Pulmonary Pathology Online
Pathology of Idiopathic Pulmonary Fibrosis
(Idiopathic Interstitial Pneumonia; Cryptogenic Fibrosing Alveolitis)
Idiopathic interstitial pneumonia (IIP) represents a diverse group of lung disorders with variable prognoses and responses to therapy.
As the name implies, the etiology is unknown.
There is confusion regarding the definition and classification of some idiopathic fibrotic lung disorders.
Various terms have been used for the group, for example, Hamman-Rich disease, fibrosing alveolitis, idiopathic pulmonary fibrosis, and usual interstitial pneumonia.
The term Cryptogenic was added to emphasize the hidden/unknown cause, and the condition became known as cryptogenic fibrosing alveolitis (CFA) in some countries.
It is important to note that although the various forms of interstitial pneumonia may be morphologically and, to some extent, clinically distinctive, they all progress to a nonspecific "honeycomb" or "end-stage" lung.
Because of alveolitis and subsequent fibrosis, the distal part of the acinus shrinks. In a sense the lesion represent a form of atelectasis and is the reverse of emphysema. With shrinkage of lung parenchyma, the bronchioles dilate.
Because of epithelial damage the bronchiolar epithelium grows into the dilated airspaces that may have been proximal respiratory bronchioles but are no longer recognized as such.
The end stage is characterized by multiple cyst-like spaces separated from each other by dense scars. Retraction of the scar, especially of lobular septa, gives the external surface of the lung a hob-nailed appearance, hence the term "pseudocirrhosis".
On histologic examination, the "cyst" (in reality, dilated bronchioles) are found to contain mucus and other debris. Bronchi may also be slightly dilated and irregular.
Extensive vascular changes, particularly intimal fibrosis and thickening of the media, are caused by inflammation , fibrosis, and pulmonary hypertension.
Liebow preferred the term “idiopathic interstitial pneumonias”, which is classified into five entities namely :
1. Usual interstitial pneumonia (UIP); usual being the commonest type encountered,
2. Bronchiolitis obliterans with interstitial pneumonia (BIP)
5. Giant cell interstitial pneumonia (GIP)
More recently, it became apparent that some of these entities are not idiopathic or cryptogenic.
GIP is now generally classified as a manifestation of hard metal pneumoconiosis , although rare idiopathic cases exist. BIP is now categorized as bronchiolitis obliterans organizing pneumonia (BOOP), and lymphocytic interstitial pneumonia is now classified mainly among the lymphoproliferative disease.
Katzenstein and Katzenstein and Myers have recently classified idiopathic interstitial pneumonia/IPF into five entities (subsequently grouped two of which into a single category). [Idiopathic pulmonary fibrosis: clinical relevance of pathologic classification.Am J Respir Crit Care Med. 1998 Apr;157(4 Pt 1):1301-15. Review.]
Their histological classification which is based on the differences in the distribution , intensity and nature of the fibrosis and inflammation was recommended on the basis of its apparent reproducibility and prognostic significance.
The classification depends on the recognition that the histological changes can usually be divided into:
- "Temporally and spatially heterogeneous" ; being characterized by heterogeneity in time and space, with extreme variation in the appearances in the same biopsy, with areas of honeycomb change, indicative of repeated episodes of focal damage to the lung over a long period of time. This is characteristic of UIP, which is the most common cause of honeycomb lung.
- "Temporally uniform" ; with histological appearances indicative of lung injury occurring at a specific time scale.
Katzenstein’s classification appears to have gained acceptance by many histopathologists, respiratory physicians and radiologists.
Classification of idiopathic pulmonary fibrosis according to histological patterns of lung injury:
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