For some the pain is so great it is hugely debilitating.”
The origin of the pain is not well understood. There may be many factors that lead to the pain, including injured nerve endings where the limb was lost and changes in the brain areas connected with the missing limb.
The Oxford University team used magnetic resonance imaging (MRI) to study how the phantom limb pain felt by people who have had an arm amputated is related to changes in the brain. They compared MRI data for 18 amputees, with differing levels of phantom pain, with 11 individuals born with one hand through a limb deficiency and a control group of 22 adults with two full limbs. The amputations had been done 18 years ago on average, but the participants still experienced sensations for the missing arm. By asking them to move the fingers of the phantom limb while in the MRI scanner, the researchers were able to look at how the missing hand is represented in the brain.
They found that the brain maintained its representation of the amputated limb, and those experiencing the most phantom limb pain maintained a stronger representation of the missing hand in the brain-to the point where it was indistinguishable from people with both hands. The researchers also found that the amount of grey matter in the phantom hand area of the brain was reduced in amputees compared to those with two hands. But again this was linked to the amount of pain amputees felt. Those experiencing stronger pain showed less structural degeneration in the missing hand area following the loss of the limb. However, while those with strong phantom limb pain maintained the local brain structure and function for the missing hand, there was evidence that connections to other parts of the brain were disrupted more. In particular, the representation of the missing hand was more out of synch with the area looking after the other hand on the opposite side of the brain.
“Most people experience ‘phantom’ sensations in a missing limb after amputation,” said Makin. “This disconnect between the physical world and what they are experiencing appears to be linked to a functional detachment in the brain…. Our results may encourage rehabilitation approaches that aim to recouple the representation of the phantom hand with the external sensory environment.”
While the Oxford University researchers have found brain changes that are correlated with phantom limb pain in this study, they cannot tell if they are causal. To get a better understanding of that, they are beginning a trial with people who’ve had an arm amputation to see if a brain stimulation technique can influence the phantom pain they experience, taking MRI scans before, during and after the technique. The brain stimulation approach uses a tiny electric current from two electrodes placed at appropriate places on the outside of the head to try and boost the connection of the phantom limb area of the brain to the rest of the cortex.
Editor’s note: This story was adapted from materials provided by Oxford University.