Anatomic Alterations of the Lungs

A lung abscess is defined as a necrosis of lung tissue that in severe cases leads to a localized air- and fluid-filled cavity. The fluid in the cavity is a collection of purulent exudate that is composed of liquefied white blood cell remains, proteins, and tissue debris. The air- and fluid-filled cavity is encapsulated in a so-called pyogenic membrane that consists of a layer of fibrin, inflammatory cells, and granulation tissue.

During the early stages of a lung abscess, the pathology is indistinguishable from that of any acute pneumonia. Polymorphonuclear leukocytes and macrophages move into the infected area to engulf any invading organisms. This action causes the pulmonary capillaries to dilate, the interstitium to fill with fluid, and the alveolar epithelium to swell from the edema fluid. In response to this inflammatory reaction, the alveoli in the infected area become consolidated (see Figure 15-1).

As the inflammatory process progresses, tissue necrosis involving all the lung structures occurs. In severe cases the tissue necrosis ruptures into a bronchus and allows a partial or total drainage of the liquefied contents into the cavity. An air- and fluid-filled cavity also may rupture into the intrapleural space via a bronchopleural fistula and cause pleural effusion and empyema (see Chapter 23, Pleural Diseases). This may lead to inflammation of the parietal pleura, chest pain, atelectasis, and decreased chest expansion. After a period of time, fibrosis and calcification of the tissues around the cavity encapsulate the abscess (see Figure 16-1).

The major pathologic or structural changes associated with a lung abscess are as follows:

Etiology and Epidemiology

A lung abscess is commonly associated with the aspiration of gastric and oral fluids. Aspiration can cause either (1) chemical pneumonia, (2) anaerobic bacterial pneumonia, or (3) a combination of both (see Chapter 15). The aspiration of acidic gastric fluids is associated with immediate injury to the tracheobronchial tree and lung parenchyma—often likened to a flash burn. Common anaerobic organisms found in the normal flora of the mouth, upper respiratory tract, and gastrointestinal tract include the following:

Anaerobic organisms often colonize in the small grooves and spaces between the teeth and gums in patients with poor oral hygiene; anaerobic organisms are frequently associated with gingivitis and dead or abscessed teeth. Aspiration often occurs in the patient with a decreased level of consciousness. Predisposing factors include (1) alcohol abuse, (2) seizure disorders, (3) general anesthesia, (4) head trauma, (5) cerebrovascular accidents, and (6) swallowing disorders. The incidence of lung abscesses caused by anaerobic organisms is also high in patients with poor oral hygiene. Anaerobic organisms are cultured in 62% to 87% of cases of aspiration pneumonia, in 85% to 93% of lung abscess cases, in 62% to 76% of patients with empyema, and as many as 94% of patients with exacerbations of bronchiectasis.

Other organisms known to cause a lung abscess are Klebsiella, Staphylococcus, Mycobacterium tuberculosis (including the atypical organisms Mycobacterium kansasii and Mycobacterium avium), Histoplasma capsulatum, Coccidioides immitis, Blastomyces, and Aspergillus fumigatus. Some parasites such as Paragonimus westermani, Echinococcus, and Entamoeba histolytica may also cause lung abscess formation. On rare occasions a lung abscess may also be caused by Streptococcus pneumoniae, Pseudomonas aeruginosa, or Legionella pneumophila. Typically, more than one type of bacterium is involved, as in an infection with anaerobic organisms mixed with aerobic ones.

Finally, a lung abscess may develop as a result of (1) bronchial obstruction with secondary cavitating infection (e.g., distal to bronchogenic carcinoma or an aspirated foreign body), (2) vascular obstruction with tissue infarction (e.g., septic embolism, vasculitis), (3) interstitial lung disease with cavity formation (e.g., pneumoconiosis [silicosis], Wegener’s granulomatosis, and rheumatoid nodules), (4) bullae or cysts that become infected (e.g., congenital or bronchogenic cysts), or (5) penetrating chest wounds that lead to an infection (e.g., bullet wound).

Anatomically, a lung abscess most commonly forms in the superior segments of the lower lobes and the posterior segments of the upper lobes. The tendency for an abscess to form in these areas is because of the effect of gravity and the dependent position of the tracheobronchial tree at the time of aspiration, which commonly occurs while the patient is in the supine position. The right lung is more commonly involved than the left.

Box 16-1 on p. 247 summarizes organisms known to cause lung abscess.