Pulmonary complications in patients with hematological disorders: pathobiological bases and practical approach

Pulmonary complications occur in up to 40 to 60% of patients with hematological disorders during the disease course and considerably influence morbidity and mortality. The main factors making the lung a clinically significant targeted organ in these patients may be summarized as follows. In the lung parenchyma a variety of inflammatory cells whose precursors are in the bone marrow pass through, park in, proliferate, and release microbicidal and cytohistotoxic substances. Constitutive parenchymal lung cells (bronchiolar and alveolar epithelial cells, endothelial cells, "interstitial" cells) may be a distinctive target for toxic substances or may have an important part in the inflammatory/reactive and reparative processes after an injury event. Pathogenic agents are allowed to reach the lung very easily through either or both the airways and the vascular bed and accumulate there in large amounts. Inflammatory/immunologic reactions may be particularly weak or, on the contrary strong, in the lungs either spontaneously or due to toxic action of drugs and radiation or to the immunodeficiency induced by hematological disorders, and finally to the presence of immunomodulatory viruses. The distinctive anatomical structure and function of the lung parenchyma (interactions between air spaces and capillary bed-gas exchange units) may render localized parenchymal damage clinically relevant. Allogeneic reactions may be overexpressed in the lung or the kinetics of the developing of graft versus host disease (GVHD)-related lung injury may be markedly different from the kinetics of GVHD in other organs. Hematological disorders may harbor in lung parenchymal structures at the onset (i.e., lympho-/myeloproliferative disorders primary in the lung) or during the disease course. Genetic predisposition, although probably involved, is not yet well understood. This article reviews the pathobiological bases of lung injury occurring in subjects with hematological disorders and suggests a practical diagnostic approach to these pulmonary complications.