| Community-Acquired PneumoniaThe 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.
- 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.
- 8001500 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. 6069 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 ALPSalcoholism, leukopenia, and pneumococcal sepsisis 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!
 |
| |