Emphysema is defined as a condition of the lung characterised by permanent, abnormal enlargement of airspaces distal to the terminal bronchiole, accompanied by the destruction of their walls without obvious fibrosis. The most important factor by far is cigarette smoking. There is also a causal relationship between HIV infection and the development of early emphysema. Various genetic disorders may be associated with emphysema, including α1-antitrypsin deficiency, heritable diseases of connective tissue such as cutix laxa, Marfan syndrome and familial emphysema.
Emphysema is thought to result from the destruction of elastic fibres caused by an imbalance between proteases and protease inhibitors in the lung and from the mechanical stresses of ventilation and coughing. Proteases are normally released in low concentration by phagocytes in the lung. Protease inhibitors, mainly α1-protease inhibitor (α1-antitrypsin), prevent them from causing structural damage to the lung. Imbalance in the protease–antiprotease activity may result from antiprotease deficiency (α1-antitrypsin deficiency), from excess release of protease stimulated by environmental agents, or from the defective repair of protease-induced damage. Tobacco smoke increases the number of pulmonary macrophages and neutrophils, reduces antiprotease activity and may impair the synthesis of elastin. As emphysema develops, lung destruction progresses, airspaces enlarge, and elastic recoil declines, reducing radial traction on bronchial walls and on blood vessels and allowing airways and vessels to collapse.
Emphysema is traditionally based on the microscopic region of disease within the secondary pulmonary lobule. The principal types are centrilobular, panlobular, paraseptal and irregular emphysema. Centrilobular (centriacinar) emphysema affects mainly the proximal respiratory bronchioles and alveoli in the central part of the acinus. The process tends to be most developed in upper parts of the lungs. It is strongly associated with cigarette smoking. Inflammatory changes in the small airways are common with plugging, mural infiltration and fibrosis leading to stenosis, distortion and destruction. Paraseptal emphysema selectively involves the alveoli adjacent to connective tissues septa and bronchovascular bundles, particularly at the margins of the acinus and lobule but also subpleurally and adjacent to the bronchovascular bundles. Airspaces in paraseptal emphysema may become confluent and develop into bullae, which may be large.
Airway obstruction and physiological disturbance may be minor. Panlobular (panacinar) emphysema is characterised by a dilatation of the airspaces of the entire acinus and lobule. With progressive destruction, all that eventually remains are thin strands of deranged tissue surrounding blood vessels. It is the most widespread and severe type of emphysema. Pathological changes are distributed throughout the lungs, but they are often basely predominant. Panlobular emphysema is the type occurring in α1-antitrypsin deficiency and in familial cases. Irregular emphysema is referred to as para-cicatricial emphysema or irregular airspace enlargement, and occurs in patients with pulmonary fibrosis. It is commonly seen adjacent to localised parenchymal scars, diffuse pulmonary fibrosis, and in the pneumoconiosis, particularly progressive massive fibrosis.
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