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Overview

Aspiration pneumonia, lung abscess, and necrotic lung are parenchymal lung diseases. Aspiration pneumonia refers to the pulmonary consequences that follow abnormal entry of fluid, particulate substances, or endogenous secretions from the upper airways or gastric contents into the lower airways (see also Chapter 69). To develop aspiration pneumonia, a series of formidable host defense mechanisms that normally protect the lower airways must be overcome, including glottic closure via the cricopharyngeus muscle, the cough reflex, ciliary clearance, and other defense mechanisms. The material aspirated must generate an inflammatory response or cause obstruction. The nature of the pneumonia that develops depends on the inoculum and the host response. Anaerobic bacteria are the most common pathogens in this setting, reflecting both pathogenic potential and importance in the normal flora of the upper airways. Risk factors for aspiration may be transient (anesthesia, intoxication) or persistent (e.g., neuromuscular disorders, achalasia) with the risk for recurrence depending on recognition and resolution of the inciting defect.1,2

Lung abscesses reflect infection with an unusual microbial burden (e.g., acute aspiration), a failure in microbial clearance mechanisms (e.g., bronchial obstruction), or both, leading to necrosis of pulmonary tissue and formation of cavities containing necrotic debris or fluid (Fig. 127-1). The formation of multiple smaller (less than 2 cm) abscesses in pulmonary tissue is occasionally referred to as necrotizing pneumonia or lung gangrene. Both lung abscess and necrotizing pneumonia are manifestations of the same pathologic processes, and the distinction is, therefore, arbitrary.3

Figure 127-1

A. Anaerobic pneumonia with abscess formation in a 48-year-old alcoholic man. The abscesses are located in the posterior segment of right upper lobe, a dependent segment that is seen best on lateral view (B).

Empyema refers to a purulent collection in any body site but is commonly used to indicate infection of the pleural space.4 Empyema is typically associated with underlying pulmonary parenchymal infection but may also be associated with blood-borne infection, thoracic surgery, trauma, abdominal infection, or neoplasm.3 Failure to recognize and treat either empyema or lung abscess is associated with a poor clinical outcome.5,6 In the preantibiotic era, lung abscess was associated with a mortality approaching 40%.7 However, controversy exists over the best approaches to both processes in terms of antimicrobial selection and physical drainage.5,6

History

In 1893, Veillon8 published a review of “fetid infections,” first marking the published record of infections due to anaerobic pathogens. However, anaerobes are now largely forgotten potential pathogens in pulmonary infection, including in both community- and healthcare-associated pneumonia.1 The clinical and bacteriologic features of anaerobic infections of the lung have been documented by extensive studies during two periods of investigation.1 The first was at the turn of the century, when anaerobic bacteria ...

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