**2.3. Classification of idiopathic interstitial pneumonia**

As noted above, if ILD is found to have no association with specific causes, such as chemical exposure, underlying systemic diseases, or genetic causes, the disease is classified as "idiopathic interstitial pneumonia" (IIP). IIPs are a heterogenous group of nonneoplastic disorders involving damage to the lung parenchyma with varying patterns of inflammation and fibrosis. The interstitium includes the space between epithelial and endothelial basement membranes and is the primary site of injury in IIPs. Frequently affecting not only the interstitium, but IIPs

bronchiolitis interstitial lung disease" (RB-ILD), or "desquamative interstitial pneumonia" (DIP) [1, 2]. "Lymphoid interstitial pneumonia" (LIP) is occasionally associated with other processes, such as connective tissue diseases or immunosuppression, while idiopathic LIP is rare. Among the IIPs, idiopathic pulmonary fibrosis (IPF) is the most common prototypic IIP. "Usual interstitial pneumonia" (UIP) is the pathologic pattern of lung injury seen in IPF. Categorization of major IIP and corresponding histological patterns defining each entity are shown in **Table 1** [2].

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*2.3.1. The importance of HRCT findings in the classification and diagnosis of interstitial lung disease*

The ATS/ERS/JRS/ALAT 2011 guidelines for IPF have assigned a primary diagnostic role to HRCT, and the HRCT criteria outlined was the same as those in the revised guideline published in 2015 [8–10]. The diagnosis of IPF should be based on the exclusion of other known causes of ILDs (environmental exposures, drugs, and CTDs) and presence of UIP on HRCT; UIP is characterized on HRCT by the presence of reticular abnormalities with subpleural and basal distribution, honeycombing with or without traction bronchiectasis, and absence of features inconsistent with UIP such as extensive ground-glass opacities, diffuse mosaic attenuation, profuse micronodules, and consolidations. Certain HRCT features predict histopathologic patterns of the different forms of ILD. **Table 2** outlines the classification of histological and corresponding radiological patterns defining each entity of IIP, which is also applicable

**3. Clinical landscape of interstitial lung disease in connective tissue** 

**Category Clinical–radiologic–pathologic diagnoses Associated radiologic and/or pathologic** 

Idiopathic pulmonary fibrosis (IPF) Usual interstitial pneumonia (UIP)

Cryptogenic organizing pneumonia (COP) Organizing pneumonia (OP) Acute interstitial pneumonia (AIP) Diffuse alveolar damage (DAD)

Desquamative interstitial pneumonia (DIP) Desquamative interstitial pneumonia (DIP)

**morphologic patterns**

Nonspecific interstitial pneumonia (NSIP)

Respiratory bronchiolitis-ILD (RB-ILD)

The connective tissue disease (CTD) is a systemic, inflammatory, autoimmune disorder characterized by immune-mediated multiple organ dysfunction. The category of CTD includes a

to CTD-associated ILD, to be explained later [5, 11].

(NSIP)

disease (RB-ILD)

Definition of abbreviation: IP = interstitial pneumonia.

Idiopathic nonspecific interstitial pneumonia

Respiratory bronchiolitis-interstitial lung

Desquamative interstitial pneumonia can occasionally occur in nonsmokers.

**disease**

Chronic fibrosing IP

IP\*

IP

\*

Smoking-related

Acute/subacute

Adapted from Travis et al. [2].

**Table 1.** Categorization of major IIP.

**Figure 1.** Classification of interstitial lung disease. Interstitial lung diseases (ILDs), also termed as diffuse parenchymal lung diseases (DPLDs), consist of disorders of known causes (connective tissue diseases, environmental, or drug related) as well as disorders of unknown cause. The latter include idiopathic interstitial pneumonias (IIPs), granulomatous lung disorders (e.g., sarcoidosis), and other forms of interstitial lung disease (ILD) including lymphangioleiomyomatosis (LAM), pulmonary Langerhans cell histiocytosis/histiocytosis X (PLCH), and eosinophilic pneumonia. The idiopathic interstitial pneumonias (IIPs) are further categorized as idiopathic pulmonary fibrosis (IPF), nonspecific interstitial pneumonia (NSIP), cryptogenic organizing pneumnia (COP), acute interstitial pneumonia (AIP), respiratory bronchiolitis interstitial lung disease (RB-ILD), or desquamative interstitial pneumonia (DIP). Lymphoid interstitial pneumonia (LIP) is occasionally associated with other disease processes, such as connective tissue diseases or immunosuppression while idiopathic LIP is rare.

may also involve the airspace, peripheral airways, and vessels, along with their respective epithelial and endothelial linings [1, 2].

As shown in **Figure 1**, the IIPs are further categorized into "idiopathic pulmonary fibrosis" (IPF), "nonspecific interstitial pneumonia" (NSIP: further subdivided into cellular and fibrotic NSIP), "cryptogenic organizing pneumonia" (COP), "acute interstitial pneumonia" (AIP), "respiratory bronchiolitis interstitial lung disease" (RB-ILD), or "desquamative interstitial pneumonia" (DIP) [1, 2]. "Lymphoid interstitial pneumonia" (LIP) is occasionally associated with other processes, such as connective tissue diseases or immunosuppression, while idiopathic LIP is rare. Among the IIPs, idiopathic pulmonary fibrosis (IPF) is the most common prototypic IIP. "Usual interstitial pneumonia" (UIP) is the pathologic pattern of lung injury seen in IPF. Categorization of major IIP and corresponding histological patterns defining each entity are shown in **Table 1** [2].

#### *2.3.1. The importance of HRCT findings in the classification and diagnosis of interstitial lung disease*

The ATS/ERS/JRS/ALAT 2011 guidelines for IPF have assigned a primary diagnostic role to HRCT, and the HRCT criteria outlined was the same as those in the revised guideline published in 2015 [8–10]. The diagnosis of IPF should be based on the exclusion of other known causes of ILDs (environmental exposures, drugs, and CTDs) and presence of UIP on HRCT; UIP is characterized on HRCT by the presence of reticular abnormalities with subpleural and basal distribution, honeycombing with or without traction bronchiectasis, and absence of features inconsistent with UIP such as extensive ground-glass opacities, diffuse mosaic attenuation, profuse micronodules, and consolidations. Certain HRCT features predict histopathologic patterns of the different forms of ILD. **Table 2** outlines the classification of histological and corresponding radiological patterns defining each entity of IIP, which is also applicable to CTD-associated ILD, to be explained later [5, 11].


Definition of abbreviation: IP = interstitial pneumonia.

\* Desquamative interstitial pneumonia can occasionally occur in nonsmokers.

Adapted from Travis et al. [2].

may also involve the airspace, peripheral airways, and vessels, along with their respective

**Figure 1.** Classification of interstitial lung disease. Interstitial lung diseases (ILDs), also termed as diffuse parenchymal lung diseases (DPLDs), consist of disorders of known causes (connective tissue diseases, environmental, or drug related) as well as disorders of unknown cause. The latter include idiopathic interstitial pneumonias (IIPs), granulomatous lung disorders (e.g., sarcoidosis), and other forms of interstitial lung disease (ILD) including lymphangioleiomyomatosis (LAM), pulmonary Langerhans cell histiocytosis/histiocytosis X (PLCH), and eosinophilic pneumonia. The idiopathic interstitial pneumonias (IIPs) are further categorized as idiopathic pulmonary fibrosis (IPF), nonspecific interstitial pneumonia (NSIP), cryptogenic organizing pneumnia (COP), acute interstitial pneumonia (AIP), respiratory bronchiolitis interstitial lung disease (RB-ILD), or desquamative interstitial pneumonia (DIP). Lymphoid interstitial pneumonia (LIP) is occasionally associated with other disease processes, such as connective tissue diseases or immunosuppression while

As shown in **Figure 1**, the IIPs are further categorized into "idiopathic pulmonary fibrosis" (IPF), "nonspecific interstitial pneumonia" (NSIP: further subdivided into cellular and fibrotic NSIP), "cryptogenic organizing pneumonia" (COP), "acute interstitial pneumonia" (AIP), "respiratory

epithelial and endothelial linings [1, 2].

idiopathic LIP is rare.

148 Contemporary Topics of Pneumonia

**Table 1.** Categorization of major IIP.
