GENERAL FEATURES
• Community-acquired pneumonia (CAP) is an acute infection of the pulmonary parenchyma occurring in a patient not hospitalized or residing in a long-term-care facility within 14 days before onset of symptoms.
• Host defense mechanisms are diminished by disease (viral upper respiratory tract infections) or habits (smoking or alcohol abuse).
• The causative organism is not identified in 50% of cases.
• Causative pathogens include
– Streptococcus pneumoniae: Most common causative pathogen (30%-60% of all cases)
- A particularly important cause of pneumonia in the elderly and patients requiring hospitalization
– Hemophilus influenzae: Second most common bacterial cause (10% of cases)
- Of particular importance in patients with chronic obstructive pulmonary disease (COPD)
– Mycoplasma pneumoniae: Most
frequent atypical pathogen, accounting for 1% to 10% of cases
- More often seen in young adults
– Chlamydophila (formerly Chlamydia) pneumoniae: Atypical pathogen recognized as a frequent cause of hospital-acquired pneumonia
– Legionella pneumophilia: Opportunistic bacterium that infects patients with renal failure, COPD, and organ-transplant recipients.
- Infection results in severe illness; patients decline rapidly and often require hospitalization.
CLINICAL ASSESSEMENT
• History and physical examination
– Patients are usually in their mid-50s and 60s with one or more chronic diseases (COPD, diabetes mellitus, as well as cardiovascular or neurologic conditions).
– Typical bacterial pneumonias are likely to induce a productive cough with large amounts of sputum.
– Viral pneumonias and M pneumoniae and C pneumoniae infections induce a severe, hacking cough that rarely produces sputum
– Patients infected with Legionella species may have either a mucopurulent or nonproductive cough.
– Dyspnea is reported, and one-third of patients will have pleurisy.
– 80% of patients will have fever, with half experiencing chills. High fever is characteristic of Legionella species.
– Other findings are rales (crackles) and rhonchi, tachypnea, bronchial breath sounds, and tachycardia.
– Consolidation signs are possible (increased fremitus, egophony, percussion dullness).
• More difficult to diagnose in the elderly because fever or cough is less frequent and dyspnea not evident.
– Confusion or falls are presenting symptoms.
• Atypical pathogens often have associated extrapulmonary symptoms.
– Legionella species: diarrhea and abdominal pain accompanied by CNS symptoms, particularly confusion
– M pneumoniae: diarrhea associated with coryza, myringitis, and pharyngitis
– C pneumoniae: pharyngitis and sinusitis
• Onset of symptoms in viral pneumonia is less abrupt and is associated with myalgias and flulike symptoms.
– Respiratory viruses rarely cause frank pneumonia, except in children.
– Influenza is the most common viral cause of pneumonia in adults.
DIAGNOSIS
• Chest radiography essential for patients with significant, productive cough or other symptoms suggestive of pneumonia
– Any pneumonia may produce
any image on radiography,
including an initially normal radiograph.
• Routine diagnostic tests (sputum Gram's stain and culture) are not necessary for treatment in the outpatient setting.
• Sputum Gram's stain and culture should be obtained for hospitalized patients, particularly those in the ICU, before initiating antibiotic therapy.
– Acceptable sputum sample: >25 neutrophils and <10 squamous epithelial cells per low-power field
– Therapy should not be delayed beyond 6 hours by attempts to obtain an adequate sputum sample.
– Obtain an endotracheal aspirate from intubated patients.
• Urinary antigen tests for Legionella species and S pneumoniae should be obtained for severely ill patients, particularly those in the ICU.
• In CAP caused by Legionella species, M pneumoniae, C pneumoniae, serologic studies are available but are mainly of epidemiologic use.