One morning in March, at the Nerve Injury Unit that Oaklander runs at Massachusetts General Hospital, I met an R.S.D. patient I will call Elena. She had entered the police academy in her home town, in western Massachusetts, at the age of forty-four, after bringing up several children, and she had been an officer on active duty for five years. During her spare time, she worked out, running and attending kickboxing classes for several hours a day, six times a week. One night in November, 2001, Elena had been patrolling bars in her town with seven other officers when a man with a history of assault-and-battery convictions punched one of her colleagues. Elena tackled the man. Then the other policemen piled on top of her, pinning her right forearm under the weight of their struggling bodies. Eventually, the suspect was subdued and handcuffed. Afterward, Elena recalled, “My arm felt heavy, as though I’d been working out at the gym with weights for hours.” For several months, the discomfort persisted, and she sometimes felt a tingling sensation. In late March, 2002, she awoke from a deep sleep convinced that her house was on fire: her arm was burning. Her hand was hot, puffy, and purplish-red, and the veins had swollen so much that they looked like fingers. “I thought they were going to explode,” Elena said. “I was so scared. It was as if I had the same pain as the night of the injury, and it had come back, only a hundred times worse.” After a few hours, the swelling decreased and the pain began to subside. Her doctor sent her to a hand surgeon, and when she arrived for the appointment her hand was freezing. She told the surgeon that when the hand was hot and swollen the skin was too sensitive to touch. “He just said, ‘R.S.D.,’” Elena said.