Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis (EAA), is a syndrome caused by repetitive inhalation of environmental antigens in susceptible individuals.

Common antigens include moulds, bacteria, bird droppings, feathers, agricultural dusts, bioaerosols, and chemicals from paints or plastics. These antigens trigger an immune response, leading to inflammation of the alveoli and small airways, which may progress to interstitial lung disease.

High magnification photomicrograph of a lung biopsy showing chronic hypersensitivity pneumonitis (H&E), with mild thickening of the walls of the small air sacs by invasion of white blood cells.

Signs and Symptoms

HP can be classified as acute, subacute, and chronic based on the duration of the illness.

Acute

In acute HP, exposure to a high dose of antigens occurs over a short duration. Symptoms such as fever, chills, malaise, cough, chest tightness, dyspnea, rash, swelling, and headache develop 4-6 hours after exposure and typically resolve within 12 hours to several days upon cessation of exposure.

Subacute

Subacute HP features a gradual onset of productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy. Symptoms are similar to those of acute HP but are less severe and last longer.

Chronic

Chronic HP results from prolonged low-dose antigen exposure. Symptoms include an insidious onset of cough, progressive dyspnea, fatigue, and weight loss. Clubbing is observed in 50% of patients, and they may also present with tachypnea, respiratory distress, and inspiratory crackles over lower lung fields.

Epidemiology

Although the prevalence of HP is not well established, it is generally considered low. The most common types are bird fancier's and farmer's lung. Interestingly, cigarette smoking appears to be protective against the disease.

Pathophysiology

HP is caused by an exaggerated immune response, involving both type III and type IV hypersensitivity reactions. Acute HP is typically associated with type III hypersensitivity, while subacute and chronic HP involve T cell infiltration and granuloma formation. Genetic susceptibility and environmental factors are proposed to contribute to the disease.

Diagnosis

Diagnosis relies on clinical judgement using a combination of findings, as there is no single universal diagnostic criterion for HP.

Exposure History

A detailed occupational, home, and environmental exposure history is very important in diagnosis. Standardised questionnaires tailored to the region may aid in identifying potential antigen exposure.

Imaging

Chest Radiographs

Acute HP may show a micro-nodular interstitial pattern and ground-glass opacities. Chronic HP may mimic idiopathic pulmonary fibrosis with reticular nodular opacities and fibrotic changes.

High-Resolution Computed Tomography

High-resolution CT is commonly used and can show fibrotic or non-fibrotic patterns. Non-fibrotic HP is characterised by ground-glass opacities and mosaic attenuation, while fibrotic HP shows irregular linear opacities, honeycombing, and traction bronchiectasis.

Histopathology

Acute HP features poorly formed noncaseating granulomas and mononuclear cell infiltration. Chronic HP presents with interstitial expansion by lymphocytes and occasional multinucleated giant cells or loose granulomas.

Pulmonary Function Testing

PFTs generally reveal a restrictive pattern but may show either restrictive or obstructive patterns. They are useful for monitoring lung function changes after antigen exposure.

Bronchoscopy

Bronchoalveolar lavage (BAL) often shows elevated cell counts, particularly T lymphocytes, aiding in excluding other lung diseases.

Lung Biopsy

Lung biopsies can be diagnostic in chronic HP and help differentiate it from idiopathic interstitial pneumonias.

Low magnification view of the histology of chronic hypersensitivity pneumonitis. The interstitium is expanded by chronic inflammatory infiltrate.

Differential Diagnosis

HP must be differentiated from conditions such as organic dust toxic syndrome, idiopathic pulmonary fibrosis, occupational asthma, and sarcoidosis.

Treatment

The primary treatment is avoiding the provoking antigen. Corticosteroids such as prednisolone may help control symptoms but can cause side effects. In severe cases, lung transplantation may be necessary. Other immunosuppressants and anti-fibrotic agents may be used, although their effectiveness is uncertain.

Prognosis

Acute HP generally has a good prognosis if exposure is avoided. However, subacute and chronic HP, particularly with fibrosis, have poorer outcomes, with death rates comparable to idiopathic pulmonary fibrosis.

Self-assessment MCQs (single best answer)

Which of the following is NOT a common antigen associated with hypersensitivity pneumonitis?

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What type of hypersensitivity reaction is primarily involved in acute hypersensitivity pneumonitis?

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In chronic hypersensitivity pneumonitis, what percentage of patients typically present with clubbing?

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Which imaging modality is commonly used to show fibrotic or non-fibrotic patterns in hypersensitivity pneumonitis?

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Which of the following histopathological features is associated with acute hypersensitivity pneumonitis?

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What is the primary treatment strategy for managing hypersensitivity pneumonitis?

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Which condition needs to be differentiated from hypersensitivity pneumonitis?

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Which of the following is a common symptom of acute hypersensitivity pneumonitis?

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What is a common finding in pulmonary function testing (PFT) for hypersensitivity pneumonitis?

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Which of the following types of hypersensitivity pneumonitis has the poorest prognosis?

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