Interstitial Lung Disease

ILDs are a diverse group of disorders affecting the lung interstitium (the tissue between the air sacs). They are broadly classified into those with known causes (e.g., connective tissue diseases like Rheumatoid Arthritis, occupational exposures like Asbestosis, or drug-induced like Amiodarone) and idiopathic forms, with Idiopathic Pulmonary Fibrosis (IPF) being the most common and aggressive. Clinically, patients present with progressive exertional dyspnoea and a persistent dry cough. Key examination findings include finger clubbing (especially in IPF) and fine, bibasal, end-inspiratory 'Velcro' crackles. Pulmonary Function Tests (PFTs) consistently show a restrictive pattern (reduced FVC, normal/increased FEV1/FVC ratio, reduced lung volumes) and, crucially, reduced gas transfer (DLCO). High-Resolution CT (HRCT) of the thorax is the gold standard for imaging, often revealing a 'UIP pattern' (Usual Interstitial Pneumonia) in IPF, characterised by subpleural honeycombing and traction bronchiectasis, predominantly at the lung bases. Management is subtype-specific: antifibrotic agents (Pirfenidone, Nintedanib) for IPF to slow disease progression, and immunosuppression (e.g., corticosteroids, mycophenolate) for inflammatory ILDs like Sarcoidosis or CTD-ILD. Oxygen therapy and lung transplantation are options for advanced disease.

The core pathology in ILD is injury to the alveolar epithelium, leading to an abnormal repair process. In IPF, this involves chronic micro-injuries, often linked to genetic predisposition (e.g., MUC5B promoter variants, telomerase mutations) and ageing. This triggers excessive activation and proliferation of fibroblasts and myofibroblasts, which deposit vast amounts of extracellular matrix (collagen) in the interstitium. This fibrotic remodelling thickens the alveolar-capillary membrane, impeding gas exchange (reduced DLCO), and increases lung stiffness (reduced compliance), leading to the restrictive physiology seen on PFTs. In contrast, other ILDs (e.g., Sarcoidosis, Hypersensitivity Pneumonitis) primarily involve a granulomatous inflammatory response, which can later progress to irreversible fibrosis.

Finals often test the recognition of ILD. The classic presentation is progressive exertional dyspnoea and a persistent dry cough. Look for finger clubbing (highly suggestive of IPF or asbestosis, less common in sarcoidosis/HP) and the characteristic 'Velcro' crackles on auscultation, which are coarse, bibasal, and end-inspiratory, not clearing with coughing. Always ask about occupational exposures (asbestos, silica, birds/pets for Hypersensitivity Pneumonitis), drug history (Amiodarone, Methotrexate, Nitrofurantoin), and systemic symptoms (joint pain, rashes for CTD-ILD). Red flags include rapid progression of symptoms, hypoxaemia, and signs of pulmonary hypertension. Complications include Type 1 respiratory failure, pulmonary hypertension, and increased risk of lung cancer. IPF has a poor prognosis, with a median survival of 3-5 years from diagnosis.

1. **Bloods**: FBC, CRP, ESR, ANA, RF, anti-CCP (for CTD-ILDs), Serum ACE (for Sarcoidosis), Avian/Aspergillus precipitins (for HP).
2. **Pulmonary Function Tests (PFTs)**: Essential for diagnosis and monitoring. Show a **restrictive pattern**: reduced FVC, normal or increased FEV1/FVC ratio, reduced Total Lung Capacity (TLC), and crucially, significantly **reduced DLCO** (gas transfer).
3. **Imaging**:
- **Chest X-ray**: May show reduced lung volumes, reticular or reticulonodular shadowing, but often normal in early disease.
- **High-Resolution CT (HRCT) Thorax**: **Gold standard**. Look for:
- **UIP pattern (classic for IPF)**: subpleural, basal predominance, honeycombing (clustered cystic airspaces), traction bronchiectasis/bronchiolectasis, reticulation.
- **NSIP pattern (Non-Specific Interstitial Pneumonia)**: more ground-glass opacities, less honeycombing, often associated with CTD-ILDs.
- **Hypersensitivity Pneumonitis**: centrilobular nodules, air trapping, mosaic attenuation.
4. **Bronchoalveolar Lavage (BAL)**: Used to exclude infection or specific inflammatory patterns (e.g., lymphocytosis in HP or Sarcoidosis, neutrophilia in IPF).
5. **Lung Biopsy (Surgical/Transbronchial)**: Considered when HRCT is not definitive, especially to differentiate between ILD subtypes. Histology is key for definitive diagnosis in complex cases.

Management is highly dependent on the ILD subtype:
1. **Idiopathic Pulmonary Fibrosis (IPF)**:
- **Antifibrotic agents**: Pirfenidone or Nintedanib. These slow the rate of FVC decline but do not cure the disease. Indicated for FVC 50-80% predicted.
- **Supportive care**: Pulmonary rehabilitation, long-term oxygen therapy for hypoxaemia, management of comorbidities (e.g., GORD).
- **Lung transplantation**: Considered for eligible patients <65 years with progressive disease.
- **Palliative care**: Essential for symptom management and quality of life.
2. **Non-IPF ILDs (e.g., Sarcoidosis, CTD-ILD, Hypersensitivity Pneumonitis)**:
- **Corticosteroids**: Prednisolone (often a prolonged, tapered course) is the first-line for inflammatory ILDs.
- **Immunosuppressants**: Mycophenolate Mofetil, Azathioprine, or Cyclophosphamide may be used as steroid-sparing agents or for refractory disease.
- **Removal of offending agent**: Crucial for Hypersensitivity Pneumonitis (e.g., avoid birds, mould).
3. **General measures for all ILDs**: Smoking cessation, annual influenza and pneumococcal vaccination, GORD management (common comorbidity).