Background: Neuromyelitis optica spectrum disorders (NMOSD) are autoimmune neurological diseases of the central nervous system, which are characterized by the presence of serum anti-aquaporin-4 autoantibodies (AQP4-IgG). An association between Sjögren syndrome (SjS) and AQP4-IgG-positive NMOSD has been proposed, but the rate of coexistence has not been determined. Methods: In this study, 4,447 patients suspected of having NMOSD with acute neurological episodes were evaluated for the positivity of serum AQP4-IgG, serum SS-A/Ro antibody, and the presence of SjS-related symptoms (dry eye, dry mouth). Results: Of the 4,447 patients, 1,651 were positive for serum AQP4-IgG, and the remaining 2,796 were negative. A significantly higher proportion of AQP4-IgG-positive patients were positive for serum anti-SSA/Ro antibody (26.3 vs. 4.5%; p < 0.0001) and anti-SSB/La antibody (7.2 vs. 1.2%; p < 0.0001) and had dry eye (9.1 vs.4.9%; p < 0.0001) and dry mouth symptoms (8.9 vs. 3.7%; p < 0.0001). More than 80% of the patients with SjS with acute neurological events such as myelitis or optic neuritis were AQP4-IgG positive. AQ4-IgG-positive patients with comorbid SjS showed a higher female rate (97.1 vs. 89.0%; p = 0.0062), a higher positivity rate for oligoclonal bands (15.4 vs. 7.5%; p = 0.029), and a higher relapse frequency (p = 0.027) than AQP4-IgG-positive patients without comorbid SjS. Conclusions: The prevalence of SjS is higher among AQP4-IgG-positive than AQP4-IgG-negative patients, with the potential prevalence of 10–20% at the diagnosis of AQP4-IgG-positive NMOSD. Comorbid SjS is more prevalent in females, and it has a higher relapse frequency among AQP4-IgG-positive patients.
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