Emphysema is a condition of the lung characterized by abnormal permanent enlargement of the airspaces distal to the terminal bronchiole, accompanied by destruction of their walls, and without obvious fibrosis. In contrast the enlargement of airspaces unaccompanied by destruction is termed overinflation, for example, the distention of air spaces in the opposite lung after unilater pneumonectomy.

Types of Emphysema

Emphysema is defined in terms of the anatomic nature of the lesion and it can be further classified acording to its anatomic distribution within the lobule. Recall that the lobule is a cluster of acini, the alveolated terminal respiratory uits. Although the term emphysema is sometimes loosely applied to diverse conditions there are four major types : (1) centriacinar (2) panacinar (3) paraseptal and (4) irregular. Of these only the first two cause clinically significant airflow obstruction. Centriacinar emphysema is far more common than the panacinar form, constituting greater than 95% of cases. Clinical management does nor rely on precise anatomic diagnosis and classification. The anatomic subtypes of emphysema, however, provide important clues to pathogenesis.

Centriacinar (Centrilobular Emphysema)

The distinctive feature of this type of emphysema is the pattern of involvement of the lubules; the central or proximal parts of acini formed by respiratory bronchioles are affected, whereas distal alveoli are apared. Thus both emphysematous and normal airspaces exist within the same acinus and lobule. The lesions are more common and usually more severe in the upper lobes, particularly in the apical segments. The walls of the emphysematous spaces often contain large amounts of blace pigment. Inflammation around bronchi and bronchioles and in the septa is common. In severe centriacinar emphysema, the distal acinus may be involved, and differentiation from panacinar emphysema occurs predominantly in heavy smokers, often lesions of so-called coal workers pneumoconiosis bear a striking resemblance to centriacinar ephysema. These points suggest an important role for tobacco products and coal dust in the genesis of this type emphysema.

Panacinar (panlobular) Emphysema

In this type , the acini are uniformly enlarged from the level of the respiratory bronchiole to the terminal blind alveoli. The prefix pan-refers to the entire acinus but nor to the entire lung. In contrast to centriacinar emphysema, panacinar emphysema tends to occur more commonly in the lower zones and in the anterior margins of the lung and it is usually most severe at the bases. This type of emphysema is associated with ? antitrypsin deficiency

Paraseptal (Distal Acinar) Emphysema

In this type the proximal portion of the acinus is normal but the distal part is dominantly involved. The emphysema is more striking adjacent to the pleura along the lobular connective tissue septa and at the upper half of the lungs. The characteristics findings are of multiple continious enlarged airspaces from less than 0.5 mm to more than 2.0 cm in diameter sometimes forming cyustlike structures. This type of emphysema probably underlies many of the cases of spontaneous pneumothorax in young aged.

Irregular Emphysema

Irregular emphysema, so named because the acinus is irregularly involved is almost invariably associated with scarring. Thus it may be the most common form of emphysema becauise careful search of most lungs at autopsy shows one or more scars from a healed inflammatory process. In most instances, these foci of irregular emphysema are asymptomatic.

Although details of genesis of the two common forms of emphysema ? centriacinar and panacinar remain unsettled the most plausible hypothesis to account for the destruction of alveolar walls is the protease antiprotease mechanism. This hypothesis is based on two important observations, one clinical and one experimental. The first is that homozygous patients with a genetic deficiency of the protease inhibitor Alpha-AT is a major inhibitor of protease, particularly elastase secreted by neutrophils during inflammation. The normal Alpha ?AT phenotype called PiMM is present in 90% of the population. Of the several phenotypes associated with alpha-AT deficiency, PiZZ is the most common. More than 80% of PiZZ phenotypes develo9p symptomatic emphysema that occurs at an earlier age and with greater severity if the individual smokes. Therefore the most important therapeutic intervention in alpha-AT deficiency is cessation of smoking . The second observationbearing on the protease antiprotease hypothesis is experimental, in that intracheal instillation of the proteolytic enzyme papain, which degrades elastin causes emphysema in experimental animals.