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INTRODUCTION

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Patients with interstitial lung diseases (ILDs) come to medical attention mainly because of the onset of progressive exertional dyspnea or a persistent nonproductive cough. Hemoptysis, wheezing, and chest pain may be present. Often, the identification of interstitial opacities on chest x-ray focuses the diagnostic approach on one of the ILDs.

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ILDs represent a large number of conditions that involve the parenchyma of the lung—the alveoli, the alveolar epithelium, the capillary endothelium, and the spaces between those structures—as well as the perivascular and lymphatic tissues. The disorders in this heterogeneous group are classified together because of similar clinical, roentgenographic, physiologic, or pathologic manifestations. These disorders often are associated with considerable rates of morbidity and mortality, and there is little consensus regarding the best management of most of them.

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ILDs have been difficult to classify because >200 known individual diseases are characterized by diffuse parenchymal lung involvement, either as the primary condition or as a significant part of a multiorgan process, as may occur in the connective tissue diseases (CTDs). One useful approach to classification is to separate the ILDs into two groups based on the major underlying histopathology: (1) those associated with predominant inflammation and fibrosis and (2) those with a predominantly granulomatous reaction in interstitial or vascular areas (Table 315-1). Each of these groups can be subdivided further according to whether the cause is known or unknown. For each ILD there may be an acute phase, and there is usually a chronic one as well. Rarely, some are recurrent, with intervals of subclinical disease.

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Sarcoidosis (Chap. 390), idiopathic pulmonary fibrosis (IPF), and pulmonary fibrosis associated with CTDs (Chaps. 378, 382, 388, and 427) are the most common ILDs of unknown etiology. Among the ILDs of known cause, the largest group includes occupational and environmental exposures, especially the inhalation of inorganic dusts, organic dusts, and various fumes or gases (Chap. 311). A multidisciplinary approach—requiring close communication between clinician, radiologist, and when appropriate, pathologist—is often required to make the diagnosis. High-resolution computed tomography (HRCT) scanning improves the diagnostic accuracy and may eliminate the need for tissue examination in many cases, especially in IPF. For other forms, tissue examination, usually obtained by thoracoscopic lung biopsy, is critical to confirmation of the diagnosis.

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PATHOGENESIS

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The ILDs are nonmalignant disorders and are not caused by identified infectious agents. The precise pathway(s) leading from injury to fibrosis is not known. Although there are multiple initiating agent(s) of injury, the immunopathogenic responses of lung tissue are limited, and the mechanisms of repair have common features (Fig. 315-1).

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FIGURE 315-1

Proposed mechanism for the pathogenesis of pulmonary fibrosis. The lung is naturally exposed to repetitive injury from a variety of exogenous and endogenous stimuli. Several local and systemic factors (e.g., fibroblasts, circulating fibrocytes, chemokines, growth ...

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