Pathology of Usual Interstitial Pneumonia (UIP)
Dr Sampurna Roy MD January 2016
Usual interstitial pneumonia is the most common form of idiopathic interstitial fibrosis (70% of all cases), and unfortunately carries a poor prognosis.
This condition is characteristically most prominent subpleurally in the lower lobes of the lung.
The most important histological feature is the highly variegated lung architecture often including the entire spectrum from normal alveolar walls to end stage fibrotic lesions (honeycomb) in the same section.
[ Each of the terms usual interstitial pneumonia, cryptogenic fibrosing alveolitis, and idiopathic pulmonary fibrosis stresses certain features - the cause is unknown, alveolar inflammation is an important part of the disease, fibrosis is the usual sequel, and there may be unusual forms of interstitial pneumonia. ]
The clinical, radiologic and functional features are those of restrictive lung disease.
The disease is usually diagnosed in the sixth decade.
Dyspnea of gradual onset, often over 5 to 10 years, is customary.
Electron microscopic examination reveals gross distortion and infolding of the alveolar basal lamina in the fibrotic areas.
Loose granulation tissue in the alveolar spaces leads to alveolar collapse and contraction of fibrous tissue.
About one quarter of all cases date their illness to an acute broncho-pulmonary infection, an observation that raises the possibility that the disease is sequel to viral infection.
The classic auscultatory finding consists of fine crackles at the lung bases in late inspiration.
The prognosis is bleak, with an average survival of 5 years.
The response to treatment, usually corticosteroids, is generally poor.
The etiology of usual interstitial pneumonia is not known, but the condition is usually thought of as an immunologically mediated disorder.
Evidence includes the association with collagen-vascular disease and serum protein abnormalities, the presence of circulating immune complexes, the presence of immunoglobulins in alveolar walls, and the release of a lymphokine, migration inhibitory factor, when lymphocytes of patients with the disease are exposed to collagen.
According to one theory, macrophages play a central role in the pathogenesis of usual interstitial pneumonia, and the initial damage is to collagen, by an unknown agent.
Macrophages engulf collagen fragments and secrete a fibroblast-stimulating factor, thereby leading to fibrosis.
They also release a chemotactic factor for polymorphonuclear leukocytes, which then do further damage.
Histologically, there is subpleural accentuation of the fibrotic and inflammatory process with relative sparing of centri-acinar structures.
The key morphologic feature of usual interstitial pneumonia is heterogeneity of lesions, that is, different appearances in different parts of the lung, in different lung biopsies, and even in different fields of the same lung biopsy.
The variation is so great that in some fields the alveolar walls are entirely normal.
Inflammation varies from subtle increased cellularity (mainly lymphocytes) of otherwise apparently normal alveolar walls to diffuse alveolar damage with obvious alveolar wall inflammation and hyperplasia of type II cells.
Lymphoid aggregates are also seen. By the time a lung biopsy is performed fibrosis is always present, but its severity varies.
There may be subtle alveolar wall thickening, demonstrated only by special stains for collagen.
In other cases fibrosis may be obvious but alveolar walls may be maintained, although the acinar structure is somewhat simplified.
At the extreme is honeycomb lung, brought about by alveolar wall inflammation and collapse.
Neutrophils are not a prominent feature, except in the infected cystic spaces.
However, the bronchoalveolar lavage fluid contains increased numbers of neutrophils.
The epithelial lining ranges from normal alveolar cells in undamaged areas to large rounded hyperplastic type 2 cells in diseased areas with focal ciliated columnar, goblet cells and occasionally squamous metaplasia in severely affected areas.
In areas of advanced disease, most of the fibrosis consists of eosinophilic collagen with scanty fibroblastic cells.
The cystically dilated air spaces of contain inspissated mucus admixed with histiocytes, inflammatory cells and cell debris.
As a result of inflammation and scarring, there is also some bronchioloectasis which is well recognized by the radiologists as "traction bronchiectasis".
UIP is also characterized by the presence of “fibroblast foci”.
These are areas of active disease, consisting of fibroblastic and myofibroblastic proliferation set in a myxoid background, which represent organization of intra-alveolar exudates (ongoing fibrosis).
Fibroblast foci are an important component of the temporal heterogeneity characteristic of the disease process.
Inflammatory cellular infiltration is usually mild, although focal lymphoid hyperplasia with prominent lymphoid follicles may be present, particularly in cases associated with systemic disease.
Reactive smooth muscle hyperplasia is often marked and is particularly prominent around small bronchioles. The smooth muscle fibers are of normal morphology, unlike those seen in lymphangioleiomyomatosis which is characterized by immature smooth muscle proliferation.
Accumulation of tight clusters or loosely dispersed macrophages within alveolar spaces is commonly seen in UIP, particularly in cigarette smokers, and this DIP-like reaction in an otherwise typical case of UIP shows no correlation with the stage of the disease.
Pulmonary arterial vascular changes are often seen in fibrotic areas, but these are of little clinical significance as there is no correlation between these changes and the presence of pulmonary hypertension.
Typically, UIP is characterized by the absence of vasculitis , granulomata and asbestos bodies.
Pleural fibrosis is very rare in UIP.
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