Hypersensitivity Pneumonitis
Definition
- Pulmonary disease secondary to inhaled allergens
Alternate/Historical Names
- Extrinsic allergic alveolitis (equvalent term)
- Many allergen specific names, a few of which are:
- Bird fancier's lung
- Farmer's lung
- Hot tub lung
- Humidifier lung
Diagnostic Criteria
Clinical
- May present as acute, subacute or chronic disease
- Acute
- Usually follows 2-9 hours after a heavy exposure
- Patient must be presensitized (frequently unknowingly)
- Flu-like symptoms
- Fever, chills, malaise, nausea
- Cough may progress to respiratory insufficiency
- If purely acute, usually resolves in 1-2 days with supportive treatment
- Subacute and chronic patients may present with acute exacerbation
- Usually follows 2-9 hours after a heavy exposure
- Subacute
- Gradual onset of dyspnea, cough over days or weeks
- Follows continued intermittent exposure
- Good response to steroids and avoidance/removal of antigen
- Chronic
- Chronic dyspnea, fatigue, weight loss
- Usually continued low level antigen exposure
- Antigen may not be identifiable
- Generally due to irreversible fibrosis
- Most identified allergens are exogenous fungi (molds) and mycobacteria (hot tubs)
- Allergic bronchopulmonary aspergillosis may be an allergen source
- A few examples of molecular antigens have been identified (isocyanates, zinc)
- Acute
High Resolution Computed Tomography
- Centrilobular nodularity and ground glass opacities
- Chronic disease shows reticular fibrosis
- May progress to honeycomb change
- Usually central, but may be peripheral
- Emphysema and cysts may be seen
- May progress to honeycomb change
Pathology
- Diagnosis can frequently be made based on history, physical findings, radiology and serologic reaction to suspected allergen, without biopsy
- Bronchoalveolar lavage shows lymphocytosis, frequently >50%
- Mast cells >1% are very suggestive
- Role of the biopsy is to suggest the diagnosis when it was not suspected clinically, to rule out iinfection or to evaluate cases with atypical presentation
- Most cases are not biopsied
- Even if the biopsy suggests hypersensitivity pneumonitis, the final diagnosis must be made clinically
- Acute phase is rarely biopsied
- Neutrophils fill alveoli and respiratory bronchioles
- Frequent loose, non-caseating granulomas
- Diffuse alveolar damage pattern with hyaline membranes may be present
- Subacute and chronic phases show a spectrum of changes
- Bronchiolocentric cellular interstitial infiltrate
- Occasional lymphoid follicles may be seen
- Poorly formed non-caseating granulomas
- May be associated with multinucleated giant cells
- Frequently contain cholesterol clefts, Schaumann bodies, asteroid bodies
- Usually infrequent in chronic disease
- May be associated with multinucleated giant cells
- BOOP-like plugs of intralumenal organizing granulation tissue and interstitial fibroblastic foci
- Both are decreased in chronic phase but frequently present focally
- Fibrosis
is present in chronic phase
- May be NSIP-like with homogeneous linear pattern
- May be UIP-like with subpleural patchy pattern leading to honeycomb change
- Centrilobular accentuation of peribronchiolar fibrosis is very suggestive of hypersensitivity pneumonitis
- Surrounds and may occlude bronchiole
- Fibrosis may bridge from centrilobular to peripheral zones
- Fibrosis is generally not reversible and imparts a poor prognosis
- Heavy re-exposure to antigen may lead to a superimposed acute reaction
- Bronchiolocentric cellular interstitial infiltrate
Gerald J Berry MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
