Diffuse pulmonary alveolar fibrosis.

, , 271-281. The problems of diagnostic categorization of diffuse pulmonary alveolar fibrosis are outlined, and attention is drawn to the different implications of categories defined aetiologically and histopathologically. The pattern of pulmonary insufficiency associated with changes of these sorts is described, and the importance of using a terminology of pulmonary insufficiency which refers unequivocally to disorders of function is emphasized. The usage of the word interstitial' in the context of inflammations of the lung is critically examined. It seems to be used to imply predominant involvement of alveolar walls, which in other contexts would be regarded as parenchyma'. This unresolved conflict which accepted anatomical usage can be avoided by general acceptance of complementary conventions that pneumonia' refers to inflammations of the lung characterized by exudation into alveolar spaces leading to consolidation, and alveolitis' to those affecting predominantly alveolar walls. Some cases of alveolitis, so defined, can be shown to be due to reactions with inhaled organic dusts in the gas-exchanging part of the lungs in specifically sensitized individuals, constituting a group of aetiologically defined categories extrinsic allergic alveolitis'. A few are due to ingested toxic substances. Many remain of unknown cause and show a strong and early tendency to alveolar wall fibrosis: they constitute a histopathologically defined category cryptogenic fibrosing alveolitis'.