What is pneumonia?

What is Pneumonia

Pneumonia is an acute infection of the lower respiratory tract that causes inflammation of the alveoli and distal airways, resulting in fluid accumulation in the lung parenchyma. 1, 2

Clinical Definition and Diagnosis

Pneumonia is clinically defined by the presence of:

• Fever and/or acute respiratory symptoms (cough, dyspnea) 1
• New lung infiltrate on chest radiography combined with clinical evidence of infection including fever, purulent sputum, leukocytosis, and decline in oxygenation 3
• Compatible physical examination findings such as rales, evidence of pulmonary consolidation, or respiratory distress 1

The diagnosis combines clinical presentation, laboratory data, and microbiologic findings, with chest radiography serving as confirmatory imaging 1.

Causative Organisms

Community-Acquired Pneumonia

The causative pathogens vary significantly by age group and host factors 1:

Children and Young Adults:

• Viral agents (respiratory syncytial virus, parainfluenza) predominate in children <5 years of age 1
• Mycoplasma pneumoniae is the major cause in North Americans aged 5-25 years 1
• Streptococcus pneumoniae is the most common bacterial pathogen in school-aged children 1

Middle-Aged and Older Adults:

• Streptococcus pneumoniae accounts for 50-90% of pyogenic pneumonia cases with acute onset 1
• Haemophilus influenzae (both ampicillin-susceptible and resistant strains) is less common but important in smokers and COPD patients 1
• Staphylococcus aureus occurs particularly following influenza infection 1
• Legionella species account for 1% of outpatient cases and 5-20% of hospitalized cases 1
• Atypical pathogens include M. pneumoniae, Chlamydophila pneumoniae, and Coxiella burnetii 1

Hospital-Acquired Pneumonia

The pathogen spectrum differs significantly 1:

• Pseudomonas aeruginosa (16.9%) is most common 1
• Staphylococcus aureus (12.9%) 1
• Klebsiella species (11.6%) and Enterobacter species (9.4%) 1
• Gram-negative bacilli predominate in ventilator-associated cases 1

Clinical Presentation Patterns

Typical (Bacterial) Pneumonia

• Acute onset with high fever 1
• Productive cough with purulent sputum 3
• Lobar consolidation on chest radiography 1, 4
• Leukocytosis 3

Atypical Pneumonia

• Gradual onset with low-grade fever 1
• Dry or minimally productive cough 1
• Interstitial pattern on chest radiography 4
• Caused by M. pneumoniae, Legionella, Chlamydophila, or viruses 1

Severe Pneumonia

• Respiratory distress with tachypnea, hypoxemia (SpO2 <90%), lower chest indrawing, or cyanosis 1
• Bilateral infiltrates suggesting primary viral pneumonia or ARDS 1
• High mortality (>40%) particularly with primary viral or mixed viral-bacterial pneumonia 1

Important Clinical Considerations

Risk Factors for Severe Disease:

• Extremes of age (infants <3-6 months, elderly >65 years) 1, 4
• Chronic conditions (COPD, heart disease, diabetes) 1
• Tobacco smoking (single most important preventable risk factor in adults) 4
• Immunocompromised states 1

Common Pitfall: Clinical symptoms often lack sensitivity and specificity for pneumonia diagnosis, and radiographic resolution typically lags behind clinical improvement 1. Imaging cannot reliably distinguish between causative organisms 1.

Critical Warning: In non-resolving pneumonia (lack of clinical improvement after 72 hours), malignancy is found in 0.4-9.2% of cases, particularly in older patients with smoking history and COPD 5. Follow-up imaging is essential in high-risk patients even after apparent clinical resolution 5.

Global Impact

Pneumonia remains the most deadly communicable disease and the single greatest infectious cause of death in children worldwide, accounting for 16% of deaths in children under 5 years of age 1, 3. It is a major cause of mortality and morbidity across all age groups globally 6, 2.