Community-acquired Pneumonia

Overview
  • Community-acquired Pneumonia (CAP) is a sub-categorization of Pneumonia and is defined as Pneumonia acquired in the general community or within 72 hr of hospital admission. Traditionally, the etiological pathogens of CAPs have been sub-divided into Typical and Atypical Organisms as Atypical Organisms cannot be observed with standard gram stains. Additionally, Atypical organisms tend to produce a slightly different clinical and radiological presentation. However, given the significant clinical overlap, the typical-atypical distinction may be of largely didactic usefulness.
Epidemiology
  • In general, patients who develop CAPs often possess some underlying disorder which reduces their pulmonary immunity. Consequently, CAPs occur more frequently in certain susceptible populations of patients. These include alcoholics, smokers, the elderly or those with asthma or COPD. Defects in generalized immunity also increase risk such as in asplenic or AIDS patients.
Typical CAP
  • Overview
    • Typical CAP organisms are gram-stainable bacteria with S. pneumoniae infection representing the vast majority of cases and thus the most common cause.
  • Etiologies
  • Morphology
    • Two basic gross patterns of CAP are observed
    • Lobar Pneumonia is characterized by consolidation of a contiguous area of parenchyma that may represent a part or an entire lobe of the lung. Bronchopneuonia is characterized by smaller foci of infection scattered throughout multiple lobes. Histopathologically, affected areas possess alveoli filled with neutrophils and proteinacious fluid leaked from pulmonary capillaries. Pulmonary capillaries may also display significant congestion which may result in extravasation of erythrocytes into the alveolar space.
  • Clinical Consequences
    • Consistent with the presence of large amounts of exudate in the alveoli, Typical CAPs frequently present with significant productive cough. The purulent sputum typically contains large numbers of neutrophils and gram negative bacteria may be present. Acute onset of intense constitutional symptoms such as high fever and intense headache is frequent along with dyspnea. When consolidation occurs in a lobar pattern, affected areas may display dullness upon percussion.
  • Radiography
    • Although chest radiography is usually not diagnostic, a lobar pneumonia will often be apparent as a dense consolidation on radiography.
Atypical CAP
  • Overview
    • Atypical CAP organisms cannot be detected by gram stain and represent a wide variety of etiological agents including bacteria and viruses.
  • Etiologies
  • Morphology
    • The Atypical CAP organisms largely cause pathology by adhering to and destroying type I pneumocytes. The primary morphological change is an inflammation and thickening of the alveolar septa frequently with macrophages and lymphocytes. Consequently, the alveolar space is often free of proteinacious exudate or inflammatory cells.
  • Clinical Consequences
    • Consistent with the lack of exudate within alveoli, atypical CAPs are characterized by a non-productive cough. Acute onset constitutional symptoms are frequent and include fever and headache and may be associated with dyspnea. Historically, Atypical CAP was thought to have a more insidious onset and display milder symptoms; however, severe illness is common.
  • Radiography
    • Although chest radiography is usually not diagnostic, atypical CAP is more likely to be associated with diffuse, patchy infiltrate.
Complications
  • Pleural Effusion: Involvement of the pleura may result in pleural effusion which is generally purulent (i.e. empyema) and is heralded by the development of pleuritic chest pain
  • ARDS: May be a consequence of severe infection
Treatment
  • In many cases the precise organismal etiology of CAP is never determined. Therefore, treatment is largely empiric and guided by the patient's medical history and clinical presentation which may suggest a possible organismal etiology.
  • In general, a third generation cephalosporin, such as ceftriaxone, is used to cover typical organisms, especially Streptococcus pneumoniae, along with a macrolide such as azithromycin. If the case is suspected to be mild, single therapy with a oral fluoroquinolone such as moxifloxacin or levofloxacin can be used.