Pneumonia for USMLE Step 1
Pneumonia for the USMLE Step 1 Exam
Etiology
- Bacterial Pathogens:
- Streptococcus pneumoniae: The most common cause of community-acquired pneumonia (CAP). It presents with fever, productive cough, pleuritic chest pain, and lobar consolidation.
- Haemophilus influenzae: Frequently seen in patients with chronic obstructive pulmonary disease (COPD).
- Mycoplasma pneumoniae: Causes atypical pneumonia, with dry cough, fever, and extrapulmonary manifestations, such as rash and cold agglutinins.
- Legionella pneumophila: Causes severe pneumonia with high fever, GI symptoms, and hyponatremia. Seen in outbreaks related to water systems.
- Viral Pathogens:
- Influenza virus: A common cause of viral pneumonia, especially during the winter.
- Respiratory syncytial virus (RSV): More common in infants and older adults.
- SARS-CoV-2: The virus responsible for COVID-19, associated with viral pneumonia and acute respiratory distress syndrome (ARDS) in severe cases.
- Aspiration Pneumonia:
- Occurs when oropharyngeal or gastric contents are inhaled into the lungs. Risk factors include altered mental status, alcohol use, and gastroesophageal reflux.
Pathophysiology
- Bacterial Pneumonia:
- Bacterial pathogens bypass defense mechanisms and reach the alveoli, causing an intense inflammatory response. Neutrophil infiltration leads to alveolar exudates and consolidation, resulting in impaired gas exchange.
- Atypical Pneumonia:
- Atypical pathogens like Mycoplasma affect the interstitium and alveolar walls, leading to diffuse inflammation with milder clinical symptoms.
Clinical Features
- Typical Pneumonia:
- Sudden onset of high fever, productive cough with purulent sputum, pleuritic chest pain, and dyspnea.
- Physical exam may reveal crackles (rales), bronchial breath sounds, and dullness to percussion over affected areas.
- Atypical Pneumonia:
- Gradual onset of low-grade fever, dry cough, headache, and myalgias. Extrapulmonary symptoms (e.g., rash, cold agglutinins) may be present in cases like Mycoplasma pneumoniae.
Diagnosis
- Chest X-ray:
- Shows lobar consolidation in typical bacterial pneumonia (e.g., S. pneumoniae).
- Diffuse, interstitial infiltrates are characteristic of atypical pneumonia (e.g., Mycoplasma pneumoniae, viruses).
- Microbiological Testing:
- Sputum Gram stain and culture: Can identify the bacterial pathogen.
- Urine antigen tests: Useful for detecting S. pneumoniae and Legionella.
- Laboratory Findings:
- Leukocytosis is common in bacterial pneumonia.
- Cold agglutinins are seen in Mycoplasma pneumoniae infections.
Management
- Empiric Antibiotic Therapy:
- Outpatient treatment: For uncomplicated CAP, amoxicillin, macrolides (e.g., azithromycin), or doxycycline are recommended.
- Inpatient treatment: For severe CAP, combination therapy with a beta-lactam (e.g., ceftriaxone) plus a macrolide or a respiratory fluoroquinolone is preferred.
- Adjunctive Therapy:
- Oxygen therapy for hypoxia.
- Fluids and supportive care for dehydration or sepsis.
Complications
- Parapneumonic Effusion and Empyema:
- Collection of pus in the pleural cavity may require drainage.
- Lung Abscess:
- Cavitary lesions can develop, especially in aspiration pneumonia, requiring prolonged antibiotics.
- Sepsis and ARDS:
- Severe pneumonia can lead to systemic inflammation, septic shock, and ARDS, particularly in immunocompromised or critically ill patients.
Key Points
- Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, with typical symptoms of high fever, productive cough, and pleuritic chest pain.
- Atypical pathogens, such as Mycoplasma pneumoniae, present with milder respiratory symptoms and extrapulmonary manifestations.
- Chest X-ray is the primary imaging modality for diagnosis, showing lobar consolidation in typical pneumonia and interstitial infiltrates in atypical or viral pneumonia.
- Management includes empiric antibiotics tailored to the patient’s clinical setting, with macrolides, beta-lactams, and fluoroquinolones commonly used.
- Complications include parapneumonic effusion, lung abscess, and ARDS in severe cases.