Pneumonia for USMLE Step 3
Pneumonia for the USMLE Step 3 Exam
Etiology
- Bacterial Pathogens:
- Streptococcus pneumoniae: The most common cause of community-acquired pneumonia (CAP). Presents with acute onset of fever, productive cough with purulent sputum, and pleuritic chest pain.
- Haemophilus influenzae: Common in patients with chronic obstructive pulmonary disease (COPD).
- Mycoplasma pneumoniae: Causes atypical pneumonia, presenting with non-productive cough, low-grade fever, and extrapulmonary manifestations like rash and hemolysis.
- Legionella pneumophila: Associated with contaminated water sources (e.g., air conditioning systems). Symptoms include high fever, diarrhea, and hyponatremia.
- Viral Pathogens:
- Influenza virus: A common cause of viral pneumonia, particularly during flu season.
- SARS-CoV-2: Responsible for COVID-19, which can progress to severe pneumonia and acute respiratory distress syndrome (ARDS).
- Aspiration Pneumonia:
- Occurs due to inhalation of oral or gastric contents into the lungs, typically in patients with altered mental status or swallowing dysfunction. Often involves anaerobic bacteria.
Pathophysiology
- Bacterial Pneumonia:
- Bacteria invade the alveoli, triggering an immune response. Neutrophils and inflammatory mediators accumulate, causing alveolar consolidation, impaired gas exchange, and clinical symptoms of pneumonia.
- Atypical Pneumonia:
- Atypical pathogens (e.g., Mycoplasma, Legionella) affect the alveolar interstitium, leading to a more diffuse inflammatory response and less dramatic clinical findings compared to typical pneumonia.
Clinical Features
- Typical Pneumonia:
- Sudden onset of high fever, productive cough, pleuritic chest pain, and dyspnea.
- Physical exam: Crackles (rales), dullness to percussion, and increased tactile fremitus over affected areas.
- Atypical Pneumonia:
- Presents with gradual onset of dry cough, mild fever, and extrapulmonary symptoms like headache, myalgias, and fatigue. Legionella may also cause gastrointestinal symptoms.
- Aspiration Pneumonia:
- Affects the right lower lobe due to the anatomy of the bronchial tree. Symptoms include productive cough, foul-smelling sputum, and fever.
Diagnosis
- Chest X-ray:
- Typical pneumonia shows lobar consolidation (e.g., S. pneumoniae).
- Atypical pneumonia presents with diffuse interstitial infiltrates (e.g., Mycoplasma, viral).
- Microbiologic Testing:
- Sputum Gram stain and culture can help identify bacterial pathogens.
- Urinary antigen tests: Useful for S. pneumoniae and Legionella.
- Laboratory Findings:
- Leukocytosis is common in bacterial infections, while atypical pathogens may show a normal or slightly elevated white blood cell count.
Management
- Outpatient Treatment:
- For previously healthy patients with CAP, amoxicillin, macrolides (e.g., azithromycin), or doxycycline are first-line therapies.
- Inpatient Treatment:
- For severe CAP requiring hospitalization, a combination of a beta-lactam (e.g., ceftriaxone) and a macrolide (e.g., azithromycin) or a fluoroquinolone (e.g., levofloxacin) is recommended.
- Supportive Care:
- Oxygen therapy for hypoxia, fluids for dehydration, and antipyretics for fever control.
- Aspiration Pneumonia:
- Empiric treatment includes antibiotics effective against anaerobes, such as clindamycin or amoxicillin-clavulanate.
Complications
- Parapneumonic Effusion and Empyema:
- Accumulation of infected fluid in the pleural space. Requires drainage and prolonged antibiotic therapy.
- Lung Abscess:
- Localized pus formation within lung tissue, often due to aspiration. Treated with prolonged antibiotics or surgical drainage.
- ARDS (Acute Respiratory Distress Syndrome):
- A severe complication in which inflammation causes alveolar damage, leading to hypoxemia and respiratory failure, requiring mechanical ventilation.
Prevention
- Vaccination:
- Pneumococcal vaccines (PCV13, PPSV23) are recommended for adults over 65 or younger individuals with chronic illnesses.
- Annual influenza vaccination reduces the risk of pneumonia due to influenza.
- Smoking Cessation:
- Smoking cessation lowers the risk of pneumonia by improving respiratory function and reducing mucosal damage.
Key Points
- Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, characterized by fever, productive cough, and pleuritic chest pain.
- Atypical pneumonia (e.g., Mycoplasma pneumoniae) presents with milder respiratory symptoms and extrapulmonary features.
- Diagnosis is based on clinical presentation and confirmed with chest X-ray showing lobar consolidation for typical pneumonia or diffuse infiltrates for atypical pneumonia.
- Treatment includes empiric antibiotics such as amoxicillin, macrolides, or fluoroquinolones depending on the severity and setting.
- Complications include parapneumonic effusion, lung abscess, and ARDS, with prevention strategies focusing on vaccination and smoking cessation.