JG

Recognizing Airspace Versus Interstitial Lung Disease–Flashcards

Classification of Parenchymal Lung Disease

  • Two broad, partly-pathologic, partly-radiographic categories:

    • Airspace (alveolar) disease

    • Interstitial (infiltrative) disease

  • Learning the distinction narrows differentials and speeds correct diagnosis.

  • Box 4.1 canonical list (remember that several entities may show mixed patterns):

    • Airspace Diseases

      • Acute: pneumonia, pulmonary alveolar edema, hemorrhage, aspiration, near-drowning

      • Chronic: adenocarcinoma (former BAC subtype), alveolar proteinosis, sarcoidosis, lymphoma

    • Interstitial Diseases

      • Reticular: pulmonary interstitial edema, interstitial pneumonia, scleroderma, sarcoid

      • Nodular: bronchogenic carcinoma, metastases, silicosis, miliary tuberculosis, sarcoid

Characteristics of Airspace (Alveolar) Disease

  • Opacity morphology:

    • Fluffy, cloud-like, hazy

    • Confluent—merge imperceptibly; edges indistinct

  • Distribution: diffuse (e.g., pulmonary edema) or focal/lobar/segmental (e.g., pneumococcal pneumonia)

  • Key signs:

    • Air-bronchogram (almost pathognomonic): black branching air-filled bronchi become visible because surrounding alveoli are replaced by material > air.

      • Bronchi normally invisible (thin walls, air-inside, air-outside)

        • Visualized when surroundings become water/soft-tissue density.

    • Silhouette sign: loss of normal interface when two like densities abut (e.g., RML pneumonia erasing right heart border).

  • What can fill alveoli?

    • Fluid (pulmonary edema)

    • Blood (hemorrhage)

    • Gastric acid/juice (aspiration)

    • Inflammatory exudate (pneumonia)

    • Water (near-drowning)

  • Summary box (Box 4.2):

    • Fluffy, fuzzy, confluent opacities

    • Indistinct margins

    • ± air-bronchograms

    • ± silhouette sign

Signature Examples of Airspace Disease

  • Pulmonary alveolar edema

    • Bilateral, perihilar, bat-wing/angel-wing; heart often enlarged

    • Usually no air-bronchograms (bronchi also fluid-filled)

    • <48\,\text{h} clearing once treated

    • Frequently accompanied by pleural effusions + fissure fluid

  • Pneumonia

    • \approx90\% of community-acquired lobar/segmental = Streptococcus pneumoniae

    • Patchy → lobar consolidation; contains air-bronchograms

    • Clears in <10\,\text{d} (pneumococcal may clear in \le 48\,\text{h})

  • Aspiration

    • Manifests in most dependent lung at time of event

    • Bedridden: lower lobes or posterior upper lobes

    • RLL > LLL

      • Right main bronchus is SHORTER, WIDER, MORE VERTICAL

    • Radiographic appearance + timeline depend on material & infection:

    • Neutralized gastric fluid/water: clears in 24\text{–}48\,\text{h}

    • Infected material: may last weeks

Characteristics of Interstitial Lung Disease (ILD)

  • Lung interstitium composed of:

    • connective tissue, lymphatics, vessels, bronchi—the scaffolding around airspaces.

  • Radiographic appearance: discrete “particles/packets” separated by aerated lung, usually with sharp margins.

  • Key features (Box 4.3):

    • Three basic patterns: reticular (lines), nodular (dots), reticulonodular (lines + dots)

    • Can be focal or diffuse

    • Usually no air-bronchograms

  • Pattern illustrations (Fig 4.8):

    • Reticular = criss-cross lines

    • Nodular = myriad small dots

    • Reticulonodular = combination

Diagnostic Pitfalls & Approach

  • Heavy ILD may look confluent → mimics airspace disease on CXR due to summation.

  • Tip: scrutinize lesion edge/periphery for underlying lines/dots; if uncertain order chest CT (resolves superimposition).

Predominantly Reticular ILDs

  • Pulmonary interstitial edema (precursor to alveolar edema)

    • Etiologies: ↑capillary pressure (CHF), ↑permeability (allergy), ↓lymphatic drainage (lymphangitic spread)

    • 4 Key findings:

      • fluid in major/minor fissures

      • peribronchial cuffing

      • pleural effusions

      • Kerley B lines

    • Clinical–radiologic dissociation: few rales despite dramatic film

    • Rapid therapeutic clearing <48\,\text{h}

  • Interstitial pneumonias

    • Usual interstitial pneumonia (UIP)

      • Older men, smoking & GERD associations

      • CXR: may be normal early; then fine → coarse reticulation, honeycombing, basal volume loss

      • CT: subpleural basilar honeycombing, reticulation, traction bronchiectasis

      • End-stage entity: idiopathic pulmonary fibrosis

    • Nonspecific interstitial pneumonia (NSIP)

      • Commonest CT pattern in connective-tissue disease (e.g., scleroderma)

      • Prognosis > UIP

      • CT: basilar ground-glass ± subpleural sparing; traction bronchiectasis late

  • Lymphangitic carcinomatosis (see nodular section – metastasis)

Predominantly Nodular ILDs

  • Bronchogenic carcinoma

    • Four major cells: adenocarcinoma, squamous, small cell, large cell

    • Adenocarcinoma often a solitary peripheral nodule

    • CXR: sharp discrete nodule; CT: spiculation/irregular borders

  • Metastases to the lung

    • Hematogenous: ≥2 nodules; “cannonballs” when large

      • Classically from breast, colorectal, renal, bladder, testicular, head/neck, sarcoma, melanoma

    • Lymphangitic spread

      • Blood → interstitium → lymphatics OR retrograde from hilar node obstruction

      • Mimics interstitial edema but segmental/lobar/unilateral

      • Kerley lines, fissural fluid, effusion; common primaries: breast, lung, stomach, pancreas, occasionally prostate

    • Direct extension

      • Least common; subpleural mass with adjacent rib destruction; pleura usually resistant

Mixed Reticular + Nodular (Reticulonodular) ILD

  • Sarcoidosis

    • Classic adenopathy: bilateral hilar + right paratracheal

    • Pulmonary parenchymal disease in ~50 %; mixture of lines and dots

    • Staging evolution:

    • Stage I – adenopathy only

    • Stage II – adenopathy + ILD

    • Stage III – ILD persists, nodes regress

    • Stage IV – pulmonary fibrosis

    • Majority of parenchymal cases eventually resolve

Mixed Airspace & Interstitial Pattern – Tuberculosis

  • Global epidemiology: Up to \frac13 of world population infected; \approx10\% progress to active disease.

Primary TB

  • Often clinically silent; UL > LL involvement

  • Manifestations:

    • Lobar pneumonia (esp. with hilar/mediastinal nodes)

    • Isolated unilateral hilar/mediastinal adenopathy (children)

    • Large, often asymptomatic pleural effusion (adults)

    • Cavitation rare.

Post-primary (Reactivation) TB

  • Predominates in adults; favors apical & posterior UL segments, superior LL segments.

  • Pathology: caseous necrosis, tubercles (macrophages + Langerhans giant cells + lymphocytes + fibroblasts)

  • Imaging patterns:

    • Thin-walled cavities, smooth inner margin, no air-fluid level (bilateral UL common)

    • Cavitary pneumonia

    • Transbronchial spread → opposite LL or other lobes

    • Asymptomatic bronchiectasis (traction or endobronchial)

    • Late bronchostenosis ⇒ chronic lobar atelectasis (middle-lobe syndrome)

    • Tuberculoma: solitary nodule ± satellite lesions

    • Pleural effusion = empyema (worse prognosis than primary effusion)

Miliary TB

  • Hematogenous dissemination; insidious systemic symptoms (fever, chills, night sweats)

  • Radiology:

    • Initial \sim1\,\text{mm} nodules → may enlarge 2\text{–}3\,\text{mm} untreated

    • Clearing rapid with therapy

    • Rarely calcifies

  • Differential (Box 4.4): miliary TB, histoplasmosis, sarcoidosis, pneumoconioses (silicosis)

Diagnostic Pearls & Pitfalls

  • Edge analysis: confluent-looking opacity—examine periphery for hidden ILD pattern.

  • Air-bronchogram ≠ pathognomonic for pneumonia but strongly indicates airspace process; not expected in pulmonary edema (bronchi fluid-filled).

  • Bat-wing pattern ≈ pulmonary edema; add cardiomegaly, pleural effusions, fissure fluid to support CHF.

  • RML silhouette of right heart border localizes airspace disease to right middle lobe on CXR.

Case Quiz Answer (Chapter Q4)

  • Close-up shows fluffy, confluent opacity + air-bronchograms → airspace disease.

  • With cough/fever history, most likely diagnosis = pneumonia.

Take-Home Points (condensed)

  • Parenchymal lung disease divides into airspace vs interstitial; recognizing pattern streamlines diagnosis.

  • Airspace: fluffy, confluent, indistinct; may show air-bronchograms or silhouette sign.

  • Interstitial: sharp packets—reticular, nodular, or reticulonodular; seldom air-bronchograms.

  • Tuberculosis, sarcoidosis, some metastases can display mixed or evolving patterns—correlate clinically and with CT.