Identifying Pneumonia on Chest X-Ray

The Core Finding: Opacity (White Area) in the Lung

Step 1: Look at the Chest X-Ray Systematically

• Left vs. right lung fields
• Upper, middle, and lower zones bilaterally
• Look for any area that is denser (whiter) than surrounding lung

Three Main Radiological Patterns of Pneumonia

1. Lobar Pneumonia (most classic)

• Homogeneous (uniform) white opacity filling a segment or entire lobe
• Margins are sharply demarcated at fissures
• The opacity spreads via air drift (pores of Kohn) - does NOT cross fissures
• Air bronchogram is frequently seen (dark branching lines of bronchi visible inside the white opacity - because the airways remain open but alveoli are filled)
• Most common cause: Streptococcus pneumoniae (also Klebsiella, Legionella, H. influenzae)
• No significant volume loss - the lobe maintains its size

2. Bronchopneumonia (Lobular Pneumonia)

Bilateral bronchopneumonia from Mycoplasma pneumoniae (Murray & Nadel's)

Frontal and lateral views of right lower lobe bronchopneumonia (Murray & Nadel's)

• Patchy, multifocal opacities in multiple lobules/segments - does not neatly fill one lobe
• Peribronchial inflammation that spreads outward
• Poorly defined margins
• Often bilateral, lower lobe predominant
• Common causes: H. influenzae, S. aureus, Mycoplasma, gram-negative organisms

3. Interstitial (Atypical) Pneumonia

• Diffuse, bilateral hazy opacities - a "ground-glass" pattern
• Bronchovascular thickening (thickened lung markings radiating from the hilum)
• Less dense opacification than lobar pneumonia
• Can appear almost normal early in the disease
• Common causes: Mycoplasma pneumoniae, Chlamydia, Legionella, viruses, Pneumocystis jirovecii (PCP)
• Note: In PCP (especially with HIV), the CXR can be completely normal in up to 10% of cases

Key Radiological Signs to Know

Lobe Localization Using the Silhouette Sign

• Right heart border obscured → right middle lobe pneumonia
• Left heart border obscured → left upper lobe / lingula pneumonia
• Right/left diaphragm obscured → right/left lower lobe pneumonia
• Aortic knuckle obscured → left upper lobe pneumonia

Special Patterns

Round Pneumonia

Round pneumonia: A 64-year-old man. CXR shows a mass-like opacity in the left upper lobe (arrow); CT confirms consolidation - confirmed Streptococcus pneumoniae (Grainger & Allison's)

• A spherical/ball-like area of consolidation
• Can mimic a lung tumor
• More common in children; resolves with antibiotics

Cavitating Pneumonia

• Consolidation with an air-fluid level or internal lucency inside
• Caused by: S. aureus (MRSA), Klebsiella, anaerobes, TB
• Indicates necrosis/abscess formation

Bilateral Consolidation Patterns

Practical Approach Summary

1. Is there an opacity? - Compare both lungs systematically
2. Is it dense and homogeneous (lobar) or patchy (bronchopneumonia)?
3. Look for air bronchograms - confirms consolidation, not collapse
4. Apply the silhouette sign - which heart/diaphragm border is lost?
5. Any pleural effusion? - blunted costophrenic angle
6. Any cavitation? - suggests abscess (MRSA, Klebsiella, anaerobes)
7. Bilateral diffuse hazy opacities? - think atypical organisms, viral, PCP
8. Always correlate with clinical findings - fever, cough, pleuritic chest pain, sputum

Important note: The CXR cannot reliably identify the specific organism. The radiological pattern narrows the differential but microbiologic confirmation (sputum culture, urine antigen) is required for definitive diagnosis. The CXR may also lag behind clinical improvement by several weeks.