Hypersensitivity pneumonitis

Hypersensitivity pneumonitis
Synonyms	Extrinsic allergic alveolitis
Pronounce	N/A
Specialty	Pulmonology
Symptoms	Cough, dyspnea, fever, fatigue
Complications	Pulmonary fibrosis, respiratory failure
Onset	Hours to years after exposure
Duration	Variable
Types	N/A
Causes	Inhalation of organic dusts
Risks	Occupational exposure, farming, bird keeping
Diagnosis	Clinical history, imaging, lung biopsy
Differential diagnosis	N/A
Prevention	N/A
Treatment	Avoidance of antigen, corticosteroids
Medication	N/A
Prognosis	Variable, depends on chronicity and management
Frequency	N/A
Deaths	N/A

Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is an inflammatory syndrome affecting the lungs, caused by the inhalation of a variety of organic dusts. It is characterized by an immune-mediated response that leads to inflammation of the alveoli, the small air sacs in the lungs.

Etiology

Hypersensitivity pneumonitis is caused by repeated inhalation of organic antigens. These antigens are typically found in occupational or environmental settings. Common sources include:

• Bird droppings and feathers (e.g., in bird fancier's lung)
• Moldy hay (e.g., in farmer's lung)
• Contaminated humidifiers or air conditioners
• Mold spores from compost

Pathophysiology

The pathophysiology of hypersensitivity pneumonitis involves a complex immune response. Upon inhalation of the antigen, the body mounts an immune response that involves both humoral and cell-mediated immunity. Key features include:

• Formation of immune complexes
• Activation of T cells
• Release of cytokines
• Recruitment of macrophages and neutrophils

This immune response leads to inflammation and damage to the alveoli, resulting in impaired gas exchange.

Clinical Presentation

The clinical presentation of hypersensitivity pneumonitis can vary depending on the duration and intensity of exposure. It is typically classified into three forms:

Acute

• Symptoms appear 4-8 hours after exposure
• Fever, chills, cough, dyspnea
• Malaise and myalgia

Subacute

• Gradual onset of symptoms
• Chronic cough and dyspnea
• Fatigue and weight loss

Chronic

• Progressive dyspnea
• Clubbing of fingers
• Pulmonary fibrosis

Diagnosis

Diagnosis of hypersensitivity pneumonitis is based on a combination of clinical history, imaging, and sometimes lung biopsy.

History

• Detailed occupational and environmental exposure history
• Identification of potential antigens

Imaging

• Chest X-ray: May show diffuse infiltrates
• High-resolution computed tomography (HRCT): Ground-glass opacities, nodules, and fibrosis

Lung Biopsy

• May be required in uncertain cases
• Shows interstitial inflammation, granulomas

Management

The primary treatment for hypersensitivity pneumonitis is avoidance of the offending antigen. Additional treatments include:

• Corticosteroids to reduce inflammation
• Immunosuppressive therapy in severe cases

Prognosis

The prognosis of hypersensitivity pneumonitis varies. Acute forms may resolve completely with antigen avoidance, while chronic forms can lead to irreversible lung damage and pulmonary fibrosis.

Prevention

Preventive measures include:

• Identification and control of environmental sources of antigens
• Use of protective equipment in occupational settings

See also

• Interstitial lung disease
• Occupational lung disease