The burning sensation started near the top of Gregory Maassen’s left arm. It quickly spread to his upper back and chest, and within a month, his whole body was burning — from his ears to his feet.

Maassen, 51 at the time, spent most days in bed but struggled to sleep. He visited doctors who administered tests, including blood draws and an endoscopy, but none could pinpoint his medical condition. One doctor, Maassen said, thought the pain was due to mental illness.