Pulmonology · Community-Acquired Pneumonia
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Streptococcus pneumoniae is the most common cause of community-acquired pneumonia (CAP) and typically presents with acute onset, high fever, and productive cough with rust-colored sputum.
Mycoplasma pneumoniae is a common atypical pneumonia in young adults that presents with a subacute, dry, hacking cough, bullous myringitis, and cold agglutinin-mediated hemolytic anemia.
Legionella pneumophila pneumonia is associated with contaminated water sources and presents with the classic triad of pneumonia, diarrhea, and hyponatremia.
Staphylococcus aureus pneumonia is a frequent complication following influenza infection and is characterized by cavitary lesions and necrotizing pneumonia on chest imaging.
Pseudomonas aeruginosa is the primary pathogen to suspect in patients with cystic fibrosis or structural lung disease (e.g., bronchiectasis) who present with pneumonia.
CURB-65 criteria (Confusion, Urea > 19 mg/dL, Respiratory rate ≥ 30, Blood pressure < 90/60, Age ≥ 65) are used to determine the site of care and mortality risk for patients with CAP.
Empiric treatment for healthy outpatients with CAP is amoxicillin, doxycycline, or a macrolide (if local resistance is low), while patients with comorbidities require combination therapy with a beta-lactam plus a macrolide or respiratory fluoroquinolone.
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A 68-year-old male with a history of hypertension presents to the emergency department with a 3-day history of fever, productive cough, and shortness of breath. Physical examination reveals dullness to percussion and bronchial breath sounds in the right lower lobe. Laboratory studies show a serum sodium of 128 mEq/L and a blood urea nitrogen of 24 mg/dL. The patient reports he recently returned from a cruise ship vacation. A chest X-ray demonstrates patchy, multilobar infiltrates.
What is the most likely causative organism?
Legionella pneumophila
The combination of pneumonia, hyponatremia, and a history of recent travel (e.g., cruise ship) is highly suggestive of Legionella, which is associated with contaminated water systems.
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Etiology / Epidemiology
Streptococcus pneumoniae is the most common cause. Advanced age, smoking, and COPD are primary risk factors.
Clinical Manifestations
Presents with fever, productive cough, and pleuritic chest pain. Dullness to percussion and egophony are classic findings.
Diagnosis
Chest X-ray showing lobar consolidation is the diagnostic standard. CURB-65 score determines inpatient admission.
Treatment
Amoxicillin or Doxycycline for healthy outpatients. Fluoroquinolones reserved for comorbidities or resistance risk.
Prognosis
Parapneumonic effusion and empyema are major complications. Monitor for sepsis and respiratory failure.
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Epidemiology & Etiology
Community-acquired pneumonia (CAP) is most frequently caused by S. pneumoniae. Aspiration pneumonia is common in patients with impaired consciousness or dysphagia. Pseudomonas aeruginosa is a critical concern in patients with cystic fibrosis or structural lung disease.
Pertinent Anatomy
Infection typically involves the alveolar spaces, leading to inflammatory exudate. Lobar consolidation suggests a bacterial etiology, whereas diffuse interstitial patterns often suggest atypical or viral pathogens.
Pathophysiology
Pathogens reach the lower respiratory tract via microaspiration or inhalation. The host inflammatory response triggers neutrophil infiltration and alveolar capillary leakage. This results in consolidation, which impairs gas exchange and causes hypoxemia.
Clinical Manifestations
Patients present with acute onset fever, rigors, and purulent sputum. Physical exam reveals bronchial breath sounds, crackles, and tactile fremitus. Respiratory distress, cyanosis, and altered mental status are red flags requiring immediate ICU evaluation.
Diagnosis
A Chest X-ray demonstrating an infiltrate is required for diagnosis, though clinical presentation combined with radiographic evidence remains the standard for confirming pneumonia. Sputum culture is rarely diagnostic; blood cultures are indicated only in severe cases. Use the CURB-65 score (Confusion, Urea >7 mmol/L, RR ≥30, BP <90/60, Age ≥65) to guide site-of-care decisions.
Treatment
Outpatient CAP is treated with Amoxicillin or Doxycycline. Patients with comorbidities require Amoxicillin/Clavulanate plus a macrolide. Fluoroquinolones (e.g., Levofloxacin) carry risks of tendon rupture and QT prolongation. Inpatient management requires Ceftriaxone plus Azithromycin.
Prognosis
Most patients improve within 48-72 hours of appropriate therapy. Empyema requires chest tube drainage. Follow-up Chest X-ray is recommended in smokers >50 years to rule out underlying malignancy.
Differential Diagnosis
Viral pneumonia: typically presents with more diffuse, bilateral interstitial infiltrates
Pulmonary embolism: sudden onset dyspnea without fever or leukocytosis
Congestive heart failure: bilateral edema and cardiomegaly on imaging
Atelectasis: volume loss without fever or systemic inflammatory response
Tuberculosis: chronic cough, night sweats, and upper lobe cavitary lesions