Hypersensitivity pneumonitis

Overview

Hypersensitivity pneumonitis (HP) / extrinsic allergic alveolitis - inflammatory lung disease caused by sensitised immune response to inhaled organic antigens. Upper and mid-zone distribution is the critical distinguishing feature from IPF (lower zone).

Pathophysiology

•Type III hypersensitivity (immune complex deposition) + type IV hypersensitivity (T-cell mediated granulomatous inflammation) - together cause alveolitis progressing to fibrosis if exposure continues

•Early allergen removal can fully reverse acute/subacute HP - unlike IPF where fibrosis is irreversible

Presentation

•Acute HP: fever, chills, myalgia 4-8 hours after antigen exposure; symptoms resolve within 24-72 hours of leaving the exposure environment

•Chronic HP: progressive exertional dyspnoea, dry cough, weight loss, bibasal inspiratory crackles, clubbing

Investigations

•Gold standard imaging: HRCT - upper and mid-zone ground-glass opacities, centrilobular nodules (acute/subacute); honeycombing and traction bronchiectasis (chronic fibrotic)

•BAL: lymphocytosis - characteristic and supportive of HP

•Serum precipitins (IgG): positive in up to 90% of farmer's lung and bird-fancier's lung; indicates sensitisation, not necessarily active disease

•Spirometry: restrictive pattern - reduced FVC, reduced TLCO, preserved/raised FEV1/FVC ratio

•Lung biopsy: non-caseating granulomas and lymphocytic alveolitis - if diagnosis uncertain

Management

🥇 First-line

•allergen identification and removal - most important intervention; full resolution possible in acute/subacute HP

🥈 Second-line

•oral prednisolone - if symptoms persist despite allergen avoidance, or in acute severe HP

🥉 Third-line

•azathioprine or cyclophosphamide - steroid-refractory or dependent disease

•Lung transplantation - end-stage fibrotic HP

🎯

Two questions distinguish HP from IPF in almost every exam scenario: (1) Is there an identifiable inhaled antigen? (2) Which zone is affected on HRCT/CXR? Upper/mid-zone + organic antigen = HP.

Upper zone fibrosis - CHARTS mnemonic

🧠

CHARTS - causes of upper zone fibrosis: C - Coal worker's pneumoconiosis, H - Histiocytosis / Hypersensitivity pneumonitis, A - Ankylosing spondylitis, R - Radiation (post-radiotherapy), T - Tuberculosis, S - Silicosis / Sarcoidosis

•Lower zone fibrosis causes: IPF, asbestos, most connective tissue disorders (e.g. rheumatoid arthritis), drug-induced (methotrexate, amiodarone, bleomycin)

HP vs IPF - key comparison

HP vs IPF

Feature	HP	IPF
Zone affected	Upper and mid-zone	Lower zone
Cause	Identifiable organic antigen	Idiopathic
BAL	Lymphocytosis	Normal / neutrophilia
Reversibility	Yes - if antigen removed early	No - irreversible
Serum precipitins	Positive (if sensitised)	Negative