Define and classify ILD full details for md exams 15marks answer
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Interstitial Lung Disease (ILD) — Definition, Classification & Clinical Details
DEFINITION
Interstitial Lung Disease (ILD) is a heterogeneous group of disorders characterized by diffuse parenchymal involvement of the lung, with inflammation and/or fibrosis of the lung interstitium (the framework supporting the alveolar walls, capillaries, and lymphatics), ultimately resulting in impaired gas exchange and, in many cases, progressive respiratory failure.
The term "diffuse parenchymal lung disease" (DPLD) is often used synonymously, as the pathological process extends beyond the interstitium to involve the alveolar epithelium, capillary endothelium, and perivasular/perilymphatic spaces.
Murray & Nadel's Textbook of Respiratory Medicine: "Interstitial lung diseases are a heterogenous group of disorders characterized by diffuse parenchymal involvement, with the potential development of fibrosis."
Epidemiology:
- Prevalence: ~81/100,000 in men; ~67/100,000 in women
- Overall incidence: ~32/100,000/year (men) and ~26/100,000/year (women)
- Mortality from ILD doubled in the U.S. between 1980 and 2014
- Incidence increases with age
CLASSIFICATION
The most practical clinician-oriented classification is based on the presence or absence of an identifiable cause (ATS/ERS classification):
I. ILDs of KNOWN CAUSE
| Category | Examples |
|---|---|
| Connective Tissue Disease (CTD)-related | Rheumatoid arthritis (RA), Systemic sclerosis (SSc), SLE, Polymyositis/Dermatomyositis, Sjögren syndrome |
| Environmental/Occupational | Asbestosis, Silicosis, Coal worker's pneumoconiosis, Berylliosis |
| Drug- and Radiation-induced | Amiodarone, Methotrexate, Bleomycin, Nitrofurantoin, Checkpoint inhibitors, Radiation fibrosis |
| Smoking-related ILDs | Respiratory bronchiolitis-ILD (RB-ILD), Desquamative interstitial pneumonia (DIP), Pulmonary Langerhans cell histiocytosis (PLCH) |
| Rare forms | Lymphangioleiomyomatosis (LAM), Pulmonary alveolar proteinosis (PAP), Amyloidosis, Alveolar microlithiasis, Heritable disorders (Hermansky-Pudlak syndrome, tuberous sclerosis) |
II. ILDs of UNKNOWN CAUSE
| Category | Examples |
|---|---|
| Idiopathic Interstitial Pneumonias (IIPs) | IPF, NSIP, COP, AIP, DIP, RB-ILD, LIP, IPPFE |
| Sarcoidosis | Multisystem granulomatous disease |
| Idiopathic Eosinophilic Pneumonias | Acute EP, Chronic EP |
| Vasculitis | ANCA-associated vasculitis with pulmonary involvement |
| Rare forms | IgG4-related disease, Erdheim-Chester disease |
Murray & Nadel's, Table 89.1: Classification based on "presence or absence of an identifiable cause is likely the most practical option for clinicians."
DETAILED CLASSIFICATION: IDIOPATHIC INTERSTITIAL PNEUMONIAS (IIPs)
The 2013 ATS/ERS Consensus Classification divides IIPs into three groups:
A. MAJOR IIPs
1. Chronic Fibrosing IPs
| Entity | Histological Pattern | Key HRCT Features | Key Clinical Points |
|---|---|---|---|
| Idiopathic Pulmonary Fibrosis (IPF) | Usual Interstitial Pneumonia (UIP) | Honeycombing; subpleural, basal-predominant reticulation; traction bronchiectasis; geographic/heterogeneous distribution | >50 yrs; male predominance; insidious dyspnea; dry cough; poor prognosis (median survival 3–5 yrs) |
| Non-Specific Interstitial Pneumonia (NSIP) | NSIP | Ground-glass opacity; reticular opacities (lower lobe predominant); subpleural sparing (20%); NO honeycombing | Younger; female predominance; strongly associated with CTDs; responds to treatment |
2. Acute/Subacute IPs
| Entity | Histological Pattern | HRCT Features | Key Clinical Points |
|---|---|---|---|
| Cryptogenic Organizing Pneumonia (COP) | Organizing Pneumonia | Peripheral/peribronchovascular consolidation; migratory infiltrates; "reverse halo" / atoll sign | Subacute onset; responds well to corticosteroids |
| Acute Interstitial Pneumonia (AIP / Hamman-Rich) | Diffuse Alveolar Damage (DAD) | Bilateral GGO ± traction bronchiectasis | Idiopathic ARDS; acute onset; very high mortality (>50%) |
3. Smoking-Related IPs
| Entity | HRCT Features | Key Clinical Points |
|---|---|---|
| RB-ILD | Centrilobular GGO nodules; no honeycombing | Smokers; responds to smoking cessation |
| DIP | Diffuse GGO; lower lobe predominant | Smokers; responds to corticosteroids + smoking cessation |
B. RARE IIPs
| Entity | Features |
|---|---|
| Lymphoid Interstitial Pneumonia (LIP) | GGO; cysts; associated with Sjögren syndrome, HIV, autoimmune disease |
| Idiopathic Pleuroparenchymal Fibroelastosis (IPPFE) | Bilateral upper lobe irregular pleural thickening + subpleural fibrosis; bilateral upper lobe predominance |
C. UNCLASSIFIABLE IIPs
Cases that do not fit into any defined IIP category, even after thorough multidisciplinary evaluation.
Grainger & Allison's Diagnostic Radiology: "The 2013 ATS/ERS consensus classification... the clinical entity, rather than the histopathological label, is given pre-eminence... a multidisciplinary and dynamic approach requiring interaction among clinicians, radiologists and pathologists."
HISTOPATHOLOGICAL PATTERNS (Contrasting UIP vs. NSIP)
| Feature | UIP | NSIP |
|---|---|---|
| Temporal heterogeneity | Present (fibrosis at different stages) | Absent (temporally uniform) |
| Fibroblastic foci | Prominent | Sparse |
| Honeycombing | Characteristic | Absent or minimal |
| Ground-glass opacity | Minimal | Prominent |
| Subpleural sparing | No | Present in ~20% |
| Prognosis | Poor | Better (especially cellular NSIP) |
CLINICAL FEATURES
Symptoms
- Progressive exertional dyspnea (cardinal symptom)
- Persistent dry cough
- Duration of symptoms aids differentiation:
- Chronic onset (months–years): IPF, NSIP, sarcoidosis
- Acute/subacute (days–weeks): AIP, COP, acute eosinophilic pneumonia
Signs
- Bilateral fine inspiratory crackles ("Velcro crackles") — best heard posteriorly at lung bases
- Digital clubbing — in 45–75% of IPF; also in sarcoidosis and other ILDs
- Cyanosis and respiratory failure in advanced disease
- Signs of pulmonary hypertension / cor pulmonale — right ventricular heave, loud P2, peripheral edema
Extrapulmonary signs (suggesting CTD-ILD)
- Sclerodactyly, mechanic's hands, Raynaud phenomenon, telangiectasias, skin rashes, facial erythema, dry mucous membranes
DIAGNOSIS
1. History
- Occupational and environmental exposures (asbestos, silica, birds, molds)
- Drug history (amiodarone, methotrexate, bleomycin, immunotherapies)
- Smoking history — some ILDs exclusive to smokers (PLCH, DIP, RB-ILD)
- Family history of pulmonary fibrosis or autoimmune disease
- Features of systemic autoimmune disease
2. Chest X-Ray
- Common pattern: reticular opacities (linear network)
- May also show: nodular opacities, alveolar opacities, cystic changes, volume loss
- Up to 10% of ILD patients may have a normal CXR
3. HRCT Chest (Cornerstone of Diagnosis)
- 1 mm thin-slice images in prone and supine; inspiration and expiration phases
- Key HRCT patterns:
| Pattern | Likely Diagnosis |
|---|---|
| Honeycombing + basal subpleural reticulation | IPF (UIP) |
| GGO + reticular opacity (lower lobes, subpleural sparing) | NSIP |
| Peripheral/peribronchial consolidation ± reverse halo sign | COP |
| Bilateral GGO + consolidation (acute) | AIP/ARDS |
| Centrilobular nodules + GGO | RB-ILD / HP |
| Bilateral hilar lymphadenopathy + perilymphatic nodules | Sarcoidosis |
| Upper lobe pleural thickening + subjacent fibrosis | IPPFE |
4. Pulmonary Function Tests (PFTs)
- Classic pattern: Restrictive (↓ TLC, ↓ FVC, preserved or ↑ FEV1/FVC ratio)
- ↓ DLCO (diffusing capacity) — correlates with severity
- Hypoxemia at rest, worsening with exercise (↑ A-a gradient)
- Obstructive PFTs may be seen in: RB-ILD, DIP, PLCH, LAM, sarcoidosis
5. Laboratory Tests
- CBC (eosinophilia in eosinophilic pneumonia)
- ANA, RF, anti-CCP, ENA panel (CTD screen)
- Anti-Jo-1 antibody, aldolase, CK (antisynthetase syndrome)
- VEGF-D (>800 pg/mL diagnostic for LAM)
- ACE level, 24-hr urine calcium (sarcoidosis)
6. Bronchoalveolar Lavage (BAL)
- Useful to exclude infection and malignancy
- Diagnose: diffuse alveolar hemorrhage, eosinophilic lung disease, PAP (milky fluid)
- BAL differentials (lymphocytosis → HP/NSIP; eosinophilia → eosinophilic pneumonia)
7. Lung Biopsy
- Transbronchial biopsy (TBBx): best for bronchiolocentric diseases (sarcoidosis, berylliosis, lymphangitic carcinomatosis); inadequate for UIP vs. NSIP
- VATS (Video-Assisted Thoracoscopic Surgery) biopsy: preferred for definitive IIP diagnosis
- Multidisciplinary discussion (clinician + radiologist + pathologist) is mandatory for diagnosis
TREATMENT
| ILD Type | Treatment |
|---|---|
| IPF | Pirfenidone (anti-fibrotic) or Nintedanib (pan-tyrosine kinase inhibitor); lung transplantation |
| NSIP/COP (CTD-associated) | Corticosteroids ± immunosuppressives (azathioprine, mycophenolate, cyclophosphamide, rituximab) |
| Medication-induced ILD | Discontinue culprit drug; corticosteroids |
| RB-ILD / DIP | Smoking cessation; corticosteroids (limited benefit) |
| Sarcoidosis | Corticosteroids; methotrexate, azathioprine, infliximab |
| HP | Antigen avoidance; corticosteroids |
| All ILD | Supplemental O₂, pulmonary rehabilitation, vaccination |
PROGNOSIS
| ILD | Prognosis |
|---|---|
| IPF | Poor — median survival 3–5 years; no cure |
| NSIP (cellular) | Good — responds to treatment |
| COP | Good — responds well to corticosteroids |
| AIP | Very poor — mortality >50% |
| DIP/RB-ILD | Good with smoking cessation |
| CTD-ILD | Variable; UIP pattern → worse than NSIP pattern |
MEMORY AID — IIP Classification (ATS 2013)
"CANS are RARE"
Chronic fibrosing: IPF, NSIP
Acute/subacute: COP, AIP
Smoking-related: RB-ILD, DIP
RARE: LIP, IPPFE
And Unclassifiable IIPs
Sources: Murray & Nadel's Textbook of Respiratory Medicine, 2-Vol; Washington Manual of Medical Therapeutics; Grainger & Allison's Diagnostic Radiology — all reflect the 2013 ATS/ERS International Multidisciplinary Consensus Classification of IIPs.
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