For people missing an arm or a leg, a sophisticated artificial limb may do more than just restore some of their capacities. A report in the June issue of Nature Neuroscience suggests that a so-called "myo-electric prosthesis"--which picks up electrical signals from muscles in the stump to drive an artificial hand--can prevent phantom pains, the feeling that a limb hurts even though it's gone.
Almost all patients who undergo an amputation can still feel their missing limb during the first few weeks after the amputation, and often the absent arm or leg hurts. In two out of three patients, the pain persists for years or even decades. The source of phantom pain is enigmatic, but seems to involve a rewiring of the cortex, the higher control center of the brain; neurons from neighboring areas in the cortex appear to make connections with the area that once controlled the severed limb but now lies fallow. Sensory stimulation of a body part, on the other hand, is known to expand the area of the cortex involved in its control.
In a myo-electric prosthesis, electrodes sense the muscle contractions and send electrical signals to a motor driving the hand movements. Neurologist Martin Lotze and his colleagues at the University of Tübingen, Germany, wondered if the tactile stimulation such a prosthesis provides, or the muscle activity it requires, could prevent the expansion into the unused cortical center by a neighboring region, which controls movement of the lips, and thus reduce phantom pains.
The team interviewed 14 patients about the severity of their pain and their prosthesis use. To assess cortical rewiring, they asked the patients to purse their lips, while brain activity in their lip-control area was being measured using functional magnetic resonance imaging. Although almost all patients recalled having phantom pain immediately after the amputation, the team found that five patients who wore their functional prosthesis for the longest periods and used it most were more or less pain-free, and showed almost no expansion of the lip-control area in the cortex.
Psychologist Ronald Melzack of McGill University in Montreal, Canada, applauds the work. "This is good stuff," he says. "There's no doubt that using prosthetic limbs should be instituted as soon as possible," he says, as it could spare patients from having to take potent pain killers with sometimes severe side effects. Melzack adds, however, that the highly sophisticated myo-electric devices are still a "very expensive luxury treatment" for amputees.