INTRODUCTION

Lung abscess is defined as necrosis of the pulmonary parenchyma caused by microbial infection. Some authorities use the term "necrotizing pneumonia" or "lung gangrene" to distinguish pulmonary necrosis with multiple small abscesses from a larger cavitary lesion, but this actually represents a continuum of the same process.

The classification, clinical features, diagnosis, and treatment of lung abscess will be reviewed here. Aspiration pneumonia, which may precede the development of a lung abscess, is discussed separately. (See "Aspiration pneumonia in adults".)

CLASSIFICATION

The term "lung abscess" is often applied to reflect the clinical features that are useful in management decisions, such as duration of prior symptoms, presence of associated conditions, or microbial etiology.

• Lung abscesses can be classified as acute or chronic based upon the duration of symptoms prior to presentation for medical care; symptoms present for one month or more are considered chronic.
• Lung abscess may be primary or secondary based upon the presence or absence of common associated conditions. Abscesses in patients prone to aspiration or patients who have been healthy previously are usually considered primary; the term "secondary lung abscess" typically indicates an associated bronchogenic neoplasm or systemic disease that compromises immune defenses, such as HIV infection or organ transplantation.
• Lung abscess can also be defined by the responsible microbial pathogen (eg, Pseudomonas lung abscess, anaerobic bacterial lung abscess, or Aspergillus lung abscess).

When no pathogen was recovered from expectorated sputum five decades ago, the lesion was referred to as a "nonspecific lung abscess." It is now thought that these infections were caused by anaerobic bacteria. Putrid lung abscess refers to the offensive odor that is often found in patients with lung abscesses and is thought to be diagnostic of anaerobic bacterial infection. A review of more than 1000 reported cases of lung abscess during the antibiotic era indicates that approximately 80 percent were considered primary; 60 percent were putrid; 40 percent were "nonspecific"; and 40 percent were chronic [1].