Emphysema, along with chronic bronchitis, is in a category of disease known collectively as chronic obstructive pulmonary disease. It is characterized by destruction of alveolar septa, which contributes to irreversible airflow obstruction by lost of elastic recoil of peribronchiolar tissue. Emphysema is classified into one of three varieties based on the part of the acinus or lobule that it primarily involves: centriacinar/centrilobular, panacinar/panlobular, or distal acinar/paraseptal.

Centrilobular emphysema accounts for more than 75% of total cases of emphysema and is defined by abnormal enlargement of airspaces centered on the respiratory bronchiole with eventual coalescence of destroyed lobules within the secondary lobule.^1,2 The enlargement of alveoli into cystic spaces together with small airway obstruction results in air trapping and marked increases in total lung volume.³

Emphysema is defined by imaging or pathologic findings of alveolar wall destruction, with confirmation of obstruction and increased lung volume by pulmonary function testing. Chronic bronchitis, which is typically also present, is defined by symptoms of chronic cough and sputum production for at least 3 months per year for 2 consecutive years.² Symptoms of emphysema are somewhat nonspecific and include dyspnea and exercise intolerance. Other findings include hyperinflation of the lungs with increased total lung capacity, decreased diffusing capacity, and increased peripheral white blood cell counts.¹ Although primarily an obstructive disease, manifest by decreased forced expiratory volume, it has been recently appreciated that approximately 8% of patients with centrilobular emphysema have interstitial findings that may result in a component of restrictive lung disease.⁸ This finding reflects the discovery of smoking-related interstitial fibrosis (see Chapter 18), which is distinct from other types of interstitial lung disease.⁹

Pulmonary hypertension with cor pulmonale occurs relatively commonly in emphysema and occurs at a greater rate than with pulmonary fibrosis.¹⁰ Progression of pulmonary hypertension occurs slowly, unless there is significant hypoxemia.¹¹ There is likely a direct effect of tobacco smoke on small muscular arteries and arterioles, with alterations in vasoconstriction and vasodilatation, and increased vascular smooth muscle cell proliferation.³

Radiologic findings include increased lung volumes and diffuse decreased in lung density, predominantly in the upper lobes.¹ This upper lobe distribution is helpful in discriminating centrilobular emphysema from panlobular and paraseptal forms. By computed tomography, there are focal regions of low attenuation, surrounded by normal lung attenuation, located within the central portion of secondary pulmonary lobules. In advanced stages, there is pruning of vessels with enlarging areas of low attenuation. Pathologic-imaging correlation has shown a high correlation between areas of low attenuation and histologic emphysema.¹²

The pathologic assessment of emphysema is often made as a secondary finding in specimens resected for lung cancer. It can also be seen in specimens removed during lung allotransplantation or in specimens taken for lung reduction surgery. Pulmonary function tests and imaging, rather than histologic assessment, are the mainstays of diagnosis.

Grossly, emphysematous lungs have a porous or spider web appearance, owing to the destruction of the underlying pulmonary parenchyma (Fig. 44.1). The disease has a very characteristic upper lung zone distribution (Fig. 44.2). The centrilobular distribution can sometimes be difficult to assess by routine visual inspection (Fig. 44.3); however, examining a parasagittal section of lung under water or alcohol can help to demonstrate the pathology and distribution. For research purposes, the tissue loss of emphysema can also be seen by parasagittal sections of the lung, mounted on paper (Gough section) (Fig. 44.3).^13,14,15 Distention of the airways with fixative is helpful in the overall assessment (both grossly and histologically), but overdistention can result in artifactual changes that resemble emphysema. There have been several methods of quantification of the extent of emphysema, using pointcounting methods and, more recently, computerized imaging.^{12,15,16,17,18}