Chronic hypersensitivity pneumonitis or idiopathic pulmonary fibrosis? Diagnostic role of high resolution Computed Tomography (HRCT)

Abstract

Purpose: In the diagnosis of extrinsic allergic alveolitis, high-resolution computed tomography (HRCT) is more sensitive and more specific than radiography; however, the accuracy of HRCT is not absolute. The role of HRCT in this field has scarcely been investigated in the literature. The purpose of this paper is to explore the possibilities of HRCT in this field and to analyse the correlations between HRCT and functional parameters.

Materials and methods: We performed a retrospective blind evaluation the HRCT scans of 24 patients (19 males and 5 females). Twelve were affected by idiopathic pulmonary fibrosis (IPF); 12 had chronic extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis. The HRCT findings were studied by two radiologists, who were not aware of the diagnoses. The possible differences in evaluation were subsequently resolved by consensus. In the presence of interstitial fibrosis, we recorded the prevailing site and the anatomical extension (with the method of the visual percentage score, described in the literature). In all the cases, we recorded the blood gas values (PaO2, PaCO2, and pH), total lung capacity (TLC), forced expiratory volume in 1 second (FEV1), Vital Capacity (VC), Tiffeneau index (FEV1/VC) and monoxide carbon diffusion (DLCO). The statistical significance of the correlations was investigated using the Mann-Whitney and Wilcoxon tests. Student "t"-test and Pearson's chi squared were used to compare the variables of the different groups. A value of p< or =0.05 was considered significant.

Results: Overall, the diagnosis was formulated with a high degree of confidence in 13 of 24 cases (54.1%); in these patients, the result was correct in 84.6% of cases (11 of 13). In the diagnosis of EAA, HRCT sensitivity was 50%, with 91.6% specificity, 70.8% accuracy. In the diagnosis of IPF, HRCT sensitivity was 75%, with 83.3% specificity, 79.1% accuracy. The HRCT signs of interstitial fibrosis were visible in 11 cases of IPF (91.6%) and in 4 cases of EAA (33.3%), with significant statistical difference (p<0.005). In the analysis of the whole series (24 patients) we found a significant inverse correlation between the HRCT score of fibrosis and the extension of the areas with air-trapping (p<0.003). There was no significant difference between IPF and EAA considering age, FEV1, PaO2, PaCO2 and pH. We found inverse correlation (p<0.001) between HRCT extension of the fibrosis and DLCO. There was a significant difference between pulmonary fibrosis and extrinsic allergic alveolitis considering TLC (in average, respectively: 65% and 84.3%; p<0.02), and DLCO (respectively 40.4% and 74.7%; p<0.03).

Discussion and conclusions: HRCT is a reliable and strong method to be used in particularly complex fields. We found signs of interstitial fibrosis more frequently in IPF than in EAA (91.6% versus 33.3%). In IPF cases, the extension of the fibrosis--evaluated with the visual score method--was higher then in EAA (34.7% versus 6%). The presence of fibrosis with basal and peripheral distribution was characteristic of idiopathic pulmonary fibrosis, with good sensitivity and specificity (75%), whereas in chronic extrinsic allergic alveolitis the areas of fibrosis often presented an irregular and heterogeneous distribution, in 91.6% of cases. However, 25% of extrinsic allergic alveolitis cases had a distribution mimicking idiopathic pulmonary fibrosis. The presence of areas with increased ground-glass opacity is more common in EAA than in IPF (66.6% versus 33.3%). Areas of mosaic attenuation, visible in inspiratory CT scans, are not rare in IPF (41.6% of cases), and generally have a basal distribution. The presence of hyperlucent lobules inside the fibrotic areas does not exclude the diagnosis of IPF. The finding of the expiratory air-trapping is more common in chronic EAA than in IPF, and the extension of this areas is greater in EAA (17.3% versus 6.5%). We found an inverse correlation between the extension of the fibrosis and the expiratory trapping. Signs of emphysema were observed in 25% of cases, wiema were observed in 25% of cases, with equal prevalence and the same extension in IPF and in EAA. All patients were smokers or ex-smokers. Smoking has not been demonstrated to have a protective effect against EAA. The areas of mosaic perfusion during inspiration do not represent a sound criterion for the differential diagnosis between IPF and chronic EAA. In fact, they can also be frequently found in IPF. HRCT can be employed in clinical practice for the differential diagnosis between IPF and chronic EAA with good (but not absolute) accuracy.