[Seroconversion of anti-aquaporin-4 antibody in a patient with neuromyelitis optica spectrum disorder: a case report].

We report a case of a patient with neuromyelitis optica spectrum disorder (NMOSD) who was originally treated for multiple sclerosis (MS) due to a negative anti-aquaporin-4 (AQP4) antibody test, but later the antibody titer became positive. A 58-year-old woman without prior medical history developed acute left facial pain, vomiting, and hiccups. MRI showed an intraparenchymal lesion extending from the medulla oblongata to cervical cord with high T-weighted signal intensity. The patient responded to steroid pulse therapy. However, she developed diplopia and gait disturbance after six months, and follow-up MRI revealed a new lesion in the left middle cerebellar peduncle. The patient was diagnosed with MS due to the dissemination in the time and space of her lesions and negative anti-AQP4 antibody status. There was no relapse for three and a half years on fingolimod therapy. However, a severe relapse occurred four years later that involved optic neuritis and multiple new brain lesions. During this episode, the anti-AQP4 antibody test was positive, and the patient was diagnosed with NMOSD. This case highlights the possibility that seroconversion of anti-AQP4 antibody may occur at any time in NMOSD patients. Therefore, this diagnostically paramount antibody should be measured several times during the treatment of relapsing-remitting MS in patients with repeat frequent recurrences and uncommon symptoms of MS.