Community Acquired Pneumonia

Jud W. Gurney, MD, FACR

Key Facts

Imaging Findings

Best diagnostic clue: Focal parenchymal abnormality in patient with fever
If initial radiograph normal in patient strongly suspected of having pneumonia, repeat radiograph in 24 hours or do CT
Centrilobular nodules in patchy distribution: Most helpful finding distinguishing infectious vs. noninfectious disease
All patients > 40 years old (or younger smokers) should have follow-up until resolution
50% resolution 2 weeks; 66% 4 weeks; 75% 6 weeks
Mortality associated with 2 radiographic abnormalities: Bilateral pleural effusions and multilobar disease

Top Differential Diagnoses

Cardiogenic Pulmonary Edema
Hemorrhage
Hypersensitivity Pneumonitis

Pathology

Offending organism cultured in < 50%

Clinical Issues

No individual or combinations of signs and symptoms from history and physical examination can reliably confirm or refute presence of pneumonia

Diagnostic Checklist

Diagnosis based on culture (grayscale image does not substitute for Gram stain)

Axial NECT shows lobular consolidation surrounded by ground-glass halos

and bronchial wall thickening

. The patient scheduled for lung cancer screening was diagnosed with mycoplasma pneumonia.

Axial CECT shows segmental distribution of centrilobular consolidation

in the right upper lobe. The consolidation is surrounded by ground-glass opacities

. The diagnosis was bacterial pneumonia.

TERMINOLOGY

Definitions

Community acquired pneumonia (CAP): Lung infection that occurs outside hospital setting

IMAGING FINDINGS

General Features

Best diagnostic clue: Focal parenchymal abnormality in patient with fever

Imaging Recommendations

Best imaging tool: Radiographs usually suffice, CT use increasing to reduce diagnostic uncertainty
Protocol advice
- Indications for chest radiograph: Fever, cough, sputum production, coarse crackles
- If initial radiograph normal in patient strongly suspected of having pneumonia, repeat radiograph in 24 hours or do CT

CT Findings

More sensitive (˜ 100%) than radiographs (50-70%)
- More expensive and increased radiation dose
HRCT patterns: Frequency in bacterial vs. atypical (mycoplasma and viral) pneumonia
- Centrilobular clustered nodules
  - Bacterial 10%, viral & mycoplasma 66%
  - Patchy distribution: Most helpful finding distinguishing infectious vs. noninfectious disease
- Lobular consolidation
  - Bacterial 33%, mycoplasma 85%, uncommon in viral pneumonia
- Segmental consolidation
  - Bacterial 75%, mycoplasma 40%
- Lobular ground-glass opacities
  - Bacterial 10%, viral & mycoplasma 60%
- Crazy-paving pattern
  - Bacterial 30%, viral & mycoplasma 15%
- Bronchovascular bundle thickening
  - Bacterial 55%, viral & mycoplasma 70%
- Ground-glass halo around consolidation
  - Bacterial 45%, viral & mycoplasma 30%
- Inner zone of lung
  - Bacterial 33%, viral & mycoplasma 85%
- Pleural effusions
  - Bacterial 40%, viral & mycoplasma 20%
- Lymphadenopathy uncommon (3%) in any community acquired pneumonia

Radiographic Findings

High sensitivity: May not have visible abnormality in
- Immunocompromised, especially if neutropenic
- Dehydration: Controversial; rare if it exists at all
Typical distribution segmental consolidation: Unilateral or bilateral
Significant interobserver variability in pattern recognition
- May have nearly any pattern from ground-glass, interstitial, to consolidation
- Pattern not diagnostic of organism; single organism may cause multiple patterns
- Poor agreement between readers for pattern of disease, presence of air bronchogram, bronchial wall thickening
- Good to excellent agreement between readers for pleural effusion, extent of radiographic abnormalities
Lobar vs. bronchopneumonia
- Pathologic designation; difficult to reliably identify on radiographs
Unusual patterns
- Hyperinflation common with viral pneumonia (due to obstruction of distal airways)
- Lobar enlargement with bulging fissures: Klebsiella pneumonia
- Round pneumonia more common pattern in children
- Pneumatoceles
  - Develop later in course of pneumonia (classically in S. aureus), may persist for months, resolve spontaneously
- Hilar adenopathy
  - Rare, limits differential: Tuberculosis, mycoplasma, fungi, mononucleosis, measles, plague, tularemia, anthrax, pertussis
Complications
- Cavitation: Suggests bacterial disease (S. aureus, Gram-negative bacteria, anaerobes)
- Empyema
  - Reactive parapneumonic effusions in 20-60%
  - Up to 5% go on to empyema
  - Suspect empyema if effusion enlarging or becomes loculated
Resolution
- Delayed with advancing age and involvement of multiple lobes
  - Faster resolution in nonsmokers and outpatients
- All patients > 40 years old (or younger smokers) should have follow-up until resolution
  - 2% of hospitalized patients with CAP will have bronchogenic carcinoma
  - 50% of these cancers diagnosed on initial chest radiograph
  - 50% diagnosed as failure of resolution on follow-up
- Expected time table
  - 50% see resolution in 2 weeks; 66% in 4 weeks; 75% in 6 weeks
Mortality associated with 2 radiographic abnormalities: Bilateral pleural effusions and multilobar disease
Recurrent pneumonia
- 10-15% of hospitalized patients with CAP have recurrence within 2 years
- Same location suggests bronchial obstruction or aspiration as etiology
- Different location in otherwise healthy patient, evaluate for immunodeficiency