In an "unprecedented" clinical trial, US researchers have successfully reversed the long-term debilitating effects in some of the stroke patients by injecting stem cells directly into their brains.

"This wasn't just, 'They couldn't move their thumb, and now they can,'" lead author Gary Steinberg, professor and chair of neurosurgery at the Stanford University School of Medicine, said in a statement.

"Patients who were in wheelchairs are walking now," said Steinberg, who led the 18-patient trial and conducted 12 of the procedures himself.

The small study, published this week in the US journal Stroke, was designed primarily to test the procedure's safety, but the promising results set the stage for an expanded trial now getting underway.

The therapy involved drilling a small hole into the patients' skulls to allow for the injection of stem cells, accomplished with a syringe, into a number of spots at the periphery of the stroke-damaged area, which varied from patient to patient.

The stem cells, called SB623, are derived from the bone marrow of adult donors and modified to beneficially alter the cells' ability to restore neurologic function.

The patients, whose average age was 61, remained conscious under light anesthesia throughout the procedure, and the next day they all went home.

All the patients had suffered their first and only stroke between six months and three years before receiving the injections.

In each case, the stroke had taken place beneath the brain's outermost layer, or cortex, and had severely affected motor function. For example, some patients couldn't walk and others couldn't move their arm.

The results showed that all these patients showed significant recovery by a number of measures within a month's time, and they continued improving for several months afterward, sustaining these improvements at six and 12 months after surgery.