To determine the incidence and prevalence of myasthenia gravis (MG) associated with acetylcholine receptor (AchR)-IgG and muscle tyrosine kinase (MuSK)-IgG in the district of Sassari (Sardinia; Italy).

The global mean incidence and prevalence of MG have been assessed at ≈15 (range, 4-29)/million and 20 (range, 2-37)/100,000, respectively. The island of Sardinia is a recognized area at higher risk for different immune-mediated disorders (e.g., multiple sclerosis), but the epidemiology of MG remains unclear.

From the Neurochemistry Laboratory of the University-Hospital of Sassari (reference laboratory for AchR-IgG/MuSK-IgG testing in the island since 1998) we retrospectively identified patients with: 1) AchR-IgG (titer ≥0.5 nmol/L) and/or MuSK-IgG (titer ≥100 pg/ml) positivity by radioimmunoprecipitation assay; 2) available medical records; and 3) residence within the district of Sassari. Incidence (January 2010-December 2019) and prevalence (on January 1, 2020; population 325,288) were calculated.

Among 517 antibody-positive patients identified, 183 were included (incident, 97; prevalent, 165). We excluded 334 patients due to: 1) missing clinical information (n=65); 2) residency outside the Sassari district (n=168); 3) not classifiable as incident or prevalent (n=56); and 4) clinical phenotype not consistent with MG (n=45; median AchR-IgG titer, 0.7 [range, 0.5-5.5] nmol/L). The crude MG prevalence was 50.7/100,000 (95% CI, 43.4-58.9), whereas incidence was 29.8/1,000,000 person-years (95% CI, 24.3-36.2). Antibody specificities were AchR in 170 (92.9%; median titer 5.7 [range, 0.5-22]) and MuSK in 13 (7.1%; median titer 1,255 [range, 136-1,730]). Among incidence cases, age at disease onset was distributed as: <18 years (n=2; 2%); 18-50 years (n=14; 14.4%); 51-65 years (n=25; 25.8%); and >65 years (n=56; 57.7%). In 45 (46.4%) incident cases, MG was initially restricted to ocular muscles.

We report the highest incidence and prevalence of MG worldwide. The environmental and genetic determinants of MG risk in the Sardinian population need to be investigated.