Sarcoidosis is a systemic inflammatory disorder of unknown etiology that affects multiple organs and is characterized by the formation of noncaseating granulomas. Intrathoracic lymph node and pulmonary parenchymal involvement occurs in more than 90% of cases. The granulomas have a characteristic distribution along the lymphatics in the pleura, interlobular septa, and bronchovascular bundles. They may resolve spontaneously or with treatment or progress to fibrosis. Etiology The…

Acute interstitial pneumonia (AIP) is a severe acute disease of unknown etiology that usually occurs in a previously healthy individual and produces histologic findings of diffuse alveolar damage. The clinical, radiologic, and pathologic manifestations are identical to those of acute respiratory distress syndrome (ARDS); the only distinction is that no etiology is found. AIP is essentially idiopathic ARDS. Prevalence and Epidemiology AIP is rare, has no…

Organizing pneumonia (OP) is a histologic pattern characterized by the presence of intraluminal granulation tissue polyps within alveolar ducts and surrounding alveoli associated with chronic inflammation of the surrounding lung parenchyma. Because the granulation tissue polyps frequently also involve the bronchioles, the pattern was previously known as bronchiolitis obliterans organizing pneumonia (BOOP). However, given the confusion of this entity with bronchiolitis obliterans (synonym: obliterative or constrictive…

Etiology, Prevalence, and Epidemiology Nonspecific interstitial pneumonia (NSIP) is a chronic interstitial lung disease characterized by homogeneous expansion of the alveolar walls by inflammation or fibrosis or both. NSIP is the second most common chronic interstitial pneumonia, after usual interstitial pneumonia (UIP), accounting for 14% to 35% of cases. NSIP may be idiopathic but more commonly occurs as a manifestation of connective tissue disease, hypersensitivity pneumonitis,…

Etiology, Prevalence, and Epidemiology Idiopathic pulmonary fibrosis (IPF) has been defined as “a specific form of chronic fibrosing interstitial pneumonia limited to the lung and associated with the histologic and/or CT appearance of usual interstitial pneumonia (UIP).” IPF is the most common idiopathic interstitial lung disease. Data from the late 1980s estimated a prevalence of 20 per 100,000 for men and 13 per 100,000 for women,…

Pulmonary Lymphoid Hyperplasia (Follicular Bronchiolitis) Etiology, Prevalence, and Epidemiology Pulmonary lymphoid hyperplasia, also known as follicular bronchiolitis or hyperplasia of bronchus-associated lymphoid tissue, is a benign condition characterized histologically by the presence of polyclonal lymphoid aggregates along the bifurcation of the bronchioles and along the pulmonary lymphatics and is mainly differentiated from lymphocytic interstitial pneumonia (LIP) by its typical distribution limited mainly to the airways. It…

Etiology, Prevalence, and Epidemiology Inflammatory pseudotumor, also known as inflammatory myofibroblastic tumor, is a quasineoplastic lesion that clinically and radiologically tends to mimic a malignant neoplasm and that histologically consists of a mixture of inflammatory cells, myofibroblastic spindle cells, and plasma cells. The proportion of the various cells varies considerably from pseudotumor to pseudotumor. Pseudotumors with a predominance of plasma cells are commonly referred to as…

Etiology, Prevalence, and Epidemiology Pulmonary hamartomas are benign neoplasms, probably derived from bronchial wall mesenchymal cells. Hamartomas are the most common benign pulmonary neoplasm and account for about 8% of primary lung tumors. Although they may be seen in adolescents and young adults, most cases occur in patients older than 40 years; the peak incidence is in the seventh decade of life. Men are affected two…

Parasites are defined as organisms that live in or on another living organism, obtain part or all of their organic nutriment from that organism, and cause some degree of damage to their host. Parasitic infestations occur mainly in tropical and subtropical regions. Pulmonary disease secondary to parasites in North America and Europe is seen in individuals who traveled to endemic areas and in recent immigrants. Parasites…

Viruses are the most common cause of respiratory infection and may result in rhinitis, pharyngitis, laryngotracheitis, bronchitis, bronchiolitis, and, less commonly, pneumonia. Most viral pneumonias in immunocompetent adults are due to influenza viruses; other common viral etiologies include respiratory syncytial virus (RSV) and adenovirus. Immunocompromised hosts are particularly susceptible to pneumonias caused by cytomegalovirus (CMV) and herpesviruses. Risk factors associated with increased incidence and severity of…