Clinical Features of Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis (HP) is characterized by an immunologically mediated inflammatory response in the lungs following inhalation of environmental or occupational antigens, presenting with distinct clinical, radiological, and histopathological features that vary based on disease stage and chronicity. 1

Clinical Presentation

General Features

Dyspnea, cough, and inspiratory crackles are common clinical manifestations across all forms of HP 1
Gastroesophageal reflux disease is frequently present in patients with HP 1
Inspiratory squawks (squeaky sounds during inspiration) may be present on auscultation 1
Weight loss is a significant predictor of HP and is more common in chronic forms 2

Acute HP

Fever, chills, malaise, and myalgias occurring 4-8 hours after antigen exposure 2
Symptoms resemble influenza-like illness with nonproductive cough and dyspnea 2, 3
Symptoms typically resolve within 24-48 hours if exposure is discontinued 4

Subacute/Chronic HP

Insidious and chronic onset of isolated pulmonary symptoms 1
Progressive dyspnea, fatigue, and weight loss over months 3
Symptoms may not clearly correlate with specific exposures 1
May progress to respiratory insufficiency and death if exposure continues 5

Radiological Features

High-Resolution CT Findings

Nonfibrotic (Acute/Subacute) HP:
- Centrilobular nodules 1, 4
- Mosaic attenuation on inspiratory scans 1
- Air trapping on expiratory scans 1, 4
- Ground-glass opacities and occasional cysts 1
Fibrotic (Chronic) HP:
- Coarse reticulation with minimal honeycombing in random distribution 1
- Evidence of small airway disease 1
- Variable patterns including basal/subpleural predominance, upper-lung-zone predominance, or central (peribronchovascular) predominance 1
- May mimic usual interstitial pneumonia (UIP) or nonspecific interstitial pneumonia (NSIP) patterns 6

Histopathological Features

Nonfibrotic (Cellular) HP

Cellular interstitial pneumonia with bronchiolocentric (airway-centered) distribution 1
Lymphocyte-predominant inflammation 1
Cellular bronchiolitis with lymphocyte predominance 1
Poorly formed non-necrotizing granulomas 1
- Loose clusters of epithelioid cells and/or multinucleated giant cells 1
- Located in peribronchiolar interstitium, terminal air spaces, and/or organizing pneumonia 1

Fibrotic HP

Chronic fibrosing interstitial pneumonia with architectural distortion 1
Airway-centered fibrosis with peribronchiolar metaplasia 1
May show fibrotic NSIP-like pattern 1
May retain some features of cellular HP in less fibrotic areas 1
Bridging fibrosis spanning subpleural and centriacinar regions 1

Laboratory Findings

Bronchoalveolar lavage (BAL) typically shows lymphocytosis (especially >30%) 1, 2
CD4+/CD8+ ratio <1 in BAL fluid 2
Positive precipitating antibodies to the offending antigen may be present 2
Pulmonary function tests show restrictive pattern in most cases, but may show obstruction or mixed pattern with avian exposure 2

Diagnostic Approach

The diagnosis of HP relies on three primary domains 1:

Identification of exposure to a causative antigen 1
Characteristic imaging pattern on HRCT 1
BAL lymphocytosis or compatible histopathological findings 1

Common Pitfalls and Caveats

HP is frequently misdiagnosed as idiopathic interstitial pneumonia, particularly in chronic forms 1
Fibrotic HP can be difficult to distinguish from idiopathic pulmonary fibrosis (IPF) due to overlapping features 1
Not all exposed individuals develop the disease, suggesting genetic susceptibility factors 6
Absence of an identifiable exposure does not rule out HP (no clear antigen identified in approximately 50% of cases) 1
Histopathological findings of HP may overlap with other interstitial lung diseases, requiring multidisciplinary discussion for accurate diagnosis 1
Early diagnosis and antigen avoidance are critical for preventing progression to irreversible fibrosis 4, 5

What are the clinical features of hypersensitive pneumonitis?

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