Epidemiology of aquaporin-4-IgG-positive NMOSD in Sardinia

Purpose: The Italian Island of Sardinia (population, 1,578,146) is recognized for the high risk of multiple sclerosis (MS) but the epidemiological burden of other less common demyelinating diseases of the central nervous system (CNS), such as aquaporin-4-IgG-positive neuromyelitis optica spectrum disorder (AQP4-IgG+NMOSD), is unknown. In this study, we determined the incidence and prevalence of AQP4-IgG+NMOSD in Sardinia over a ten-year study period (2013–2022). Methods: Patients with a diagnosis of AQP4-IgG+NMOSD (per 2015 IPND diagnostic criteria) were retrospectively identified using two sources: (1) Archives of the reference and only laboratory for AQP4-IgG testing in Sardinia; and (2) medical records of the four MS units in the island. Incidence (January 2013–December 2022) and prevalence (December 31, 2022) were calculated. Results: A total of 45 cases were included: incident, 31; prevalent, 41. The median age (range) at disease presentation was 51 (6–78) years; female/male ratio was 9:1. The crude (95 % CI) incidence and prevalence were 1.9 (1.3–2.7) per million and 2.6 (1.9–3.5) per 100,000, respectively. Prevalence increased from 2013 (1.1 per 100,000) to 2022 (2.6 per 100,000); p = 0.002. After age-standardization to the world, incidence and prevalence (95 % CI) decreased to 1.3 (0.7–2) per million and 1.8 (1.3–2.3) per 100,000, respectively. Coexisting immune-mediated disorders, mostly autoimmune thyroiditis, were reported in 50 % of patients. Conclusions: The epidemiology of AQP4-IgG+NMOSD in Sardinia is overall in line with other Caucasian populations. The high MS risk in the island seems disease-specific and not associated with an increased risk of other CNS demyelinating disorders, confirming different pathophysiology.