Catastrophic relapses following initiation of dimethyl fumarate in two patients with neuromyelitis optica spectrum disorder.

We report two cases of neuromyelitis optica spectrum disorder (NMOSD) who were misdiagnosed as multiple sclerosis (MS) and developed catastrophic relapses following initiation of dimethyl fumarate. Both patients developed a severe myelitis extending from the cervical cord to the medulla with significant cord swelling, resulting in complete quadriplegia and respiratory difficulties, in addition to severe bilateral visual loss in one patient. It is of note that both catastrophic relapses occurred 2 and 3 months following initiation of dimethyl fumarate.