Researchers at the University of Toronto and The Toronto Hospital have discovered a biological basis for the phantom sensations that are frequently experienced on the missing limbs of amputees. The findings of the study are published in the Jan. 22 issue of Nature.

The researchers found that the neurons in the brain that used to represent sensation in the lost limb were still functional but now driven by the stimulation of other body parts, usually the part of the body closest to the amputated limb. The investigators also found that in patients experiencing phantom pain, the sensation can be recreated by stimulating within the brain. Phantom sensations could not be elicited, however, in amputees without a history of phantom sensations.

"Many amputees have a sense of their missing limb and frequently these sensations are painful," says investigator Andres Lozano, an associate professor in the department of surgery at U of T and a neurosurgeon at TTH. "Phantom pain can severely compromise the quality of life of patients who have already had to adjust to a change in body image and quite often their activities of daily living."

An area of the brain called the thalamus functions as a relay centre in the brain, receiving messages from the surface of the body (input) and sending these impulses to higher centres in the brain (output) to interpret where and how the sensation is perceived. "Amputation can change the representation of the body surface in the brain, but until now it has been unclear how these changes relate to phantom sensation," says Professor Jonathan Dostrovsky of the department of physiology at U of T, who was also involved in conducting the study.

The team of scientists and clinicians at the U of T and TTH treating people experiencing chronic pain and movement disorders through the electrical stimulation of the thalamus identified an opportunity to investigate phantom sensations. They hypothesized that in amputees who experience phantom sensations, the area in the thalamus originally representing the missing limb remains functional and stimulating this area of the thalamus results in phantom sensations.

The study involved six amputees in the pain/movement management program who all had chronic pain following amputation; four had experienced phantom pain and two people experienced pain in their stump but had not experienced phantom sensations. As part of their treatment for chronic pain, the patients underwent surgery to map the sensory areas in the brain. During the mapping process the investigators were able to stimulate the patients' thalamus and the patients were able to report what they felt because they were conscious, enabling the researchers to learn about sensory output from the thalamus.

"In addition to providing patients and caregivers with a sense of reassurance knowing that there is a biological basis for phantom pain, the research has helped us understand the ability of neurons to adapt to change, a characteristic referred to as plasticity," says Karen Davis, an assistant professor in the department of surgery at U of T and neurophysiologist at TTH who, along with Dostrovsky and U of T postdoctoral physiology fellow Lei Luo, were the basic scientists involved in the study. "Although the results won't immediately change how we treat phantom sensations, we can now study a larger group of patients and try to further understand the plasticity of neurons."

Some amputees develop pain in their phantom or stump and this is frequently very difficult to control with conventional therapies. Continuous electrical stimulation through electrodes surgically implanted into the thalamus has been found to provide relief of this pain in some patients. "This technique blocks spontaneous neuronal activity in the thalamus that is thought to cause phantom sensations," explains Lozano who notes the bursting' activity of neurons may be responsible for spontaneous phantom pain, an hypothesis the investigators plan to pursue.

Additional investigators involved in the study were Professor Ron Tasker of the department of surgery and a neurosurgeon at TTH and Zelma Kiss, a U of T PhD student and neurosurgeon at TTH. Funding for the study was provided by the Medical Research Council.