Segmental and Lobar Consolidations

Lobar Consolidation

Lobar consolidation results from alveolar filling with fluid, exudate, or tumor that solidifies the lung. ²²¹ The radiologic appearance of a consolidated lobe is a homogeneous confluent opacity that obliterates the normal vascular markings and often contains air bronchograms (see Fig. 14.1, A and B ). Lobar pneumonia is the most common cause of lobar consolidation ( Charts 14.1 and 14.2 ). The opacity is frequently bounded by the fissures, and stretched fissures may even lead to the appearance of an expanded lobe (see Fig. 14.2, A and B ). Because the airways are frequently air filled and surrounded by airless lung, air bronchograms are another component of the classic appearance of lobar consolidation.

From the foregoing description, the distinction of lobar consolidation from atelectasis would appear to be simple. Fraser et al. ¹⁷⁵ have emphasized that consolidation of a lobe with fluid is the antithesis of lobar atelectasis. Therefore, bulging fissures that indicate lobar expansion are unequivocal evidence of consolidation. As mentioned in Chapter 13 , the displacement of fissures together indicates volume loss in the involved lobe. The problem with applying this approach to the evaluation of a lobar opacity for the purpose of distinguishing atelectasis from consolidation is not in the evaluation of the lobe that is expanded, but rather in the evaluation of the lobe of diminished size. It will become apparent in this discussion that a number of the processes that consolidate the lung also produce small airway obstruction, so that the volume loss produced by the process may be greater than the quantity of fluid that is consolidating the lung. Thus, a consolidating process may also be the cause of atelectasis. For example, bronchopneumonia, which causes air space consolidations, also leads to small airway obstructions that may result in atelectasis.

Lobar Pneumonia

The distinction between lobar pneumonia and bronchopneumonia has been deemphasized in favor of a bacteriologic classification of pneumonias, which is more relevant in determining appropriate therapy. However, knowledge of the gross morphologic distinctions between the two types of pneumonia is important for understanding the variety of radiologic patterns that may be encountered in acute pneumonias.

The pathogenesis of lobar pneumonia is the key to understanding the spectrum of radiologic patterns. Experimental evidence indicates that the pathogens, in the form of small, infected, mucous particles, are inhaled to the periphery of the lung, where they set up foci of reaction. Initially, the tissue reaction involves the exudation of watery edema fluid into alveolar spaces. The exudate then spreads into the air passages and their alveoli. As the alveoli fill, the exudate spreads into adjacent lobules and segments. This movement of the exudate occurs via the pores of Kohn, canals of Lambert, and small airways, but does not appear to spread via the bronchovascular bundle or interstitium of the lung. The watery edema fluid serves as a culture medium for the rapid multiplication of the bacteria. The alveolar walls respond to the organisms by releasing polymorphonuclear leukocytes. The spread of the process through the collateral channels, rather than the bronchioles, explains why lobar pneumonia often does not follow a segmental distribution. Rather, lobar pneumonia produces opacities that appear to involve multiple segments early in the course of the process. During these early phases, the radiologic appearance is that of a nonsegmental sublobar consolidation, which may appear rather sharply circumscribed because of the uniform involvement of contiguous alveoli. This leads to so-called round pneumonia. ⁶⁰⁸ Round pneumonia is more commonly seen in children but may occur as an early stage of lobar pneumonia in adults ( Fig. 14.3, A and B ). Fully developed, classic lobar consolidation is less commonly encountered because early diagnosis of bacterial pneumonia followed by appropriate antibiotic therapy frequently arrests the process in its early phases. Since the initial focus of infection is the periphery of the lung, it is not surprising that pleural effusion and empyema are potential complications. However, the occurrence of these complications is also drastically reduced by early antibiotic therapy.

Streptococcus pneumoniae, formally Diplococcus pneumoniae, is the most common bacteriologic cause of community-acquired lobar pneumonia (answer to question 1 is b ), but the organism involved cannot be determined from the radiologic appearance. K. pneumoniae is another cause of lobar pneumonia that tends to follow a more aggressive course than S. pneumoniae . It is more likely to expand a lobe and produce necrosis of lung tissue with cavitation (see Chapter 23 ; answer to question 2 is c .)

The clinical course of lobar pneumonia is characterized primarily by nonspecific symptoms, such as productive cough and temperature elevation, so additional history should be obtained. For example, is the patient debilitated by alcoholism or immunologic suppression? Considering the radiologic appearance of an expanding lobar consolidation with cavitation and a history of alcoholism allows the diagnosis of K. pneumoniae to be proposed with confidence, but the definitive diagnosis of a bacterial pneumonia can be made only by analysis of smears and cultures of the sputum, blood, and lung aspirate.