Interstitial lung diseases: an epidemiological overview.

Epidemiological studies on interstitial lung diseases (ILDs) may be schematically subdivided into the following major types: 1) quantifications of disease, broken down into incidence, prevalence and mortality data; 2) identification of aetiological factors; and 3) clinical epidemiological studies. Epidemiological data may be obtained from different sources or population groups, using different study designs such as systematic national statistics, population-based data and registries, and large case series of specific diseases. Differences in results between epidemiological studies may be due to real differences in incidence, but may also be due to changes in disease definitions and classifications, differences in the epidemiological design of the studies, or even registration bias. Comparative epidemiological data of different ILDs are almost limited to the general population study in Bernalillo County and to national mortality statistics, which should be interpreted with great caution. Also, some, mostly national registries of the different ILDs have been carried out by specific medical profession groups (especially pulmonologists), which clearly underestimate the real incidence of ILDs, but in which the comparison of the relative frequencies is probably accurate. Based on all these comparative studies, sarcoidosis and idiopathic pulmonary fibrosis appear to be the most frequent ILDs, followed by hypersensitivity pneumonitis and ILD in collagen vascular disease, when classical pneumoconioses are not included. There is also a relatively large group of nonspecific fibrosis. Much more data have been published on the epidemiology of specific forms of interstitial lung disease. Most information is available on the epidemiology of sarcoidosis, and those data are probably the most accurate. Data on idiopathic pulmonary fibrosis have the disadvantage of the recent changes in definition and classification of this disease. Hypersensitivity pneumonitis has been studied epidemiologically, especially in some exposure groups such as farmers and pigeon breeders, and in some regions in North America, UK, France and Scandinavia. Estimates of frequencies of interstitial lung disease in collagen vascular disease or of drug-induced interstitial lung disease are less accurate and more variable, depending on diagnostic criteria. Notwithstanding the aforementioned problems, this report tries to provide a balanced overview of the epidemiology of different interstitial lung diseases.