Overview of Idiopathic Interstitial Pneumonias
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 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
Idiopathic pulmonary fibrosis (identified histologically as usual interstitial pneumonia)
These subtypes are characterized by varying degrees of interstitial inflammation and fibrosis (1). 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) and respond differently to treatment.
Key Features of Idiopathic Interstitial Pneumonias
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.
IIP 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) are assessed systematically. For maximum diagnostic yield, history should address the following criteria:
A chronologic listing of the patient's entire employment history, including specific duties and known exposures to organic and inorganic agents (see 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 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 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, other more specific serologic tests for connective tissue diseases (eg, RNP, SSA, SSB, scl70, Jo-1), hypersensitivity panel (a collection of tests for antibodies to common antigens from microbial, fungal, and animal sources), antineutrophil cytoplasmic antibodies, and anti-basement membrane antibody.
Bronchoscopic transbronchial biopsy can help differentiate certain interstitial lung diseases, such as 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.
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 varies by disorder (see Table: Treatment and Prognosis of Idiopathic Interstitial Pneumonias*). Smoking cessation is always recommended to avoid potentially accelerating disease progression and to limit respiratory comorbidities.
Lung transplantation may be recommended for selected patients with end-stage disorders.