This article first appeared on Mosaic, a weekly outlet published by the Wellcome Trust.
In the summer of 2010, Ryan Clark twisted his ankle during a gym class. It was painful, but inconvenient more than anything. He was put on crutches for a week and his ankle healed. Then, six weeks later, the pain returned—only this time, it was a lot worse. Ryan ended up in a wheelchair, unable to bear the agony of walking. Drugs and rehab helped and after six weeks or so he recovered. Then he injured himself again, and a third time, each minor accident triggering pain that became horrendous. "They were just normal injuries for a nine-year-old," says Ryan's father, Vince, "but for him it was huge. As well as the pain, he got tremors. His muscles locked up. He'd go into full body spasms, and just curl up on the floor."
Ryan was eventually diagnosed with complex regional pain syndrome, a disorder that affects one in a million children his age. Vince Clark, who directs the Psychology Clinical Neuroscience Center at the University of New Mexico in Albuquerque, threw himself into understanding the syndrome and finding ways to help Ryan. Traditional painkillers had provided no relief, so Clark wondered about what he'd been researching in his lab. It's called transcranial direct-current stimulation (tDCS) and it involves applying mild electrical currents to the head.