Regional differences in the prevalence of generalized pustular psoriasis in Japan

Generalized pustular psoriasis (GPP), a rare form of psoriasis, is characterized by neutrophil-rich, sterile pustules. In Japan, GPP has intractable and rare disease designation, which allows patients to access support from national and local governments for medical expenses. Previously, similar numbers of patients in Tokyo and Hokkaido have been shown to have GPP designation, despite different population sizes. Here, we determine whether there are regional differences in the proportion of patients receiving GPP designation status in Japan and aim to identify causal factors. In this descriptive, retrospective cohort study, publicly available data were collected on the number of patients with intractable and rare disease designation for GPP in each prefecture and age classification (April 2018-March 2021). Three other designated intractable and rare disease cohorts were included: pemphigus, rare skin diseases, and all diseases. The primary outcome was the standardized morbidity ratio (SMR) of patients at prefecture level (observed numbers divided by expected). Regional differences were compared with the statistical expectation for the total population and age distribution of each prefecture. Regional differences were observed in all cohorts. Overall, 1910 patients had GPP as a designated intractable and rare disease in 2020. Regional differences in SMRs for GPP were observed with high SMRs (≥1.5) in Hokkaido, Tottori, Kagawa, and Miyazaki, and low SMRs (<0.6) in Gunma and Kanagawa. Regional differences in SMRs for GPP did not correlate with the number of medical doctors or dermatologists or internal migration. The number of medical doctors or dermatologists correlated with SMRs in the rare skin diseases and total cohorts. Regional differences in Japan exist in the number of patients with GPP who have an intractable and rare disease designation. Managing rare diseases is an important public health issue, and further research is required to elucidate the factors contributing to these differences.