USMLE Guide: Pneumonia

Introduction

Pneumonia is a common and potentially serious infection that affects the lungs. It can be caused by various pathogens, including bacteria, viruses, fungi, and parasites. This USMLE guide aims to provide a comprehensive overview of pneumonia, including its etiology, pathophysiology, clinical presentation, diagnosis, and management.

Etiology

1. Bacterial pneumonia: Common causative bacteria include Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and Klebsiella pneumoniae.
2. Viral pneumonia: Common viruses include influenza viruses, respiratory syncytial virus (RSV), and adenovirus.
3. Fungal pneumonia: Fungi such as pneumocystis jirovecii and histoplasma capsulatum can cause pneumonia, especially in immunocompromised individuals.
4. Parasitic pneumonia: Parasites like Toxoplasma gondii and Strongyloides stercoralis can cause pneumonia in certain populations.

Pathophysiology

1. Bacterial pneumonia: Pathogens enter the lungs, leading to an inflammatory response. The alveoli become filled with exudate, impairing gas exchange.
2. Viral pneumonia: Viruses infect the respiratory epithelial cells, causing damage and inflammation. Viral replication within the cells further contributes to the disease process.
3. Fungal pneumonia: Fungi can cause pneumonia by direct invasion of lung tissue or through inhalation of spores. Immune response to fungal antigens leads to tissue damage.
4. Parasitic pneumonia: Parasites can invade the lungs, causing inflammation and tissue damage. Immune responses against the parasites contribute to the pathology.

Clinical Presentation

1. General symptoms: Fever, cough (productive or non-productive), chest pain, fatigue, and shortness of breath.
2. Bacterial pneumonia: Rapid onset, high fever, productive cough with purulent sputum, pleuritic chest pain, and consolidation on chest examination.
3. Viral pneumonia: Gradual onset, low-grade fever, non-productive cough, myalgias, headache, and diffuse bilateral infiltrates on chest imaging.
4. Fungal pneumonia: Variable presentation depending on the causative agent. Individuals with Pneumocystis jirovecii pneumonia may have dyspnea, non-productive cough, and hypoxemia.
5. Parasitic pneumonia: Symptoms can mimic bacterial or viral pneumonia, but eosinophilia may be present in the peripheral blood.

Diagnosis

1. Clinical evaluation: Thorough history, physical examination, and assessment of vital signs.
2. Chest X-ray: Can reveal consolidation, infiltrates, or pleural effusion.
3. Laboratory tests: Complete blood count (CBC) to assess leukocytosis or leukopenia, C-reactive protein (CRP), procalcitonin levels, and blood cultures in suspected bacterial pneumonia.
4. Sputum culture: Collecting and culturing sputum can help identify the causative organism in bacterial pneumonia.
5. Other tests: PCR-based tests for viral pathogens, serological tests for fungal or parasitic pneumonia, and bronchoscopy with bronchoalveolar lavage in certain cases.

Management

1. Bacterial pneumonia: Empirical antibiotic therapy based on the suspected pathogen, with choices guided by local resistance patterns. Treatments may include macrolides, fluoroquinolones, or beta-lactam antibiotics.
2. Viral pneumonia: Supportive care, including rest, hydration, antipyretics, and sometimes antiviral medications (e.g., oseltamivir for influenza).
3. Fungal pneumonia: Antifungal therapy based on the specific fungal pathogen. Treatments may include trimethoprim-sulfamethoxazole for Pneumocystis jirovecii pneumonia or amphotericin B for severe fungal infections.
4. Parasitic pneumonia: Antiparasitic medications specific to the causative organism, along with supportive care.

Conclusion

Pneumonia is a diverse and complex respiratory infection. Understanding the etiology, pathophysiology, clinical presentation, diagnosis, and management is crucial for medical professionals. This USMLE guide provides a concise overview to aid in exam preparation and reinforce knowledge on pneumonia.