Hidradenitis suppurativa epidemiology: from the first Italian registry in 2009 to the most recent epidemiology updates – Italian Registry Hidradenitis Suppurativa project 2

Background The incidence of hidradenitis suppurativa (HS) is still not fully established with only a few studies reporting its estimates. Prevalence estimates range from 5:10 000 to 4:100. These extremely large variations could be explained by a combination of factors, including different selection procedures, different diagnostic criteria, and variations in the sex and age distribution of the examined samples. Objectives To analyze variations between two consecutive Italian Registries on HS. Methods Data obtained from the second Italian Registry on HS, named 'Italian Registry Hidradenitis Suppurativa (IRHIS) Project 2', are compared to the previous first Italian registry on HS. Results Data on 944 patients are reported. The more relevant aspects that characterize IRHIS 2 project, in comparison with the previous first Italian Registry on HS, are as follows: (i) the total number of patients, about fourfold higher; (ii) a more uniform national geographic distribution of the patient population; (iii) a larger number of dermatology units involved; (iv) a larger number of items considered in the data collection; (v) 6 years of difference between the onset of the two registries (2009-2013 vs. 2015-2019). Comparing data of the two registries, there are no statistically significant differences in terms of age at the time of the visit, gender, BMI, smoking habits, age at onset and age at first diagnosis by physician. Interestingly, the mean Sartorius score in the IRHIS project 2 (58.8) was significantly lower compared to the first Italian Registry (78.4). Conclusions The importance of the registries, at both national and international levels, in collecting useful real-life data is confirmed by these two Italian projects.