Fine structural changes in idiopathic pulmonary haemosiderosis

Lung biopsies from four children and two adults with idiopathic pulmonary haemosiderosis have been examined by transmission electron microscopy. No qualitative differences were identified between the children and the adults but the changes were more severe in the children. In each case the major damage involved the capillary endothelium and its basement membrane. Capillary endothelial swelling was very noticeable and in one case the endothelium was attenuated but gaps between endothelial cells were very difficult to find. Capillary narrowing and platelet aggregation were common. The capillary endothelial basement membrane showed focal thickening, particularly on the thick side of the air/blood barrier, but no electron dense deposits were identified. Degenerative changes in the alveolar epithelium were not so marked as those in the capillary endothelium and the epithelial basement membrane was normal except for haemosiderin deposition. Haemosiderin was also noted on elastin and within intra‐alveolar macrophages. Other secondary changes included mild interstitial oedema and fibrosis. These findings indicate that the major site of damage is the alveolar capillary, but provide no evidence of the cause of the disease.