Usual interstitial pneumonia

Usual Interstitial Pneumonia (UIP) is the name of a histopathological pattern of fibrosis associated with various restrictive lung diseases, including idiopathic pulmonary fibrosis and asbestosis. Strictly speaking it is an incorrect usage, since the term "pneumonia" is reserved for lung inflammations of contagious origin.

UIP is defined histologically as a pattern of fibrosis that begins in the lower lung zones, in the subpleural part of the lung (i.e. sparing the central or hilar part), and gradually progresses in ascending fashion to involve the upper lung zones, ultimately resulting in honeycomb lung.

USUAL INTERSTITIAL PNEUMONIA

Clinical — Usual interstitial pneumonia (UIP) is the typical pathological finding in patients with suspected idiopathic pulmonary fibrosis (IPF). An ATS consensus statement defined IPF as a specific form of chronic fibrosing interstitial pneumonia limited to the lung and associated with the histologic appearance of usual interstitial pneumonia (UIP) on surgical (thoracoscopic or open) lung biopsy.

IPF has an estimated prevalence of 14 to 43 per 100,000 and an estimated incidence of 7 to 16 per 100,000, depending on the criteria used to define IPF. Both prevalence and incidence increase with advancing age. Most cases are sporadic and present with slowly progressive dyspnea and nonproductive cough. Men are affected more commonly than women. Rare familial cases of IPF have been described.

Pulmonary function studies typically demonstrate a restrictive pattern, and chest radiographs show diffuse interstitial opacities associated with reduced lung volumes. High resolution computed tomography (HRCT) demonstrates a characteristic pattern of peripheral (subpleural) and bibasilar reticulonodular opacities associated with architectural distortion, including honeycomb changes and traction bronchiectasis. In the appropriate clinical setting, radiographic findings may be sufficient to establish the diagnosis.

UIP/IPF usually follows a relentlessly progressive course, with most patients dying of respiratory failure within five to ten years after diagnosis. Periodic exacerbations are common with a two-year frequency of approximately ten percent [8]. Exacerbations are associated with increased symptoms and an accelerated decline in pulmonary function are the rule and can be related to either variation in the tempo of the underlying lesion or superimposed complications of various types.Most patients succumb to respiratory failure, although other fatal complications, such as bronchogenic carcinoma, account for a significant minority of deaths. Treatment options are limited. Pathology — UIP is a specific morphologic entity. The histologic hallmark and chief diagnostic criterion is a heterogeneous appearance with alternating areas of normal lung, interstitial inflammation, fibroblast foci, and honeycomb change. The histological changes correlate with findings on HRCT, in that the peripheral subpleural parenchyma is most severely affected.

The fibrotic zones are composed mainly of dense collagen, although scattered foci of fibroblast proliferation ("fibroblast foci") are a consistent finding. Interstitial inflammation is usually patchy and consists of an alveolar septal infiltrate of lymphocytes, plasma cells, and histiocytes associated with hyperplasia of type 2 pneumocytes. Areas of honeycomb change are composed of cystic fibrotic air spaces, which are frequently lined by bronchiolar epithelium and filled with mucin. Smooth muscle hyperplasia is commonly seen in areas of fibrosis and honeycomb change. Patients who are biopsied during an accelerated phase of their illness (accelerated IPF) may show a combination of UIP and a superimposed acute lung injury pattern such as diffuse alveolar damage. Insights into the pathogenesis of UIP have helped correlate histology with survival and response to treatment. In a carefully executed, prospective cohort study of 87 patients with biopsy-proven UIP, a greater degree of granulation/connective tissue deposition (characteristic of fibroblast foci) was correlated with shorter survival. This finding is consistent with the view that UIP is not a disease of active inflammation, but one of abnormal lung fibroblast proliferation and dysregulated fibrogenesis.

Differential diagnosis — The term UIP is generally reserved for those patients in whom the lesion is idiopathic. A pattern of interstitial inflammation and fibrosis indistinguishable from UIP can occur in patients with collagen vascular diseases (eg, rheumatoid arthritis), asbestosis, and certain drug-induced lung diseases. Hermansky-Pudlak syndrome, an autosomal recessive disorder characterized by oculocutaneous albinism and platelet abnormalities, is a rare cause of UIP.

Distinguishing the idiopathic form of UIP from lesions complicating the use of certain drugs is largely a matter of correlation with the clinical information. A diagnosis of asbestosis requires not only an appropriate occupational history but also demonstration of asbestos fibers (usually in the form of ferruginated asbestos bodies) in the tissue specimen. The differential diagnosis of UIP frequently includes the other idiopathic interstitial pneumonias, hypersensitivity pneumonitis, and pulmonary Langerhans' cell histiocytosis (also referred to as Langerhans' cell granulomatosis, pulmonary eosinophilic granuloma, and pulmonary histiocytosis X) (show table 1). (See "Approach to the adult with interstitial lung disease").