Pulmonary Infections

Bacterial Pneumonias - Bugs & Blobs

• Lobar Pneumonia:
  • Organism: Streptococcus pneumoniae (most common)
  • Pattern: Homogeneous consolidation (lobe/segment)
  • X-ray/CT: Dense opacity, air bronchograms.
• Bronchopneumonia (Patchy):
  • Organisms: Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa, Klebsiella pneumoniae
  • Pattern: Patchy, segmental, or lobular consolidation
  • X-ray/CT: Multiple foci of opacity, often bilateral, peribronchial thickening.
• Atypical Pneumonia (Interstitial):
  • Organisms: Mycoplasma pneumoniae, Legionella pneumophila, Chlamydia pneumoniae
    • 📌 Mnemonic: "My Lungs Cough" (Mycoplasma, Legionella, Chlamydia)
  • Pattern: Interstitial infiltrates (reticular, reticulonodular)
  • X-ray/CT: Ground-glass opacities, fine reticular markings.

⭐ Streptococcus pneumoniae is the most common cause of community-acquired lobar pneumonia, classically presenting with consolidation in a single lobe and air bronchograms.

Viral & Fungal Pneumonias - Hazy Invaders

• Viral Pneumonias:
  • Influenza: Non-specific bilateral infiltrates.
  • COVID-19: Peripheral, bilateral Ground-Glass Opacities (GGOs); "crazy-paving" pattern.
• Fungal Pneumonias:
  • Pneumocystis jirovecii Pneumonia (PJP):
    • Bilateral, perihilar GGOs ("batwing" appearance).

    ⭐ In immunocompromised patients, especially with HIV and CD4 count < 200 cells/µL, PJP classically presents with bilateral, diffuse, symmetrical perihilar GGOs.

  • Aspergilloma:
    • Fungus ball within a pre-existing lung cavity.
  • Invasive Aspergillosis (immunocompromised):
    • "Halo sign" (early).
    • "Air crescent sign" (later, with neutrophil recovery).

Pulmonary Tuberculosis - The Great Mimicker

• Primary TB: Often asymptomatic; immune response contains infection.
  • Ghon focus: Initial lung lesion (calcified peripheral nodule).
  • Ranke complex: Ghon focus + ipsilateral hilar/paratracheal lymphadenopathy (often calcified).
  • Lymphadenopathy: Hilar/paratracheal, can cause airway compression in children.
• Post-primary (Reactivation) TB: Symptomatic; reactivation of latent infection.
  • Predilection: Apical/posterior segments of upper lobes, superior segments of lower lobes.
  • Key findings: Cavitation, fibrocalcific changes (scarring), Rasmussen aneurysm (rare, pulmonary artery pseudoaneurysm within cavity).
• Miliary TB: Hematogenous dissemination.
  • Key findings: Diffuse, bilateral millet-seed (1-2 mm) opacities on CXR.

⭐ Post-primary (reactivation) tuberculosis most commonly affects the apical and posterior segments of the upper lobes or the superior segments of the lower lobes, often leading to cavitation.

Infection Patterns & Complications - Reading Between Lines

Key radiological signs:

• Silhouette Sign: Lost lung-soft tissue interface. Seen in: Consolidation.
• Air Bronchogram: Patent bronchi in opaque lung. Seen in: Pneumonia.
• Tree-in-bud Sign: Centrilobular nodules, branching lines. Seen in: Endobronchial spread (TB, MAC).

  ⭐ The tree-in-bud sign on CT, representing centrilobular bronchiolar dilatation and impaction, is highly suggestive of endobronchial spread of infection, commonly seen in tuberculosis or atypical mycobacterial infections.

• Halo Sign: GGO around nodule/mass. Seen in: Invasive Aspergillosis.
• Air Crescent Sign: Air in cavity, around sequestrum. Seen in: Invasive Aspergillosis (recovery).
• Reversed Halo (Atoll) Sign: Central GGO, peripheral consolidation ring. Seen in: COP, fungal, TB.

Major Complications:

• Lung Abscess: Thick-walled (>2mm) cavity, air-fluid level.
• Empyema: Pleural pus; split pleura sign.
• ARDS: Bilateral diffuse opacities (white-out); normal heart size.

Approach to Patterns:

High‑Yield Points - ⚡ Biggest Takeaways

• Lobar pneumonia: Typically S. pneumoniae; presents as lobar consolidation with air bronchograms.
• Post-primary TB: Favors apical/posterior segments of upper lobes; shows cavitation and fibrosis.
• PJP: In immunocompromised patients; bilateral, diffuse perihilar ground-glass opacities.
• Aspergilloma: A fungus ball (mycetoma) developing in a pre-existing lung cavity.
• Atypical pneumonia (Mycoplasma): Characterized by diffuse reticulonodular or interstitial patterns.
• Viral pneumonia: Often presents with bilateral, diffuse ground-glass opacities or interstitial infiltrates.
• Bronchopneumonia: Manifests as patchy, segmental consolidations, often multilobar.