Nationwide prevalence of sporadic and familial idiopathic pulmonary fibrosis: evidence of founder effect among multiplex families in Finland

Background: The prevalence of sporadic and familial idiopathic pulmonary fibrosis (IPF) cases in Finland was evaluated according to the revised recommendations of the American Thoracic Society. Methods: All Finnish pulmonary clinics (n=29) were included in the primary screening. Hospital data bases were used to identify patients with the diagnosis "alveolitis fibroticans idiopathica" (J84.1 in ICD-10 classification). The total number of patients with IPF was extrapolated based on the evaluation of random samples of case records in different centres. Families with more than one potentially affected member were identified from a questionnaire study and the diagnosis was verified from the medical records. Results: Using this approach, the nationwide prevalence of IPF in Finland was estimated to be 16-18/100 000. In 90% of the patients lung involvement was assessed by high resolution computed tomographic (HRCT) scanning and in 31% a surgical biopsy specimen was available, further confirming the diagnosis. Seventeen multiplex families with 2-5 affected family members were identified, giving a prevalence of 5.9/million for familial IPF in Finland. Both multiplex and sporadic families were clustered in Eastern Finland. This clustering reflects the demographic history of Finland in the 16th century and suggests that multiplex families may share a common ancestor in the last 20-25 generations. Conclusion: The familial form explained 3.3-3.7% of all Finnish cases of IPF diagnosed according to the revised international guidelines. Geographical clustering of multiplex families suggests a recent founder effect in patients with familial IPF.