Introduction: Hypersensitivity pneumonitis (HP) is a complex disease caused by inhalation of many different antigens. Despite exposure avoidance and immunosuppressive therapy, some patients develop a progressive phenotype (PP). Aim of this study was to investigate the prevalence of PP, describe the lung function decline and the impact on survival in patients with cHP.
Methods: We retrospectively investigated consecutive patients with cHP followed at our center between 2001 and 2021. HP was diagnosed according to the most recent guidelines, and stratified into fibrotic  and non-fibrotic. PP was defined as an absolute functional decline (FVC >5% or DLCO >10% from baseline)and/or an increase in fibrosis on HRCT with or without worsening of symptoms within one year.
Results: 75 patients with cHP (44 women, age 64±12 years) were studied, of whom 64% had fibrotic HP.  An inciting antigen was identified in 88% of patients, most frequently birds. 29 (39%) of patients had progression within 12 months and twenty patients (27%) had loss of follow-up. The median survival was 18 (IQR: 13-30) years in the overall cohort, 14 years in patients with and 19.5 years in those without PP (p=0.38).  At logistic regression analysis, BALF lymphocytosis at baseline and fibrotic HP type were significant predictors of progression at one year (OR 1.064 and 19.989 respectively), whereas specific antigen positivity against more than 1 antigen tended to be protective (OR 0.020, p=0.061).
Conclusions: Approximately one-third of patients with HP developed a PP within one year, most of them had fibrotic HP. The impact on survaival remains unclear.