Hyperlucent Thorax

Emphysema

Emphysema is a very important cause of loss of pulmonary vascularity (see Fig. 22.1, A ). 169, 174 Severe cases of emphysema will produce marked attenuation and stretching of pulmonary vessels. This process may be diffuse to such an extent that there may even be nearly complete absence of vessels. The diagnosis of emphysema by evaluation of the pulmonary vascularity requires a subjective evaluation of the vascular patterns. Thurlbeck and Simon ⁵⁹⁴ evaluated 700 patients in an effort to correlate paper-mounted, whole-lung sections with posteroanterior and lateral view roentgenograms to determine the accuracy of the radiologic diagnosis of emphysema. They measured lung length and width, size of the retrosternal clear space, heart size, and position of the diaphragm. They observed that lung length and the size of the retrosternal clear space increased, the level of the diaphragm was lowered, the heart size decreased, and the lung width was unchanged as the emphysema became more severe. They believed that lung length and diaphragm level were the most discriminating measurements for diagnostic accuracy, followed by size of the retrosternal clear space. However, they identified no combination of these radiologic variables that identified emphysema better than the subjective diagnosis of the disease based on arterial deficiency. They also emphasized that radiologic lung dimensions are related to stature and must therefore be interpreted cautiously. For example, kyphosis of the thoracic spine causes an increase in the anteroposterior diameter of the chest. They further divided vascular patterns into three main categories of abnormality: (1) the vessels are present but narrowed in most of the lung; (2) there is a normal axial pathway but fewer side branches; and (3) there may be complete absence of vessels. While these vascular alterations may be subtle on chest radiograph, HRCT is very sensitive for the detection of variations in regional perfusion and detection of emphysema ( Fig. 22.3, A and B ).

There is hardly any aspect of chest disease that generates more controversy than the role of the radiologist in the diagnosis of emphysema, but most agree that the alterations in pulmonary vascularity are the most reliable radiologic criteria for this diagnosis. When the foregoing criteria are applied, the chest radiograph ( Fig. 22.4 ) is highly specific for the diagnosis of emphysema in patients with advanced disease, but it is not sensitive. HRCT has been shown to be much more sensitive for the diagnosis of early emphysema by demonstrating vascular attenuation, areas of hyperlucency, and small bullae that may be subtle or not detectable by chest radiography. 187, 593

It is important to emphasize that emphysema is an anatomic diagnosis. The morphologic definition of emphysema requires the presence of destruction of alveolar walls and obstruction of small airways. This destruction results in increased size of the distal air spaces. Emphysema is further subdivided into two main types, panlobular (panacinar) and centrilobular (centriacinar). An emphysematous process that destroys all of the lung that is distal to the terminal bronchiole is termed panlobular emphysema, whereas incomplete destruction of lung distal to the terminal bronchiole is termed centrilobular emphysema. The destruction in centrilobular emphysema may occur in the center of the lobule, but may also be eccentrically located. Of the two types, centrilobular is the more common. Other terms frequently confused in the discussion of emphysema are bulla and bleb. A bleb is a collection of air within the layers of visceral pleura, and a bulla is an emphysematous space in the lung parenchyma with a diameter of more than 1 cm; however, these terms are frequently used interchangeably. A bulla represents a distended secondary pulmonary lobule or group of lobules involved by paraseptal or panacinar emphysema. ²³⁵ Bullous lesions have a rounded or oval configuration, indicating that there is a significant element of air trapping in the bulla ( Fig. 22.5, A and B ). Bullae must be carefully evaluated because they may be misdiagnosed as pneumothorax (see Fig. 22.1, A ). They may also rupture, causing pneumothorax and a bronchopleural communication, which may be difficult to manage (see Fig. 22.1, B ).

Radiologic distinction of the types of emphysema is frequently impossible, but there are differences in the distribution. Centrilobular emphysema (see Fig. 22.3, A and B ) tends to involve the upper lobes, whereas panacinar emphysema tends to be more severe in the bases ( Fig. 22.6 ). Bullous emphysema may accompany paraseptal or panacinar emphysema (see Fig. 22.1, A and B ).

The term chronic obstructive pulmonary disease (COPD) should not be used interchangeably with the term emphysema . 169, 174 The diagnosis of COPD is a clinical diagnosis encompassing the entire group of obstructive pulmonary diseases, including chronic bronchitis, asthma, bronchiolitis obliterans, bronchiectasis, and emphysema. Of these entities, only emphysema results in chronic bilateral loss of pulmonary vascularity and thus results in hyperlucent lungs.

In contrast to the foregoing situation, a severe acute asthmatic attack may cause bilateral air trapping with depression of the diaphragm and hyperlucent lungs, but these changes are reversible.