Interstitial Lung Disease: IPF, Sarcoidosis, and Hypersensitivity Pneumonitis

Interstitial lung diseases (ILDs) represent a heterogeneous group of over 200 disorders characterized by inflammation and/or fibrosis of the lung parenchyma, specifically affecting the alveolar epithelium, pulmonary capillary endothelium, and the spaces between them. The three most clinically significant ILDs are idiopathic pulmonary fibrosis (IPF), sarcoidosis, and hypersensitivity pneumonitis (HP).

[HIGH_YIELD] Idiopathic Pulmonary Fibrosis is the most common idiopathic interstitial pneumonia, with an incidence of 6.8-16.3 per 100,000 person-years. It predominantly affects males over 60 years of age with a history of smoking. The median survival is 2-3 years from diagnosis.

Sarcoidosis is a multisystem granulomatous disease of unknown etiology with an incidence of 10.9 per 100,000 person-years. It shows bimodal age distribution with peaks at 25-35 and 45-65 years. African Americans have a 3-4 fold higher incidence than Caucasians.

[KEY_CONCEPT] Hypersensitivity Pneumonitis is an immune-mediated lung disease triggered by inhalation of organic antigens in genetically susceptible individuals. Prevalence varies widely (0.4-12.4%) depending on environmental exposures and occupational risks.

Common Symptoms Across ILDs:

• Progressive dyspnea on exertion
• Non-productive cough
• Fatigue and exercise intolerance
• Chest discomfort

[CLINICAL_PEARL] The triad of progressive dyspnea, dry cough, and bibasilar crackles should raise suspicion for ILD.

IPF-Specific Features:

• Insidious onset over months to years
• Velcro crackles - fine, inspiratory crackles heard at lung bases
• Digital clubbing (50-75% of patients)
• Weight loss in advanced disease
• Acute exacerbations may occur

Sarcoidosis Features:

• Extrapulmonary manifestations in 90% of patients:
  • Skin: erythema nodosum, lupus pernio
  • Eyes: uveitis, conjunctivitis
  • Lymph nodes: hilar and peripheral lymphadenopathy
  • Cardiac: arrhythmias, heart block
  • Neurologic: cranial neuropathies, seizures
• Löfgren syndrome: acute sarcoidosis with erythema nodosum, hilar lymphadenopathy, and arthritis

[HIGH_YIELD] Hypersensitivity Pneumonitis Patterns:

• Acute HP: flu-like symptoms 4-6 hours after exposure (fever, chills, cough, dyspnea)
• Subacute HP: gradual onset over weeks to months
• Chronic HP: insidious fibrotic changes, may be indistinguishable from IPF

Physical Examination Findings:

• Fine inspiratory crackles (bibasilar in IPF, diffuse in HP)
• Digital clubbing (common in IPF, rare in sarcoidosis/HP)
• Lymphadenopathy (characteristic of sarcoidosis)
• Skin lesions (erythema nodosum in sarcoidosis)
• Signs of cor pulmonale in advanced disease

High-Resolution Computed Tomography (HRCT) is the cornerstone of ILD diagnosis and differentiation.

[KEY_CONCEPT] IPF Diagnostic Criteria (ATS/ERS/JRS/ALAT 2018): ✓ Exclusion of other known causes of ILD ✓ HRCT pattern consistent with usual interstitial pneumonia (UIP):

• Subpleural, basal predominance
• Honeycombing with or without traction bronchiectasis
• Reticular pattern
• Absence of features inconsistent with UIP ✓ Age typically >60 years ✓ Gradual onset of dyspnea

Sarcoidosis Diagnostic Algorithm:

1. Clinical presentation + imaging suggestive ↓
2. Tissue biopsy showing non-caseating granulomas ↓
3. Exclusion of other granulomatous diseases ↓
4. Laboratory testing:
   • Elevated ACE levels (60-70% of patients)
   • Elevated 1,25-dihydroxyvitamin D
   • Hypercalciuria/hypercalcemia ↓
5. Kveim test (historical, rarely used)

[CLINICAL_PEARL] Hypersensitivity Pneumonitis Workup:

• Detailed exposure history (occupational, environmental, hobbies)
• Serum precipitins against suspected antigens
• Bronchoalveolar lavage (BAL): lymphocytosis >20-40%
• Lymphocyte transformation test
• Inhalation challenge test (specialized centers only)

Pulmonary Function Tests:

• Restrictive pattern: ↓FVC, ↓TLC, normal or ↑FEV1/FVC ratio
• Impaired gas exchange: ↓DLCO (earliest and most sensitive)
• Exercise testing: oxygen desaturation, elevated A-a gradient

IPF Management:

[HIGH_YIELD] Antifibrotic Therapy (first-line):

• Pirfenidone: 2403 mg daily (titrated), reduces FVC decline by 50%
• Nintedanib: 150 mg BID, tyrosine kinase inhibitor
• Both medications slow disease progression but don't reverse fibrosis

Contraindications to Immunosuppression in IPF:

• Corticosteroids, immunosuppressants may worsen outcomes
• Exception: acute exacerbations may require pulse steroids

Sarcoidosis Treatment Algorithm:

Asymptomatic/Mild Disease ↓ Observation (many cases self-resolve) ↓ Progressive/Symptomatic Disease ↓ Corticosteroids (Prednisone 0.5-1 mg/kg/day) ↓ 8-12 weeks → Taper over 6-12 months ↓ Steroid-Sparing Agents (if relapse/side effects):

• Methotrexate 10-25 mg weekly
• Azathioprine 2-3 mg/kg/day
• Mycophenolate mofetil 1-3 g/day ↓ Refractory Disease:
• Anti-TNF agents (infliximab, adalimumab)
• Rituximab

[CLINICAL_PEARL] Hypersensitivity Pneumonitis Management:

1. Antigen avoidance - most critical intervention
2. Acute/Subacute HP: Prednisone 0.5-1 mg/kg/day × 2-4 weeks, then taper
3. Chronic HP: Antifibrotic therapy may be considered (off-label nintedanib)
4. Environmental controls: HEPA filters, protective equipment

Supportive Care (All ILDs):

• Pulmonary rehabilitation: improves exercise tolerance and quality of life
• Oxygen therapy: for resting or exercise-induced hypoxemia (SpO2 <88%)
• Vaccination: annual influenza, pneumococcal vaccines
• Treatment of comorbidities: GERD, sleep apnea, pulmonary hypertension
• Lung transplantation evaluation: for end-stage disease

Monitoring and Follow-up:

• PFTs every 3-6 months: track FVC and DLCO decline
• 6-minute walk test: assess functional capacity
• Pulse oximetry: detect exercise-induced hypoxemia
• HRCT: monitor disease progression (annually or as clinically indicated)

IPF Complications:

[HIGH_YIELD] Acute Exacerbations (10-15% annually):

• Rapid worsening of dyspnea and hypoxemia
• New ground-glass opacities on HRCT
• Mortality rate 50-90%
• Treatment: high-dose corticosteroids, supportive care

Progressive Respiratory Failure:

• Inexorable decline in lung function
• Pulmonary hypertension in 30-50% of patients
• Lung cancer risk increased 7-fold

Sarcoidosis Prognosis:

• Stage I (hilar lymphadenopathy only): 90% remission rate
• Stage II (hilar + parenchymal): 60-70% remission
• Stage III (parenchymal only): 30-50% remission
• Stage IV (pulmonary fibrosis): <20% remission

[CLINICAL_PEARL] Löfgren syndrome has excellent prognosis with >90% spontaneous remission.

Extrapulmonary Sarcoidosis Complications:

• Cardiac sarcoidosis: sudden cardiac death, heart failure
• Neurosarcoidosis: cranial neuropathies, seizures
• Ocular sarcoidosis: blindness if untreated
• Hypercalcemia: nephrolithiasis, chronic kidney disease

Hypersensitivity Pneumonitis Outcomes:

Prognostic Factors Across ILDs:

Poor Prognosis Indicators:

• Age >65 years
• Male gender
• Extensive fibrosis on HRCT
• Low initial DLCO (<40% predicted)
• Rapid FVC decline (>10% annually)
• Pulmonary hypertension
• Digital clubbing

[KEY_CONCEPT] Monitoring for Disease Progression:

• Serial pulmonary function tests (most reliable)
• Six-minute walk distance decline
• Oxygen desaturation during exercise
• Quality of life scores (SGRQ, K-BILD)

Transplant Considerations:

• IPF: consider listing when FVC <80% predicted or DLCO <40%
• Sarcoidosis: end-stage pulmonary fibrosis
• HP: chronic fibrotic phase unresponsive to treatment
• Contraindications: age >65, significant comorbidities, active malignancy