The Harrison's Practice Preview allows you to view 5 FREE complete topics or conduct a search that delivers abstracts for more than 450 medical topics. For full access, please subscribe today!

Definition

• Community-acquired pneumonia (CAP) is an infection of the alveoli, distal airways, and interstitium of the lungs that occurs outside the hospital setting.
  • See also Hospital-Acquired Pneumonia.
• Characterized clinically by:
  • Fever, chills, cough, pleuritic chest pain, sputum production
  • At least one opacity on chest radiography
• Manifests as four general patterns
  • Lobar pneumonia: involvement of an entire lung lobe
  • Bronchopneumonia: patchy consolidation in one or several lobes, usually in dependent lower or posterior portions centered around bronchi and bronchioles
  • Interstitial pneumonia: inflammation of the interstitium, including the alveolar walls and connective tissue around the bronchovascular tree
  • Miliary pneumonia: numerous discrete lesions due to hematogenous spread

Epidemiology

• Incidence: U.S.
  
  • 800–1500 cases per 100,000 persons annually
  • Affects 4 million adults per year
    • ~20% require hospitalization.
  • Annual cost: $9.7 billion
• Age
  • Incidence highest at extremes of age
• Sex
  • Rate higher among men than among women
• Race
  • More common among African Americans than among whites
• Seasonality
  • More common during the winter months
    

Risk Factors

• Independent risk factors for CAP include:
  • Alcoholism [relative risk (RR) 9]
  • Asthma (RR 4.2)
  • Immunosuppression (RR 1.9)
  • Age > 70 years (RR 1.5 vs. 60–69 years)
• Risk factors for pneumococcal pneumonia include:
  • Dementia
  • Seizures
  • Congestive heart failure
  • Cerebrovascular disease
  • Tobacco smoking
  • Alcoholism
  • Chronic obstructive pulmonary disease (COPD)
  • HIV infection
    • Risk up to 40 times that in age-matched patients not infected with HIV
• Risk factors for invasive pneumococcal disease include:
  • Male gender
  • African-American race
  • Chronic illness
  • Current tobacco smoking (strongest independent predictor among immunocompetent young adults)
  • Passive exposure to tobacco smoke
  • Immunologic defects
    • Multiple myeloma
    • Nephrotic syndrome with low serum immune globulin levels
    • Splenectomy
    • HIV infection
    • Others
• Risk factors for Legionnaires’ disease include:
  • Male gender
  • Current tobacco smoking
  • Diabetes
  • Hematologic malignancy
  • Cancer
  • End-stage renal disease
  • HIV infection
• Risk factors for gram-negative bacterial pneumonia (including that caused by Pseudomonas aeruginosa)
  • Probable aspiration
  • Previous hospital admission
  • Previous antimicrobial treatment
  • Bronchiectasis
  • Neutropenia
• Alcohol use
  • Heavy drinkers (i.e., those consuming > 100 g of ethanol per day for the preceding 2 years)
    • Higher incidence of gram-negative bacterial pneumonia
    • Worse clinical symptoms
    • Require longer courses of IV antibiotic therapy than do nondrinkers
  • More prolonged fever, slower resolution, and a higher rate of empyema have been noted in pneumococcal pneumonia patients with chronic alcoholism than in their nondrinking counterparts.
  • The clinical entity designated ALPS—alcoholism, leukopenia, and pneumococcal sepsis—is associated with a mortality rate of 80%.
  • Excessive alcohol use is an independent risk factor for the development of acute respiratory distress syndrome (ARDS).

Etiology

• Most cases of CAP are caused by a few common respiratory pathogens, including:
  • Streptococcus pneumoniae
    • Accounts for ~50% of all cases of CAP requiring hospital admission
  • Haemophilus influenzae
  • Staphylococcus aureus
  • Mycoplasma pneumoniae
  • Chlamydia pneumoniae
  • Moraxella catarrhalis
  • Legionella spp.
  • Aerobic gram-negative bacteria
  • Influenza viruses
  • Adenoviruses
  • Respiratory syncytial virus
  • Other rare organisms
    • Viral: hantavirus, Nipah virus, Hendra virus, metapneumovirus, severe acute respiratory syndrome (SARS) virus
    • Nonviral: Pneumocystis, Mycobacterium tuberculosis , fungi, bioterrorism agents (e.g., those of Q fever, tularemia, anthrax, plague), etc.
• The relative frequency of these pathogens differs with the age of the patient and the severity of the pneumonia.
• Pathogenesis
  • Microaspiration of oropharyngeal secretions colonized with pathogenic microorganisms (e.g., S. pneumoniae, H. influenzae) is the most common route.
  • Gross aspiration
    • Central nervous system disorders that affect swallowing (e.g., stroke, seizures)
    • Impaired consciousness (e.g., in alcoholism, IV drug use)
    • Anesthesia or intubation
    • Pathogens include anaerobic organisms and gram-negative bacilli.
  • Aerosolization (e.g., of M. tuberculosis, Legionella spp., viruses)
  • Hematogenous spread (e.g., seeding of the lungs by S. aureus during endocarditis)
  • Contiguous spread from another site

Associated Conditions

• Infections with encapsulated organisms such as S. pneumoniae, H. influenzae, and Neisseria meningitidis may suggest underlying immunodeficiency due to multiple myeloma, nephrotic syndrome, etc.
• Pneumococcal CAP is particularly common among patients with HIV infection.
  

The Harrison's Practice Preview allows you to view 5 FREE complete topics or conduct a search that delivers abstracts for more than 450 medical topics. For full access, please subscribe today!