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Researchers at Toronto Western Hospital have made a discovery about the brain's capacity to rebuild itself that could lead to new approaches to treat or prevent dementia.

The study involved 29 patients with a rare vascular disorder who needed surgery to improve blood flow to their brains and reduce the risk that they would suffer a major stroke. When researchers did brain scans nearly a year after the operations, they were astounded by what they saw: Their patients' brains had grown. On average, there was a 5-per-cent increase in the thickness of the regions that had previously been chronically starved of oxygen.

Those areas had grown progressively thinner in the months and years prior to the surgeries, losses documented in the magnetic resonance imaging scans the doctors periodically took of their patients to see if they required surgery. By 11 months after the procedure, the lost tissue had been restored, said Krembil Neuroscience Centre neurosurgeon Michael Tymianski.

"To our surprise, they experienced a restoration of their cerebral cortex," he said. He and his colleagues, who published their findings Thursday in the journal Stroke, were checking to see if there had been any further deterioration after the surgery.

The patients were relatively young; the average age was 41. Most of them suffered from Moyamoya disease, which can run in families and causes a progressive narrowing of arteries in the brain.

It is still too early to say if this discovery will lead to new ways to prevent or treat vascular dementia, Dr. Tymianski said. It is the second most common form of dementia after Alzheimer's disease and is related to stroke or other conditions that reduce blood flow to the brain.

"These discoveries are new, they may have tremendous potential, but we are still trying to figure out their role for the greater population," Dr. Tymianski said. "It is very possible that this operation … may have a broader role than for this very unique patient population. But for the time being, caution is the key word. We don't know."

He suspects the restored tissue is composed of thousands of new connections formed between neurons that had returned to a healthy state.

Think of arteries in the brain as tree trunks, he said. If they can't deliver enough water and nutrients, the branches and leaves higher in the tree wither. This is what happens with Moyamoya disease. But if the trunk is healthy, the branches and leaves can thrive, and thriving neurons are able to connect with each other.

He and his colleagues have started studies in animals in hopes of learning more about the nature of the restored tissue, but he said it is unlikely that new neurons were created after the surgery.

They have also begun testing the memory and other cognitive functions of patients before and after they undergo bypass surgery to improve blood flow to the brain. They want to see if there are any changes.

Dr. Tymianski has anecdotal evidence that this may be the case. One of his patients, a high-school teacher, had developed symptoms of dementia and could no longer work. After surgery, she was able to return to teaching, he said.

A small number of studies done in other countries suggest that patients who require the procedure often suffer from cognitive impairment, such as memory problems, and that they experience significant improvement afterward.

But not all of them do. Lindsay Chan-Kent, 25, says she wasn't experiencing any memory problems or cognitive difficulties before Dr. Tymianski performed two operations on her in June.

"My job is pretty demanding," the management consultant said in an interview from California. "I didn't notice anything like that."

There were no cognitive changes afterward, although she feels much better and no longer gets headaches. "I feel great" she said.