5.4 Interstitial Lung Disease & Idiopathic Pulmonary Fibrosis
Key Takeaways
- Idiopathic pulmonary fibrosis, sarcoidosis, radiation pneumonitis, drug-induced disease, connective tissue disease and eosinophilic pneumonia are enumerated interstitial lung disease topics.
- A definite usual interstitial pneumonia pattern on high-resolution computed tomography can establish idiopathic pulmonary fibrosis without surgical lung biopsy.
- Antifibrotic therapy slows decline in forced vital capacity in idiopathic pulmonary fibrosis but does not reverse fibrosis.
- Immunosuppression with prednisone, azathioprine and N-acetylcysteine increases mortality in idiopathic pulmonary fibrosis and is contraindicated.
- Every new interstitial lung disease evaluation requires a detailed occupational, environmental, medication and connective tissue disease history.
Interstitial lung diseases (ILDs) comprise a broad group of non-neoplastic disorders characterized by progressive inflammation and/or fibrosis of the pulmonary parenchyma and alveolar septa. On the ABIM exam, candidates must master the diagnostic integration of pulmonary function testing, high-resolution computed tomography (HRCT), clinical syndromic patterns, occupational exposures, and modern antifibrotic versus immunosuppressive therapeutics.
1. Diagnostic Approach to Interstitial Lung Disease
Pulmonary Function Testing (PFT) Patterns in ILD
- Restrictive Ventilatory Defect: Hallmark finding characterized by a reduction in Total Lung Capacity (TLC < 80% predicted) and Forced Vital Capacity (FVC < 80% predicted).
- Airflow Ratios: The FEV1/FVC ratio is normal or elevated (> 0.75–0.80) due to increased radial parenchymal traction holding airways open.
- Gas Exchange Impairment: Disproportionate reduction in the Diffusing Capacity of the Lung for Carbon Monoxide (DLCO < 70% predicted) due to alveolar-capillary membrane thickening and capillary bed obliteration. DLCO reduction often precedes FVC decline and correlates with exertional oxygen desaturation.
High-Resolution CT (HRCT) Imaging Protocol
- Thin-slice (<=1 mm) volumetric imaging at full inspiration, supplemented with expiratory scans (to detect mosaic attenuation and air-trapping in hypersensitivity pneumonitis or bronchiolitis) and prone imaging (to distinguish true subpleural interstitial fibrosis from dependent atelectasis).
| HRCT Pattern | Dominant Radiologic Features | Classical Etiologies |
|---|---|---|
| Usual Interstitial Pneumonia (UIP) | Subpleural and basilar predominance, reticular opacities, honeycombing (stacked thick-walled cysts 3–10 mm), traction bronchiectasis; absence of inconsistent features (no extensive ground-glass). | Idiopathic Pulmonary Fibrosis (IPF), Chronic Hypersensitivity Pneumonitis, Rheumatoid Arthritis-ILD, Asbestosis. |
| Non-Specific Interstitial Pneumonia (NSIP) | Symmetric, basilar-predominant ground-glass opacities with fine reticulation, traction bronchiectasis, and subpleural sparing (sparing of immediate 1–3 mm subpleural space); minimal/absent honeycombing. | Systemic Sclerosis (SSc), Polymyositis/Dermatomyositis, Mixed Connective Tissue Disease (MCTD), Idiopathic NSIP. |
| Hypersensitivity Pneumonitis (HP) | Centrilobular diffuse ground-glass nodules, mosaic perfusion, and expiratory air-trapping ("headcheese sign" / three-density sign combining lucent air-trapping, normal lung, and ground glass). | Bird fancier's lung (avian antigen), Farmer's lung (S. rectivirgula), Hot tub lung (M. avium). |
| Organizing Pneumonia (OP / COP) | Patchy, bilateral, migratory subpleural and peribronchovascular consolidations and ground glass; reverse halo / Atoll sign (central ground-glass surrounded by a crescentic or ring-like consolidation). | Cryptogenic Organizing Pneumonia (COP), post-infectious, drug toxicity (Amiodarone, Bleomycin, Nitrofurantoin), SLE. |
2. Idiopathic Pulmonary Fibrosis (IPF)
IPF is a specific form of chronic, progressive, fibrosing interstitial pneumonia of unknown etiology occurring primarily in older adults and limited to the lungs.
Clinical Presentation
- Demographics: Age > 60 years (rare <50 years), male-to-female ratio ~2:1, strong association with cigarette smoking (>20 pack-years).
- Symptoms & Signs: Insidious onset of progressive exertional dyspnea (>6 months) and chronic dry, non-productive cough. Physical examination reveals bilateral bibasilar fine end-inspiratory "Velcro-like" crackles (present in >95%) and digital clubbing (40–50%).
Diagnosis: The Definitive UIP Pattern
- When HRCT demonstrates a Definite UIP Pattern (subpleural/basilar reticular opacities, honeycombing +/- traction bronchiectasis, absence of features inconsistent with UIP) in a patient with compatible clinical features and no identifiable secondary cause (no connective tissue disease, no environmental/drug exposures), a diagnosis of IPF is established without surgical lung biopsy.
- Surgical lung biopsy (VATS) is reserved exclusively for cases with indeterminate or non-diagnostic HRCT patterns in surgical candidates.
Pharmacotherapy and Contraindications
- Approved Antifibrotic Therapies:
- Pirfenidone: Oral pyridone agent with pleiotropic antifibrotic, antioxidant, and anti-inflammatory properties; downregulates TGF-beta and procollagen production. Adverse effects: photosensitivity rash (requires sunscreen), nausea, dyspepsia, elevated transaminases.
- Nintedanib: Oral intracellular tyrosine kinase inhibitor that targets Platelet-Derived Growth Factor Receptor (PDGFR), Fibroblast Growth Factor Receptor (FGFR), and Vascular Endothelial Growth Factor Receptor (VEGFR). Proven in the INPULSIS and TOMORROW trials to slow the annual rate of FVC decline by ~50% and reduce acute IPF exacerbations. Adverse effects: diarrhea (>60%; managed with Loperamide), nausea, hepatotoxicity (monitor LFTs monthly for first 3 months).
- CRITICAL CONTRAINDICATION — The PANTHER Trial:
- The landmark PANTHER-IPF trial demonstrated that "triple immunosuppressive therapy" with Prednisone + Azathioprine + N-Acetylcysteine (NAC) caused a statistically significant increase in all-cause mortality, hospitalizations, and serious adverse events compared to placebo. Immunosuppression is strictly contraindicated in IPF.
- Non-Pharmacologic Management: Early referral for lung transplantation evaluation (IPF carries a median untreated survival of 3–5 years), supplemental oxygen for exertional or resting hypoxemia, pulmonary rehabilitation, and management of comorbid GERD (microaspiration accelerates fibrosis).
A 67-year-old man presents with progressive exertional shortness of breath and a persistent dry cough over the past 9 months. He has no known occupational or environmental exposures and no joint pain, skin changes, or muscle weakness. On exam, he has digital clubbing and bilateral bibasilar fine end-inspiratory Velcro crackles. High-resolution CT (HRCT) of the chest reveals subpleural, basilar-predominant reticular opacities, traction bronchiectasis, and extensive honeycombing without ground-glass attenuation or air-trapping, consistent with a definite Usual Interstitial Pneumonia (UIP) pattern. Pulmonary function tests show TLC 62% predicted, FVC 58% predicted, FEV1/FVC 0.84, and DLCO 42% predicted. A prior physician had started him on Prednisone 40 mg daily, Azathioprine 100 mg daily, and oral N-Acetylcysteine 600 mg three times daily 2 months ago. What is the most appropriate next step in management?
3. Progressive Pulmonary Fibrosis, Secondary ILD & the Eosinophilic Pneumonias
Idiopathic pulmonary fibrosis is only one destination in the interstitial lung disease algorithm. The remaining named blueprint entities — connective tissue disease, drug toxicity, radiation injury, and eosinophilic pneumonia — matter because most of them are treated in the opposite direction from IPF: they respond to immunosuppression that would harm an IPF patient. Sarcoidosis, hypersensitivity pneumonitis, and the pneumoconioses are developed separately in the following material.
Progressive Pulmonary Fibrosis (PPF)
The 2022 ATS/ERS/JRS/ALAT clinical practice guideline created progressive pulmonary fibrosis as a formal designation for a patient with a fibrosing interstitial lung disease other than IPF whose disease behaves like IPF. It requires at least two of the following three criteria within the past year, with no alternative explanation:
- Worsening respiratory symptoms.
- Physiologic progression: an absolute decline in FVC of >= 5% predicted, or an absolute decline in DLCO (corrected for hemoglobin) of >= 10% predicted.
- Radiologic progression on HRCT — increased traction bronchiectasis or bronchiolectasis, new ground glass with traction, new or increased honeycombing, or increased lobar volume loss.
The guideline makes a conditional recommendation for nintedanib in progressive pulmonary fibrosis after standard management of the underlying fibrotic ILD has failed, based on the INBUILD trial, which showed a slowed annual rate of FVC decline across fibrotic ILD subtypes rather than in IPF alone. Evidence for pirfenidone in this population was judged insufficient, and further research was recommended. The practical exam point is that antifibrotic therapy is no longer exclusive to IPF — it now belongs to any fibrosing ILD meeting the progression criteria.
Connective Tissue Disease-Associated ILD
Every new ILD evaluation includes a serologic and clinical screen for connective tissue disease, because the answer redirects therapy entirely.
| Disease | Usual HRCT Pattern | Management Point |
|---|---|---|
| Systemic sclerosis | NSIP far more often than UIP | ILD is the leading cause of death in systemic sclerosis. Mycophenolate was non-inferior to cyclophosphamide with better tolerability in Scleroderma Lung Study II; tocilizumab and nintedanib are approved for systemic sclerosis-ILD |
| Rheumatoid arthritis | Uniquely often a UIP pattern, which can be radiologically indistinguishable from IPF | A UIP pattern carries the worst prognosis of the connective tissue ILDs; methotrexate-induced pneumonitis is the main mimic to exclude |
| Inflammatory myopathy / antisynthetase syndrome | NSIP or organizing pneumonia | Look for mechanic's hands, Raynaud phenomenon, arthritis, and anti-Jo-1 |
| Anti-MDA5 dermatomyositis | Rapidly progressive ILD | Amyopathic skin disease with a normal creatine kinase and cutaneous ulcers; carries very high mortality and demands aggressive early combined immunosuppression |
| Sjogren syndrome | NSIP, lymphocytic interstitial pneumonia, cysts | Consider lymphoma when there is a persistent focal abnormality |
Drug-Induced Interstitial Lung Disease
Every ILD workup requires a complete medication reconciliation, including agents stopped months earlier.
- Amiodarone: onset from months to years, dose- and duration-related. Its iodine content gives affected lung and liver a characteristic high attenuation on non-contrast CT, and lavage shows foamy macrophages. Management is drug withdrawal plus corticosteroids, with the caution that the 45-60 day half-life means clinical improvement lags for months.
- Bleomycin: dose-related above roughly 400 units cumulative, potentiated by high inspired oxygen and by thoracic radiation — which is why anesthesia teams deliberately limit FiO2 in bleomycin-exposed patients.
- Methotrexate: an idiosyncratic hypersensitivity-type pneumonitis with peripheral eosinophilia and granulomas that can appear early and is not dose-related.
- Nitrofurantoin: two distinct syndromes — an acute hypersensitivity/eosinophilic reaction within days to weeks, and a chronic fibrotic form after months to years of prophylactic use in older women.
- Immune checkpoint inhibitors: pneumonitis is graded by severity; hold the drug and give corticosteroids for grade 2 or higher, escalating to infliximab or mycophenolate for steroid-refractory disease.
- Daptomycin: a well-described cause of eosinophilic pneumonia; daptomycin is also inactivated by pulmonary surfactant and must never be used to treat pneumonia.
Radiation Pneumonitis and Radiation Fibrosis
- Acute radiation pneumonitis appears 4-12 weeks after thoracic radiotherapy with dyspnea, a dry cough, and low-grade fever. The imaging signature is an opacity that conforms to the radiation port with straight, non-anatomic borders that cross fissures and do not respect segmental boundaries. Treat with prednisone 40-60 mg daily and a slow taper over weeks; abrupt tapering causes recurrence.
- Radiation fibrosis develops 6-24 months out, with volume loss, traction bronchiectasis, and dense scarring confined to the port. It is irreversible, and corticosteroids do not help.
- Radiation recall pneumonitis is an inflammatory flare in a previously irradiated field triggered by later systemic chemotherapy.
The Eosinophilic Pneumonias
| Feature | Acute Eosinophilic Pneumonia | Chronic Eosinophilic Pneumonia |
|---|---|---|
| Tempo | Under 1 month; febrile hypoxemic respiratory failure that can mimic ARDS | Weeks to months of cough, fever, night sweats, and weight loss |
| Typical patient | A recently started smoker, or after an inhalational or dust exposure | A woman with asthma or atopy |
| Peripheral eosinophilia | Often absent at presentation — this is the classic trap | Prominent peripheral eosinophilia |
| Imaging | Diffuse bilateral ground glass and consolidation, often with pleural effusions | "Photographic negative of pulmonary edema" — peripheral, subpleural, upper-lobe-predominant consolidation |
| Diagnosis | Bronchoalveolar lavage eosinophils > 25% | Lavage eosinophilia plus the characteristic imaging |
| Course | Dramatic corticosteroid response; does not relapse if the trigger is avoided | Corticosteroid-responsive but relapses in more than half during or after taper, often requiring prolonged low-dose therapy |
Before attributing pulmonary eosinophilia to one of these two idiopathic syndromes, exclude allergic bronchopulmonary aspergillosis (asthma, very high IgE, central bronchiectasis), eosinophilic granulomatosis with polyangiitis (asthma, sinus disease, neuropathy, ANCA), drug reaction, and parasitic infection. Screening for Strongyloides before giving corticosteroids to an at-risk patient is mandatory, because steroids can convert chronic infection into fatal hyperinfection syndrome.
A 63-year-old woman with rheumatoid arthritis on stable methotrexate and etanercept has fibrotic interstitial lung disease with a nonspecific interstitial pneumonia pattern on HRCT. She was started on mycophenolate 14 months ago. Over the past year her exertional dyspnea has clearly worsened, her FVC has fallen from 71% to 64% predicted, and repeat HRCT shows increased traction bronchiectasis with new areas of ground glass with architectural distortion. There is no evidence of infection, heart failure, or pulmonary embolism, and her rheumatoid joint disease is quiescent. Which of the following best describes her current status and the most appropriate addition to her regimen?