Researchers identify cause of phantom limb pain in amputees

Researchers from Osaka University in Japan have identified the underlying cause of phantom pain in amputees. The study concluded that amputees and patients with chronic pain due to severe nerve injury unconsciously activate parts of the brain that are associated with sensation and movement. Phantom limb pain typically occurs following the loss of sensation in a body part that has either suffered intense nerve damage or has undergone complete amputation. In most cases, amputees still “feel” the presence of their missing limb.

In order to test for the cause of the pain, researchers asked 10 participants to control a robotic arm using signals from their minds, relying on the help of a brain-machine interface.

These participants were chosen as subjects for the study because they were amputees. The experiment required the 10 volunteers to use prosthetics by switching between using their affected and unaffected arms. When controlling the movements of the robotic arm with the amputated arm, participants reported feeling pain. When controlling the prosthetic arm with the unaffected arm, however, they reported no pain.

In order to evaluate and accurately pinpoint the level of pain that the participants reported feeling, the researchers asked them to describe what they experienced through a visual analogue scale as well as a pain questionnaire.

The brain-machine interface was designed to show and decode magnetoencephalography sensorimotor cortex signals, which record changes in cortical activity and indicate movements and internal reactions of the participant. Usually, there is a mismatch between a signaled movement and the perception of that movement, which is what the scientists who led the study tried to identify. This mismatch has the ability to morph into the illusion of feeling an increase in pain, especially while using motor senses, such as in this experiment.

To gain the most accurate results, the researchers mandated that the participants redo the experiment with their unaffected arm for comparative standards. To use the prosthetic arm, researchers asked the patients to grasp and release a ball using their phantom hand while watching the prosthetic arm moving. Nine out of the 10 participants reported feeling as though they had retained more control of their phantom limb.

After having conducted the experimental portion of the study, researchers then classified the reactions and cortical activity quantitatively. Following practice using and concentrating on the amputated arm to operate the prosthetic arm, most participants reported feeling as though they had more motor control than before. These results emphasize the necessity behind rehabilitation and physical therapy post-amputation or nerve damage to restore motion. Although rehabilitation is key to restoring movement and control of the affected limb, it is likely to give the illusion that pain exists when it actually does not, as shown by the results of the study.

The results, however, most notably indicate that the perceived pain was reduced not as a direct consequence of increased motor function, but as patients began to exhibit variations in cortical representation.

Between 50 and 80 percent of all amputees feel chronic pain in their lost limbs. Though the range of the statistic is large, the study provides a way for scientists and doctors to learn more about the issues that amputees face.

While the sample size needs to expand in order for the study to gain greater scientific validity, this experiment proves promising. The study’s scientists have also concluded that this kind of phantom pain can be applicable in the future to patients who suffer from arthritis.

The research implies that training involving a brain-machine interface may potentially help to provide therapy to amputees. Despite its effectiveness, doctors who employ this technology in the future should consider pain in their treatment options. Brain-machine interface technology does not appear to diminish pain, and may even exacerbate it.