What is pneumonia?

Definition of Pneumonia

Pneumonia is defined clinically as the presence of fever and/or acute respiratory symptoms, including new onset of fever, purulent sputum, and leukocytosis, with confirmatory chest radiographic findings consistent with infection. 1

Clinical Definition and Presentation

Pneumonia is characterized by:

• Respiratory symptoms:

  • Cough (initially dry, then becoming productive, possibly blood-stained in severe cases) 2
  • Dyspnea (shortness of breath), particularly with exertion 2
  • Pleuritic chest pain 3

• Systemic symptoms:

  • Fever (≥38°C) (may be absent in elderly patients) 2
  • Sweating, shivers, aches, and generalized myalgia 2
  • Fatigue and loss of appetite 3

• Physical examination findings:

  • Tachypnea (particularly important in pediatric pneumonia) 2
  • New localizing chest signs such as crackles and diminished breath sounds 2
  • Dullness to percussion 2
  • Signs of consolidation 1

• Radiographic findings:

  • Chest radiography showing infiltrates, consolidation, or interstitial patterns 1

Types of Pneumonia

Based on Acquisition Setting

1. Community-acquired pneumonia (CAP):

   • Defined as the presence of signs and symptoms of pneumonia in a previously healthy person caused by an infection acquired outside of the hospital 1
   • Accounts for approximately 1.4 million emergency department visits and 740,000 hospitalizations annually in the US 4

2. Hospital-acquired pneumonia:

   • Defined as pneumonia that develops after 48 hours of hospitalization that was not present at the time of admission 1
   • Second most common nosocomial infection after bloodstream infections 1

3. Ventilator-associated pneumonia:

   • A subset of hospital-acquired pneumonia occurring in mechanically ventilated patients 1
   • Can affect up to 12% of ventilated children 1

Based on Pathophysiology

1. Primary viral pneumonia:

   • Characterized by breathlessness within the first 48 hours of fever onset 1
   • Chest radiography typically shows bilateral interstitial infiltrates 1
   • High mortality rate (>40%) despite maximum supportive treatment 1

2. Secondary bacterial pneumonia:

   • More common than primary viral pneumonia (up to four times) 1
   • Typically develops during early convalescent period (4-5 days from onset of initial symptoms) 1
   • Chest radiography usually shows lobar pattern of consolidation 1

3. Mixed viral-bacterial pneumonia:

   • Concurrent viral and bacterial infection 1
   • Chest radiograph may show lobar consolidation superimposed on bilateral diffuse lung infiltrates 1
   • High mortality rate (>40%) 1

Etiologic Agents

The causative pathogens vary by age group and setting:

• Children under 5 years: Viral agents predominate (e.g., respiratory syncytial virus, parainfluenza virus) 1

• Ages 5-25 years: Mycoplasma pneumoniae is a major cause 1

• Middle-aged and older adults: Bacterial agents predominate, with Streptococcus pneumoniae accounting for 50-90% of pyogenic pneumonia cases 1

• Other bacterial causes (in decreasing frequency):

  • Haemophilus influenzae
  • Staphylococcus aureus
  • Mixed aerobic-anaerobic bacteria
  • Gram-negative bacilli (e.g., Klebsiella pneumoniae) 1

• Atypical pneumonia agents:

  • Mycoplasma pneumoniae
  • Legionella species
  • Coxiella burnetii
  • Chlamydia pneumoniae 1

Diagnostic Approach

Diagnosis combines:

1. Clinical assessment:

   • Compatible clinical picture (fever, cough, auscultatory findings) 1
   • Physical examination for localizing signs 2

2. Radiographic confirmation:

   • Chest radiography showing infiltrates or consolidation 1, 2
   • Imaging is primarily reserved for more severe presentations, including significant respiratory distress, hypoxemia, failed antibiotic therapy, or for suspected complications 1

3. Laboratory tests:

   • C-reactive protein (CRP) ≥30 mg/L increases likelihood of pneumonia when combined with suggestive symptoms 2
   • Leukocytosis is common 1

4. Microbiologic diagnosis (when possible):

   • Isolation of causative pathogen from respiratory specimens (sputum, bronchial washings) or blood 1
   • Specific testing for atypical pathogens as indicated 1

Important Clinical Considerations

• Pneumonia is the single greatest infectious cause of death in children worldwide, accounting for 16% of all deaths in children under 5 years of age 1

• Elderly patients may present atypically with confusion, failure to thrive, worsening of underlying chronic illness, and falls 2

• Tachypnea is usually present in elderly patients despite atypical presentation 2

• Clinical symptoms often lack sensitivity and specificity for the diagnosis of pneumonia, highlighting the importance of radiographic confirmation in uncertain cases 1

• Resolution of radiographic findings may lag behind clinical improvement 1