LAUSR

In “Nationwide prevalence and incidence study of neuromyelitis optica spectrum disorder in Denmark,” Papp et al. reported that using the 2015 International Panel for NMO Diagnosis (IPND) criteria, the prevalence of NMO spectrum disorder (NMOSD) in Denmark in persons older than 16 years is 1.09 per 100,000 persons. They noted that this rate is higher than that reported in previous studies in Australia/New Zealand (0.7) and Catalonia (0.89) and lower than that reported in previous studies in Region of Southern Denmark (4.4) and Olmsted County, Minnesota (3.9). Asgari and Flanagan, who authored the articles from Region of Southern Denmark and Olmsted County, respectively, believe that Papp et al.'s calculation of the prevalence of NMOSD is low because of the (1) exclusion of 16 patients who were seropositive for aquaporin-4 IgG, (2) use of aquaporin-4 IgG data from multiple rather than a single laboratory, some of which used a less sensitive assay, and (3) use of only 2 (rather than all) of the core clinical criteria for NMOSD. Papp et al. respond that (1) their findings actually fall within Flanagan et al.'s wide interquartile range (0.8–7.1) and that (2) the 2015 IPND criteria emphasize exclusion of alternative diagnoses and all excluded seropositive cases were inconsistent with NMOSD due to weak positivity, features consistent with MS, good response to disease-modifying therapy, or long-term stability without treatment. It is clear that estimates of NMOSD prevalence vary depending on the criteria used to define NMOSD and the study population. Nonetheless, it seems to consistently be less than 5 per 100,000 persons in the regions studied.