Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory disorder of the central nervous system that predominantly targets the optic nerve and spinal cord. In 2001, we began conducting epidemiological surveys on multiple sclerosis (MS) and NMOSD in the Tokachi area of Hokkaido Island in Japan, and have since reported the data every 5 years. In the last survey in 2016, we first applied the 2015 criteria and reported that the prevalence of NMOSD in the Japanese population was similar to that in Caucasians.1 We conducted an epidemiological survey of NMOSD in 2021 to determine the prevalence and clinical characteristics of NMOSD in the Tokachi area, and to analyse its clinical characteristics. The population of Tokachi province per the Government Census Office was 334 736 (160 533 men, 174 203 women) at the end of March 2021. Processing sheets for the detailed coding of clinical data were sent to 14 MS-related and NMOSD-related institutions inside and outside this area, and were collected from April to May 2021. In this study, we used the NMOSD diagnostic criteria defined by the IPND in 2015.2 The Mann-Whitney U test or Kruskal-Wallis test was used to analyse differences in the clinical characteristics between groups.

On the day the prevalence was determined (31 March 2021), 22 patients residing in the study area were diagnosed with NMOSD, which was …