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Overview of Idiopathic Interstitial Pneumonias


Joyce Lee

, MD, MAS, University of Colorado Denver

Last full review/revision Sep 2019| Content last modified Sep 2019
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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

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*


People Most Often Affected


Chest X-ray Findings

High-Resolution CT Findings

CT Differential Diagnosis

Histologic Pattern

More frequently men >50 (> 60% smoke)

Chronic (> 12 months)

Basal-predominant reticular abnormality with volume loss and honeycombing

Peripheral, subpleural, and basal reticular honeycombing

Traction bronchiectasis or bronchiolectasis

Architectural distortion

Connective tissue disorders

Usual interstitial pneumonia

More frequently men, aged 30–50 (> 90% smoke)

Subacute to chronic (weeks to years)

Ground-glass opacification

Lower zone, peripheral predominance in most cases

Ground-glass opacification

Reticular lines

Hypersensitivity pneumonitis

Desquamative interstitial pneumonia

More frequently women, usually age 40–60 (< 40% smoke)

Subacute to chronic (months to years)

Ground-glass and reticular opacity

Peripheral, basal, symmetric

Reticular opacities

Variable ground-glass opacification

Irregular lines

Cryptogenic organizing pneumonia

Desquamative interstitial pneumonia

Hypersensitivity pneumonitis

Idiopathic pulmonary fibrosis

Nonspecific interstitial pneumonia

People of any age, usually aged 40–50 (< 50% smoke)

Subacute (< 3 months)

Patchy bilateral consolidation


Patchy consolidation, nodules, or both

Alveolar cell carcinoma


Nonspecific interstitial pneumonia


Organizing pneumonia

Slightly more men, aged 30–50 (> 90% smoke)

Subacute (weeks to months)

Bronchial wall thickening

Ground-glass opacification

Diffuse pattern

Bronchial wall thickening

Centrilobular nodules

Patchy ground-glass opacification

Desquamative interstitial pneumonia

Hypersensitivity pneumonitis


Nonspecific interstitial pneumonia

Respiratory bronchiolitis–associated interstitial lung disease

People of any age†

Abrupt (1–2 weeks)

Progressive, diffuse ground-glass opacification

Diffuse consolidation, ground-glass opacification, often with lobular sparing

Traction bronchiectasis later

Hydrostatic edema

Diffuse alveolar damage

Mostly women, of any age†

Chronic (> 12 months)

Reticular opacities


Diffuse pattern

Centrilobular nodules

Ground-glass opacification

Septal and bronchovascular thickening

Thin-walled cysts

Connective tissue disorders

Lymphangitic carcinoma


Lymphocytic interstitial pneumonia

No sex predilection, median age of 57 years

Chronic (>12 months)

Bilateral apical irregular pleural thickening

Dense subpleural consolidation

Traction bronchiectasis

Architectural distortion

Upper lobe volume loss


Connective tissue disorders

Hypersensitivity pneumonitis

Nonspecific interstitial pneumonia

Radiation-induced lung disease


Pleuroparenchymal fibroelastosis

* Listed in order of decreasing frequency.

† History of smoking unknown.

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) 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 entire employment history, including specific duties and known exposures to organic and inorganic agents (see table 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 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 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

  • Often corticosteroids

  • Sometimes lung transplantation

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.


Treatment and Prognosis of Idiopathic Interstitial Pneumonias*




Pirfenidone or nintedanib

Lung transplantation

Mortality rate: 50–70% in 5 years

Smoking cessation

Mortality rate: 5% in 5 years

Corticosteroids with or without immunosuppressive therapies (eg, azathioprine, mycophenolate mofetil)

Mortality rate: widely variable, but generally better than idiopathic pulmonary fibrosis. In purely cellular disease (rare), extremely low


Complete recovery rate: > 65%

Relapses: In many

Mortality rate: Rare

Smoking cessation

Mortality rate: Rare

Supportive care

Mortality rate: 60% in < 6 months


Not well defined

Appropriate treatment unknown; often corticosteroids

Disease progression occurs in 60%

* Listed in order of decreasing frequency.

Key Points about Idiopathic Interstitial Pneumonias

  • 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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