Pneumonia for USMLE Step 2
Pneumonia for the USMLE Step 2 Exam
Etiology
- Bacterial Causes:
- Streptococcus pneumoniae: Most common cause of community-acquired pneumonia (CAP). Classic symptoms include fever, productive cough with purulent sputum, and pleuritic chest pain.
- Haemophilus influenzae: Common in patients with chronic lung diseases such as chronic obstructive pulmonary disease (COPD).
- Mycoplasma pneumoniae: Causes atypical pneumonia, often in young adults. Symptoms are less severe, with dry cough, low-grade fever, and extrapulmonary manifestations (e.g., rash, hemolysis).
- Legionella pneumophila: Severe pneumonia, associated with water sources. Symptoms include high fever, gastrointestinal symptoms, and hyponatremia.
- Viral Causes:
- Influenza virus: A common viral cause of pneumonia, especially during flu season.
- SARS-CoV-2 (COVID-19): A major cause of viral pneumonia that can progress to acute respiratory distress syndrome (ARDS).
- Aspiration Pneumonia:
- Occurs when oropharyngeal or gastric contents are inhaled into the lungs, typically in patients with impaired consciousness, dysphagia, or alcohol use. Common pathogens include anaerobes and oral flora.
Pathophysiology
- Bacterial Pneumonia:
- Bacteria enter the alveoli and trigger an inflammatory response. Neutrophils, cytokines, and inflammatory mediators flood the alveoli, leading to consolidation, impaired gas exchange, and the clinical signs of pneumonia.
- Atypical Pneumonia:
- Atypical pathogens, like Mycoplasma and Chlamydophila pneumoniae, affect the respiratory interstitium, causing diffuse inflammation with fewer systemic symptoms.
Clinical Features
- Typical Pneumonia:
- Sudden onset of high fever, productive cough with purulent sputum, pleuritic chest pain, and dyspnea.
- Physical exam may show crackles, bronchial breath sounds, and dullness to percussion over the affected area.
- Atypical Pneumonia:
- Gradual onset with low-grade fever, dry cough, and non-respiratory symptoms such as headache, myalgias, and fatigue.
- Aspiration Pneumonia:
- Frequently involves the right lower lobe due to the anatomy of the right mainstem bronchus. Patients may have fever, cough, and foul-smelling sputum.
Diagnosis
- Chest X-ray:
- Lobar consolidation is characteristic of bacterial pneumonia (e.g., S. pneumoniae).
- Diffuse interstitial infiltrates are seen in atypical pneumonia (e.g., Mycoplasma pneumoniae, viral pneumonia).
- Microbiologic Testing:
- Sputum Gram stain and culture: Useful for bacterial identification.
- Urine antigen tests: Detect S. pneumoniae and Legionella in severe cases.
- Laboratory Findings:
- Leukocytosis with neutrophil predominance in bacterial pneumonia.
- Cold agglutinins in Mycoplasma pneumoniae infection.
Management
- Outpatient Antibiotic Therapy:
- For previously healthy individuals, amoxicillin, macrolides (e.g., azithromycin), or doxycycline are recommended for mild CAP.
- Inpatient Antibiotic Therapy:
- For patients requiring hospitalization, combination therapy with a beta-lactam (e.g., ceftriaxone) plus a macrolide or a respiratory fluoroquinolone is recommended.
- Supportive Care:
- Oxygen therapy for hypoxia.
- Fluids for dehydration or septic shock.
Complications
- Parapneumonic Effusion and Empyema:
- Accumulation of infected pleural fluid that may require drainage via thoracentesis or chest tube placement.
- Lung Abscess:
- Necrosis of lung tissue with cavitary lesions, more common in aspiration pneumonia. Requires prolonged antibiotic therapy.
- Acute Respiratory Distress Syndrome (ARDS):
- A severe complication in which inflammation causes alveolar damage, leading to hypoxemia and respiratory failure, requiring mechanical ventilation.
Prevention
- Vaccination:
- Pneumococcal vaccine (PCV13 and PPSV23) is recommended for adults over 65 or those with chronic medical conditions.
- Annual influenza vaccination reduces the risk of secondary bacterial pneumonia.
Key Points
- Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, presenting with fever, productive cough, and pleuritic chest pain.
- Atypical pneumonia (e.g., Mycoplasma pneumoniae) presents with dry cough, low-grade fever, and extra-pulmonary symptoms such as rash.
- Diagnosis is based on clinical features and confirmed by chest X-ray showing lobar consolidation in typical pneumonia or diffuse infiltrates in atypical pneumonia.
- Treatment includes empiric antibiotics, typically amoxicillin or macrolides for outpatient cases, and beta-lactams plus macrolides for hospitalized patients.
- Prevention includes pneumococcal and influenza vaccination, especially in high-risk populations.