Table 7.
Histopathological Criteria for the Diagnosis of HP (Other than “Hot-Tub Lung”*)
| HP | Probable HP | Indeterminate for HP |
|---|---|---|
|
Nonfibrotic HP (cellular HP) | ||
| Typical histopathological features of nonfibrotic HP; at least one biopsy site showing all three of the following features: | Both of the following features (1 and 2 from first column) in at least one biopsy site: | At least one biopsy site showing one of the following: |
| 1. Cellular interstitial pneumonia | 1. Cellular interstitial pneumonia | • 1 or 2 from the first column |
| • Bronchiolocentric (airway-centered) | • Bronchiolocentric (airway-centered) | • Selected IIP patterns |
| • Cellular NSIP-like pattern | • Cellular NSIP-like pattern | ○ Cellular NSIP pattern |
| • Lymphocyte-predominant | • Lymphocyte-predominant | ○ Organizing pneumonia pattern |
| ○ Peribronchiolar metaplasia without other features to suggest fibrotic HP and | ||
| 2. Cellular bronchiolitis | 2. Cellular bronchiolitis | Absence of features in any biopsy site to suggest an alternative diagnosis |
| • Lymphocyte-predominant (lymphs > plasma cells) with no more than focal peribronchiolar lymphoid aggregates with germinal centers | • Lymphocyte-predominant (lymphs > plasma cells) with no more than focal peribronchiolar lymphoid aggregates with germinal centers | • Plasma cells > lymphs |
| • ±Organizing pneumonia pattern with Masson bodies | • ±Organizing pneumonia pattern with Masson bodies | • Extensive lymphoid hyperplasia |
| • ±Foamy macrophages in terminal air spaces | • ±Foamy macrophages in terminal air spaces and | • Extensive well-formed sarcoidal granulomas and/or necrotizing granulomas |
| Absence of features in any biopsy site to suggest an alternative diagnosis | • Aspirated particulates | |
| 3. Poorly formed nonnecrotizing granulomas† | • Plasma cells > lymphs | |
| • Loose clusters of epithelioid cells and/or multinucleated giant cells ± intracytoplasmic inclusions | • Extensive lymphoid hyperplasia | |
| • Situated in peribronchiolar interstitium, terminal air spaces, and/or organizing pneumonia (Masson bodies) and | • Extensive well-formed sarcoidal granulomas and/or necrotizing granulomas | |
| Absence of features in any biopsy site to suggest an alternative diagnosis | • Aspirated particulates | |
| • Plasma cells > lymphs | ||
| • Extensive lymphoid hyperplasia | ||
| • Extensive well-formed sarcoidal granulomas and/or necrotizing granulomas | ||
| • Aspirated particulates | ||
|
Fibrotic HP‡ | ||
| Typical histopathological features of fibrotic HP; 1 or 2 and 3 in at least one biopsy site: | Both of the following features (1 or 2 from first column) in at least one biopsy site: | Either one of the following features in at least one biopsy site: |
| 1. Chronic fibrosing interstitial pneumonia | 1. Chronic fibrosing interstitial pneumonia | |
| 1. Chronic fibrosing interstitial pneumonia | • Architectural distortion, fibroblast foci ± subpleural honeycombing | • Architectural distortion, fibroblast foci ± honeycombing |
| • Fibrotic NSIP-like pattern | ||
| • Architectural distortion, fibroblast foci ± subpleural honeycombing | • Fibrotic NSIP-like pattern | ±Cellular interstitial pneumonia |
| • Fibrotic NSIP-like§ pattern | 2. Airway-centered fibrosis • ±Peribronchiolar metaplasia |
±Cellular bronchiolitis |
| • ±Bridging fibrosis‖ | ±Organizing pneumonia pattern and | |
| ±Cellular interstitial pneumonia | Absence of features in any biopsy site to suggest an alternative diagnosis | |
| 2. Airway-centered fibrosis • ±Peribronchiolar metaplasia | ±Cellular bronchiolitis | • Plasma cells > lymphs |
| • ±Bridging fibrosis‖ | ±Organizing pneumonia pattern and | • Extensive lymphoid hyperplasia |
| Absence of features in any biopsy site to suggest an alternative diagnosis | • Extensive well-formed sarcoidal granulomas and/or necrotizing granulomas | |
| 3. Poorly formed nonnecrotizing granulomas† | • Plasma cells > lymphs | • Aspirated particulates |
| ±Cellular interstitial pneumonia | • Extensive lymphoid hyperplasia | |
| ±Cellular bronchiolitis | • Extensive well-formed sarcoidal granulomas and/or necrotizing granulomas | |
| ±Organizing pneumonia pattern and | • Aspirated particulates | |
| Absence of features in any biopsy site to suggest an alternative diagnosis | ||
| • Plasma cells > lymphs | ||
| • Extensive lymphoid hyperplasia | ||
| • Extensive well-formed sarcoidal granulomas and/or necrotizing granulomas | ||
| • Aspirated particulates | ||
Definition of abbreviations: HP = hypersensitivity pneumonitis; IIP = idiopathic interstitial pneumonias; lymphs = lymphocytes; NSIP = nonspecific interstitial pneumonia; UIP = usual interstitial pneumonia.
Histological findings in hot-tub lung are distinctly different from nonfibrotic and fibrotic forms of classic HP.
Granulomas in HP are smaller, less tightly clustered, and lack the perigranulomatous hyaline fibrosis commonly seen in sarcoidosis.
Fibrotic HP may show classic features of nonfibrotic HP (cellular HP) in less fibrotic or nonfibrotic areas; if present, this combination of findings is a histological clue to the diagnosis of HP.
Updates to the classification of IIPs by Travis and colleagues (330) and diagnostic guidelines for idiopathic pulmonary fibrosis (20, 128) tightly link a UIP pattern with idiopathic pulmonary fibrosis and an NSIP pattern with idiopathic NSIP.
Bridging fibrosis spans subpleural and centriacinar or neighboring centriacinar fibrotic foci.