Interstitial Lung Diseases

ILD Overview & HRCT Patterns - Lungs in Distress

• Interstitial Lung Disease (ILD): Heterogeneous group of disorders affecting lung interstitium, often leading to inflammation & fibrosis.

• Broad Classification:

  • Known causes: Drugs (Amiodarone, Bleomycin), Connective Tissue Diseases (CTD - RA, SSc), Environmental (Silicosis, Asbestosis).
  • Idiopathic Interstitial Pneumonias (IIPs): e.g., Usual Interstitial Pneumonia (UIP), Non-Specific Interstitial Pneumonia (NSIP).
  • Granulomatous: Sarcoidosis, Hypersensitivity Pneumonitis (HP).
  • Rare ILDs: e.g., Lymphangioleiomyomatosis (LAM), Pulmonary Langerhans Cell Histiocytosis (PLCH).

• Key HRCT Patterns:

  • Ground-Glass Opacity (GGO): Hazy ↑ lung density, vessels visible.
  • Reticular pattern: Network of fine lines.
  • Honeycombing: Clustered, stacked subpleural cysts (typically 3-10 mm diameter, <3 mm walls); key for UIP.
  • Nodules (distribution): Perilymphatic (Sarcoidosis, Silicosis), Centrilobular (HP, RB-ILD), Random (Miliary TB, fungal).
  • Traction bronchiectasis/bronchiolectasis: Irreversible airway dilatation due to adjacent fibrosis.
  • Consolidation; Cysts.

• Distribution Patterns:

  • Apical: Silicosis, Sarcoidosis (chronic), PLCH, Ankylosing Spondylitis.
  • Basal & Peripheral: UIP/IPF, Asbestosis, most CTD-ILDs.

⭐ Honeycombing on HRCT is a hallmark of the Usual Interstitial Pneumonia (UIP) pattern, indicating irreversible fibrosis and generally associated with a poorer prognosis compared to other IIPs without honeycombing.

IIPs - The Usual Suspects

• Idiopathic Pulmonary Fibrosis (IPF) / Usual Interstitial Pneumonia (UIP): Most common IIP, progressive, poor prognosis.

  • UIP Pattern (HRCT): Basal, subpleural, reticular pattern; honeycombing is hallmark. +/- traction bronchiectasis. Absence of features suggesting alternative diagnoses.
  • See flowchart for diagnostic categories (Definite, Probable, Indeterminate, Alternative Dx).

• Nonspecific Interstitial Pneumonia (NSIP): Better prognosis than IPF.

  • HRCT: Ground-glass opacities (GGO), reticulation, traction bronchiectasis. Typically basal predominant; subpleural sparing can occur.
  • Cellular (more GGO, better response) vs. Fibrotic.

• UIP vs. NSIP Key Differences:

  Feature	UIP (IPF)	NSIP
  Distribution	Basal, subpleural	Basal (subpleural sparing possible)
  GGO	Minimal/Absent	Often Prominent (esp. cellular)
  Honeycombing	Common, Hallmark	Rare/Absent
  Prognosis	Poor	Better

⭐ Honeycombing on HRCT is the most specific finding for a UIP pattern.

• Other Major IIPs: Cryptogenic Organizing Pneumonia (COP), Respiratory Bronchiolitis-ILD (RB-ILD), Desquamative Interstitial Pneumonia (DIP), Lymphocytic Interstitial Pneumonia (LIP), Acute Interstitial Pneumonia (AIP). 📌 Can Really Do Lung Assessment.

Sarcoid & HP - Granuloma Spotlight

• Sarcoidosis: Multisystem granuloma; common thoracic involvement.

  • HRCT:
    • Perilymphatic nodules (along bronchovascular bundles, interlobular septa, subpleural regions).
    • Bilateral, symmetrical hilar/mediastinal lymphadenopathy (LAPs).
    • Fibrosis in later stages (upper/mid zone predominant).
  • Scadding Stages: 0 (Normal CXR) to IV (Pulmonary fibrosis).
  • 📌 Mnemonic: "1-2-3 sign" (Right paratracheal + bilateral hilar LAPs); perilymphatic "beading".

• Hypersensitivity Pneumonitis (HP): Immune response to inhaled antigens.

  • HRCT:
    • Acute/Subacute: Centrilobular GGO nodules, mosaic attenuation (air trapping on expiration).
    • Chronic: Fibrosis (often mid/upper zone or diffuse), honeycombing (can mimic UIP but distribution differs), significant air trapping.

⭐ "Three-density pattern" ("headcheese sign": normal lung, GGO, air trapping) on HRCT strongly suggests subacute HP.

Pneumoconioses & CTD‑ILDs - Occupational & Systemic

• Pneumoconioses: ILDs from inhaled inorganic dusts.
  • Silicosis: Upper lobe nodules, hilar eggshell calcification. Risk of Progressive Massive Fibrosis (PMF).
  • CWP: Similar to silicosis; simple vs. complicated (PMF).
  • Asbestosis: Lower lobe fibrosis, pleural plaques (parietal, diaphragmatic), benign effusions.
• CTD-ILD: Systemic autoimmune disease-associated ILD.
  • Patterns:

    CTD	Common ILD Pattern(s)
    Scleroderma	NSIP (most common)
    Myositis	NSIP, OP
    RA	UIP (most common)
    Sjögren's	LIP, NSIP

  ⭐ Asbestosis: only pneumoconiosis significantly ↑ mesothelioma risk, esp. in smokers.

High‑Yield Points - ⚡ Biggest Takeaways

• UIP/IPF: Characterized by basal, subpleural honeycombing.
• NSIP: Shows ground-glass opacities with subpleural sparing; linked to CTDs.
• Sarcoidosis: Presents with bilateral hilar lymphadenopathy and upper lobe predominant reticulonodular patterns.
• Silicosis/CWP: Features upper lobe nodules and eggshell calcification of hilar nodes.
• HP: Key findings include centrilobular nodules, air trapping, and the headcheese sign.
• LCH: Upper lobe cysts and nodules in smokers, typically spares costophrenic angles.
• LAM: Diffuse thin-walled cysts in women of childbearing age; associated with tuberous sclerosis.