A simple, inexpensive device that delivers electrical current to the brain noninvasively could help stroke patients recover lost motor ability. According to a new study, the treatment–transcranial direct current stimulation (tDCS)–in combination with occupational therapy boosted recovery better than either treatment on its own.
Many patients spontaneously recover some function in the weeks and months after suffering a stroke, as their brains reorganize to compensate for the damaged area. Scientists are searching for ways to both boost and focus this innate plasticity, thus improving neural repair. Electrical activity is one option under study: electrical current applied to the brain can modulate brain-cell activity–a crucial component of neural remodeling.
In tDCS, an electrical current is passed directly to the brain through the scalp and skull. (The treatment generates just a slight tingle, if anything, in the patient.) Previous research has shown that applying tDCS to the motor cortex can improve motor performance in healthy people and, to some extent, in stroke patients. But most previous studies have tested just a single treatment, and few have used it in conjunction with rehabilitation exercises.
In the current study, Gottfried Schlaug and his collaborators at Beth Israel Deaconess Medical Center, in Boston, tested 20 patients who had suffered a stroke an average of 2.5 years previously and still had moderate to severe impairments. Patients performed 60 minutes of occupational therapy each day for five days, while also receiving a 30-minute session of either active electrical stimulation or a placebo–a fake treatment designed to mimic electrical stimulation.
The researchers used a simple device–a nine-volt battery connected to large flat sponges that are moistened and then applied to the head–that has been approved by the Food and Drug Administration for delivering drugs across the skin. (The current encourages the movement of charged drug molecules across the skin.)
A week after the start of the experiment, patients given the real treatment performed much better on a number of motor tests–including tests of strength, range of movement, and practical functions such as grasping a cup–than those who received the fake treatment, improving by about 12 to 15 percent versus about 3 to 5 percent, says Schlaug. He presented the research at a conference in San Francisco this week sponsored by the Organization for Human Brain Mapping.
Using functional magnetic resonance imaging (fMRI), the researchers also found that activity in the injured part of the brain increased after the course of treatment.
While it’s not yet clear exactly how tDCS improves motor function after stroke, one theory is that it helps repair an imbalance in the interactions between the two hemispheres of the brain. In the healthy brain, the left and right sides of the motor cortex continually inhibit each other in order to carry out one-sided movements, such as writing or brushing one’s teeth. If one side is damaged by stroke, it can no longer effectively inhibit the healthy side, which in turn leads to increased inhibition of the stroke-damaged hemisphere. “There is some thought that this imbalance of inhibition actually hinders stroke recovery,” says Schlaug. “Noninvasive brain stimulation offers a potential solution to this, or at least a way to test this hypothesis.”
“It’s something that has tremendous potential,” says Leonardo Cohen, a neurologist and chief of the Human Cortical Physiology Section at the National Institute for Neurological Disorders and Stroke, in Bethesda, MD, who was not involved in the study. Researchers now need to figure out the optimal brain-stimulation parameters to trigger healing, he says.
Schlaug’s study was unique in that his team applied stimulation to both hemispheres of the brain, using one direction of current to increase brain activity on the damaged side, and the reverse current to inhibit activity on the healthy side. “Maybe with the new technique, the magnitude of the effect may be better,” says Cohen.
It’s still unclear how long the benefits of tDCS can last, and what magnitude of improvement they can provide. In Schlaug’s study, the benefits lasted for at least a week, which was as long as the scientists followed the patients. A previous study in healthy patients found lasting benefits after three months.
Most tDCS studies of stroke have focused on patients whose stroke occurred two or more years previously, when the course of spontaneous recovery has come to an end. “The question for the future is, what happens when this is applied in the subacute period [soon after a stroke]?” says Cohen. Assessing the effectiveness of treatments applied during this period is challenging because it’s difficult to determine whether improvements are triggered by the treatment or would have happened naturally.
Schlaug’s team is also trying to better understand the changes that take place in the brain as patients recover. They are using a brain-imaging method called diffusion tensor imaging to track the brain’s white matter–the neural wires that connect brain cells–before and after treatment. “We are very interested in seeing if something changes in the tracts that go from the brain to the spinal cord,” says Schlaug. “And based on those changes, can we figure out how to make the treatment better, or try to predict which patients will benefit?”