Pneumonia: Classification, Pathogenesis, Diagnosis, and Management

Introduction

Pneumonia is an acute infection of the lung parenchyma that results in inflammation of the alveoli and interstitial tissue. It remains a significant cause of global morbidity and mortality, particularly among infants, the elderly, and immunocompromised individuals. Pneumonia is not a single disease entity but a clinical syndrome caused by a variety of pathogens and influenced by host and environmental factors. Understanding its classification, pathophysiology, and evidence-based management is essential for effective care.

Types and Classification of Pneumonia

Pneumonia can be classified by the setting in which it is acquired, the causative organism, or the radiologic pattern of lung involvement.

Community-acquired pneumonia (CAP) refers to infections occurring outside of hospital settings or within 48 hours of hospital admission. The most common causative organism is Streptococcus pneumoniae, followed by Haemophilus influenzae, atypical pathogens such as Mycoplasma pneumoniae, Chlamydophila pneumoniae, and respiratory viruses including influenza and respiratory syncytial virus (RSV). CAP often presents with fever, productive cough, pleuritic chest pain, and focal lung findings on auscultation.

Hospital-acquired pneumonia (HAP) develops ≥48 hours after hospital admission and is typically caused by more resistant organisms such as Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter species, and methicillin-resistant Staphylococcus aureus (MRSA). Ventilator-associated pneumonia (VAP) is a subset of HAP that arises 48 hours or more after endotracheal intubation. Both are associated with higher morbidity, multidrug resistance, and often occur in critically ill patients.

Aspiration pneumonia occurs when oropharyngeal or gastric contents are inhaled into the lower respiratory tract, introducing a mix of aerobic and anaerobic bacteria. This is common in patients with impaired swallowing, altered mental status, or gastroesophageal reflux. Fungal pneumonias, while uncommon in immunocompetent hosts, are a significant concern in those with HIV/AIDS, organ transplants, or neutropenia. Pneumocystis jirovecii, Histoplasma capsulatum, and Aspergillus species are notable pathogens.

Pathogenesis

The pathogenesis of pneumonia begins with the invasion of the alveoli by infectious organisms. In healthy individuals, the upper airway and mucociliary clearance act as barriers to infection. However, when these defenses are compromised — by smoking, viral illness, aspiration, or immunosuppression — pathogens can reach the lower respiratory tract.

Once in the alveoli, microbes multiply and trigger a local inflammatory response. Neutrophils infiltrate the alveolar spaces, leading to consolidation, impaired gas exchange, and the classic clinical manifestations of cough, dyspnea, fever, and sometimes pleuritic chest pain. In viral or atypical pneumonia, the inflammatory process may be more interstitial than alveolar, leading to a more indolent course and diffuse findings on imaging.

Diagnosis

The diagnosis of pneumonia is primarily clinical, supported by radiographic evidence. Patients often present with fever, cough, sputum production, dyspnea, and abnormal lung auscultation (e.g., crackles, bronchial breath sounds). In elderly or immunocompromised patients, symptoms may be subtle or atypical.

Chest radiography is the cornerstone of diagnosis, typically showing a new infiltrate. Lobar consolidation suggests typical bacterial pneumonia, while patchy or interstitial infiltrates may point toward viral or atypical causes. Further evaluation depends on severity and setting. Sputum Gram stain and culture can help guide antibiotic therapy if productive cough is present. Blood cultures, urinary antigen tests (e.g., for Legionella or Streptococcus pneumoniae), and nasopharyngeal PCR panels may be used in hospitalized or severely ill patients. In cases of suspected aspiration, anaerobic coverage may be necessary even if cultures are negative.

Treatment

Treatment of pneumonia depends on the likely pathogen, severity of illness, and local resistance patterns.

For outpatient CAP in healthy adults with no comorbidities, empiric therapy includes amoxicillin, doxycycline, or a macrolide (e.g., azithromycin) if local resistance is low. In patients with comorbidities, combination therapy with a beta-lactam and macrolide, or monotherapy with a respiratory fluoroquinolone, is recommended.

Inpatient non-ICU CAP is typically treated with intravenous beta-lactam (e.g., ceftriaxone) plus a macrolide or monotherapy with a respiratory fluoroquinolone. ICU-level CAP requires broader coverage and consideration of MRSA or Pseudomonas if risk factors are present.

For HAP and VAP, empiric therapy must account for multidrug-resistant organisms and often includes antipseudomonal beta-lactams (e.g., piperacillin-tazobactam, cefepime) and MRSA coverage (e.g., vancomycin or linezolid). Antibiotics should be tailored based on culture results and clinical response.

Aspiration pneumonia is treated with agents that cover anaerobic organisms, such as ampicillin-sulbactam or metronidazole in combination with a beta-lactam. Patients with fungal pneumonia require pathogen-specific antifungal therapy, often guided by infectious disease specialists.

Supportive care — including oxygen therapy, hydration, antipyretics, and sometimes bronchodilators — is essential in all forms of pneumonia.

Conclusion

Pneumonia is a diverse and potentially serious respiratory infection with varying etiologies, clinical presentations, and outcomes. Classification based on acquisition setting and causative organisms helps guide appropriate diagnostic and therapeutic decisions. Prompt recognition, appropriate empiric antibiotic therapy, and supportive management remain the cornerstones of care. With the rise of antimicrobial resistance and emerging pathogens, ongoing research and clinical vigilance are necessary to optimize outcomes.

References

1. Metlay JP, Waterer GW, Long AC, et al. Diagnosis and treatment of adults with community-acquired pneumonia: An official clinical practice guideline of the American Thoracic Society and Infectious Diseases Society of America. Am J Respir Crit Care Med. 2019;200(7):e45–e67. doi:10.1164/rccm.201908-1581ST

2. Niederman MS, Baron RM, Mandell LA. Hospital-acquired pneumonia, ventilator-associated pneumonia, and healthcare-associated pneumonia. In: Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 9th ed. 2020.

3. Kalil AC, Metersky ML, Klompas M, et al. Management of adults with hospital-acquired and ventilator-associated pneumonia: 2016 clinical practice guidelines. Clin Infect Dis. 2016;63(5):e61–e111.

4. Musher DM, Thorner AR. Community-acquired pneumonia. N Engl J Med. 2014;371(17):1619–1628. doi:10.1056/NEJMra1312885

   
   

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