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Overview of Idiopathic Interstitial Pneumonias


Joyce Lee

, MD, MAS, University of Colorado School of Medicine

Reviewed/Revised Jun 2021 | Modified Sep 2022
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Idiopathic interstitial pneumonias (IIPs) are interstitial lung diseases Overview of Interstitial Lung Disease Interstitial lung diseases are a heterogeneous group of disorders characterized by alveolar septal thickening, fibroblast proliferation, collagen deposition, and, if the process remains unchecked... read more of unknown etiology that share similar clinical and radiologic features and are distinguished primarily by the histopathologic patterns on lung biopsy. Classified into 8 histologic subtypes, all are characterized by varying degrees of inflammation and fibrosis and all cause dyspnea. Diagnosis is based on history, physical examination, high-resolution CT imaging, pulmonary function tests, and lung biopsy. Treatment varies by subtype. Prognosis varies by subtype and ranges from excellent to nearly always fatal.

The 8 histologic subtypes of IIP, in decreasing order of frequency, are

These subtypes are characterized by varying degrees of interstitial inflammation and fibrosis (1 General reference Idiopathic interstitial pneumonias (IIPs) are interstitial lung diseases of unknown etiology that share similar clinical and radiologic features and are distinguished primarily by the histopathologic... read more ). All cause dyspnea; diffuse abnormalities on high-resolution CT (HRCT); and inflammation, fibrosis, or both on biopsy. The subtypes are important to distinguish, however, because they have different clinical features (see table Key Features of Idiopathic Interstitial Pneumonias Key Features of Idiopathic Interstitial Pneumonias* Key Features of Idiopathic Interstitial Pneumonias* ) and respond differently to treatment.


General reference

  • 1. Travis WD, Costabel U, Hansell DM, et al: An Official American Thoracic Society/European Respiratory Society Statement: Update of the International Multidisciplinary Classification of the Idiopathic Interstitial Pneumonias. Am J Respir Crit Care Med 188 (6):733–748, 2013.

Symptoms and Signs of Idiopathic Interstitial Pneumonias

Symptoms and signs of idiopathic interstitial pneumonias are usually nonspecific. Cough and dyspnea on exertion are typical, with variable onset and progression. Common signs include tachypnea, reduced chest expansion, bibasilar end-inspiratory dry crackles, and digital clubbing.

Diagnosis of Idiopathic Interstitial Pneumonias

  • High-resolution CT (HRCT)

  • Pulmonary function tests

  • Sometimes surgical lung biopsy

Idiopathic interstitial pneumonia should be suspected in any patient with unexplained interstitial lung disease. Clinicians, radiologists, and pathologists should exchange information to determine the diagnosis in individual patients. Potential causes (see table Causes of Interstitial Lung Disease Causes of Interstitial Lung Disease Causes of Interstitial Lung Disease ) are assessed systematically. For maximum diagnostic yield, history should address the following criteria:

  • Symptom duration

  • Family history of lung disease, especially lung fibrosis

  • History of tobacco use (because some diseases occur mostly among current or former smokers)

  • Current and prior drug use

  • Detailed review of home and work environments, including those of family members

A chronologic listing of the patient's employment history, including specific duties and known exposures to organic and inorganic agents (see table Causes of Interstitial Lung Disease Causes of Interstitial Lung Disease Causes of Interstitial Lung Disease ), is obtained. The degree of exposure, duration of exposure, latency of exposure, and the use of protective devices is elicited.

Chest x-ray is done and is typically abnormal, but findings are not specific enough to differentiate between the various types.

Pulmonary function tests Overview of Tests of Pulmonary Function Pulmonary function tests provide measures of airflow, lung volumes, gas exchange, response to bronchodilators, and respiratory muscle function. Basic pulmonary function tests available in the... read more are often done to estimate the severity of physiologic impairment, but they do not help differentiate between the various types. Typical results are restrictive physiology, with reduced lung volumes and diffusion capacity. Hypoxemia is common during exercise and may be present at rest.

HRCT, which distinguishes airspace from interstitial disease, is the most useful test and is always done. It provides assessment of the potential etiology, extent, and distribution of disease, and is more likely to detect underlying or coexisting disease (eg, occult mediastinal adenopathy, cancer, emphysema). HRCT should be done with the patient supine and prone and should include dynamic expiratory imaging to accentuate evidence of small airway involvement.

Laboratory tests are done for patients who have clinical features suggesting a connective tissue disorder, vasculitis, or environmental exposure. Such tests may include antinuclear antibodies, rheumatoid factor, and other more specific serologic tests for connective tissue diseases (eg, ribonucleoprotein [RNP], anti-Ro [SSA], anti-La [SSB], scleroderma antibody [Scl70], anti-Jo-1 antibody ).

Bronchoscopic transbronchial biopsy can help differentiate certain interstitial lung diseases, such as sarcoidosis Sarcoidosis Sarcoidosis is an inflammatory disorder resulting in noncaseating granulomas in one or more organs and tissues; etiology is unknown. The lungs and lymphatic system are most often affected, but... read more Sarcoidosis and hypersensitivity pneumonitis, but the biopsy does not yield enough tissue to diagnose the IIPs. Bronchoalveolar lavage helps narrow the differential diagnosis in some patients and can provide information about disease progression and response to therapy. The usefulness of this procedure in the initial clinical assessment and follow-up of most patients with these diseases has not been established, however.

Cryobiopsy, a technique that quickly freezes the lung tissue immediately prior to removal, is being investigated as an aid to diagnosis of certain interstitial lung diseases. The tissue yield is higher than that of transbronchial biopsy but lower than surgical lung biopsy. Risks of the procedure include bleeding and pneumothorax. Cryobiopsy is currently under investigation and is not recommended by international guidelines; its role in diagnosis remains to be established.

Surgical lung biopsy is needed to confirm the diagnosis when the history and HRCT are nondiagnostic. Biopsy of multiple sites with a video-assisted thoracoscopic surgery (VATS) procedure is preferred.

Treatment of Idiopathic Interstitial Pneumonias

  • Varies by disorder

  • Sometimes corticosteroids

  • Sometimes lung transplantation


Key Points

  • There are 8 histologic subtypes of idiopathic interstitial pneumonia.

  • Symptoms, signs, and chest x-ray findings are nonspecific.

  • Diagnose IIPs initially based primarily on history and high-resolution CT (HRCT).

  • When clinical evaluation and HRCT are not diagnostic, do surgical lung biopsy.

  • Treatment varies by subtype.

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