IPF, which typically includes a pathologic pattern known as usual interstitial pneumonia (UIP), is the most common idiopathic interstitial pneumonia. Cigarette smoking is a risk factor for IPF. Common respiratory symptoms include exertional dyspnea and a nonproductive cough. Physical examination is notable for inspiratory crackles at the lung bases. Clubbing may occur. High-resolution chest CT scans show subpleural reticular opacities predominantly in the lower lung fields, which are associated with honeycombing and traction bronchiectasis in advanced disease. Surgical lung biopsy is usually required to confirm the diagnosis, although pts with classic UIP patterns on CT scan may not require a biopsy. IPF can include acute exacerbations characterized by accelerated clinical deterioration over days to weeks; these exacerbations are associated with high mortality. Antifibrotic treatment with pirfenidone and nintedanib can slow the decline of pulmonary function in IPF, but immunosuppressive therapies are not effective.
Nonspecific Interstitial Pneumonia 
Nonspecific interstitial pneumonia (NSIP) is a histologic pattern that can be observed in connective tissue disease, drug-induced ILD, and chronic HP. Idiopathic NSIP is a subacute restrictive process with similar presentation to IPF. High-resolution CT (HRCT) shows bilateral ground-glass opacities, and honeycombing is rare. Unlike IPF, NSIP pts have a good prognosis and typically respond well to systemic glucocorticoid treatment, cytotoxic agents, or biologics.
Ild Associated with Connective Tissue Disorders
ILDs commonly occur in scleroderma, RA, and polymyositis/dermatomyositis, but can also occur in Sjögren syndrome and systemic lupus erythematosus (SLE). Pulmonary manifestations may precede systemic manifestations of a connective tissue disorder. In addition to direct pulmonary involvement, it is necessary to consider complications of therapy (e.g., opportunistic infections), respiratory muscle weakness, esophageal dysfunction, and associated malignancies as contributors to pulmonary parenchymal abnormalities in pts with connective tissue disorders.
Progressive systemic sclerosis (scleroderma) commonly includes ILD as well as pulmonary vascular disease. Lung involvement tends to be highly resistant to available treatment, but cyclophosphamide and mycophenolate have modest benefits. Minimizing esophageal reflux with proton pump inhibitors or antireflux surgery should be considered if progressive ILD develops.
In addition to pulmonary fibrosis (ILD), RA pts can develop a range of pulmonary complications, including pleural effusions, pulmonary nodules, and pulmonary vasculitis. ILD in RA pts is more common in men. Immunosuppressive and cytotoxic agents have been used with variable success for RA-related ILD.
Dermatomyositis/polymyositis pts with anti-synthetase antibodies often have ILD. Multiple histopathologic subtypes of ILD may be present. Immunosuppressive and cytotoxic agents have been used in pts with progressive ILD.
SLE also can involve a range of pulmonary complications, including pleural effusions, pulmonary vascular disease, pulmonary hemorrhage, and BOOP. Chronic, progressive ILD is not commonly observed.
Cryptogenic Organizing Pneumonia
When the BOOP pathologic pattern occurs without another primary pulmonary disorder, the term cryptogenic organizing pneumonia is used. COP may present with a subacute flu-like illness. Recurrent and migratory patchy consolidative and ground glass pulmonary opacities are common. COP may occur secondary to a connective tissue disorder, medications, or an underlying malignancy; COP may also occur in isolation. Glucocorticoid therapy is often effective.
Desquamative Interstitial Pneumonia and Respiratory Bronchiolitis-Associated Ild
Desquamative interstitial pneumonia (DIP) includes extensive macrophage accumulation in intraalveolar spaces with minimal fibrosis. It is seen almost exclusively in cigarette smokers and improves with smoking cessation. Respiratory bronchiolitis-associated ILD is a subset of DIP that includes central bronchial wall thickening, ground-glass opacities, and air trapping on HRCT; it also resolves in most pts after smoking cessation.
Characterized by acute onset of hypoxemia and respiratory distress, AIP (Hamman-Rich syndrome) has high mortality with frequent recurrences among survivors. Chest imaging frequently shows patchy bilateral ground glass opacities and dependent air-space consolidation. Treatment is supportive and usually includes mechanical ventilation.
Common granulomatous ILDs include sarcoidosis and HP. HP is an inflammatory lung disorder caused by repeated inhalation of an organic antigen in a susceptible individual. Treatment involves avoiding exposure to the causative antigen; systemic corticosteroids may be required in subacute or chronic HP. Granulomatous vasculitides include inflammatory infiltrates of blood vessels with associated granulomas. Hemoptysis is frequently observed. Granulomatosis with polyangiitis (Wegener's disease) commonly affects the lungs; chest imaging abnormalities include lung nodules, patchy ground glass opacities, hilar lymphadenopathy, and consolidative opacities. Eosinophilic GPA (Churg-Strauss syndrome) frequently includes asthma, peripheral blood eosinophilia, and chronic sinusitis; common chest imaging abnormalities include consolidative opacities and small pleural effusions.