He conducted a definitive clinical trial establishing the effectiveness of plasma exchange as a rescue treatment for severe attacks of acute demyelinating disease, a treatment now used worldwide. He recognized the key differences between multiple sclerosis and neuromyelitis optica, developed the first diagnostic criteria for neuromyelitis optica, and was part of the team that discovered the diagnostic biomarker, AQP4-IgG, that is used worldwide to test for this condition. He was awarded the John Dystel Prize for multiple sclerosis research in 2011.