Figure 42.1. Schematic of the classification system for diffuse parenchymal lung diseases devised by the American Thoracic Society (ATS) and European Respiratory Society (ERS) joint committee.
Note: LAM = Lymphangioleiomyomatosis. Source: Adapted from American Thoracic Society/European Respiratory Society International Multidisciplinary consensus classification of the idiopathic interstitial pneumonias. Am J Respir Crit Care Med. 2002;165:277–304.
The IIPs are the most common group of ILDs encountered in the clinical setting. With that said, the practicing clinician should bear in mind that there has been a growing academic interest in the ILDs associated with collagen vascular diseases (ILD-CVD). A greater appreciation for the complexity of this relationship has grown in light of epidemiological data implicating underlying CVD in approximately 15% of ILD patients and the fact that a full third of all patients effected by rheumatoid arthritis will showcase complications of ILD. Research surrounding diagnostic biomarkers, novel immunosuppressive treatment regimens, and screening guidelines for patients with ILD-CVD is the subject of current investigative efforts.
THE IDIOPATHIC INTERSTITIAL PNEUMONIAS
The ATS/ERS consensus statement on IIP notes that this represents “a heterogeneous group of non-neoplastic disorders resulting from damage to the lung parenchyma by varying patterns of inflammation and fibrosis.” There are seven subtypes of IIP distinguishable by clinical, radiographic, and histological characteristics. All of the IIPs cause dyspnea and varying degrees of impairment in oxygen diffusion. Most will likely exhibit characteristic radiographic findings that reflect infiltrative changes to the lung interstitium. Some of the IIPs are stereotypically rapid in progression (i.e., acute interstitial pneumonia), while others have a more variable presentation (i.e., IPF, organizing pneumonia). Given this variability (and accompanying variable prognosis), establishing a firm diagnosis is perhaps the most important step in caring for patients afflicted with these diseases.
Table 42.1 presents common presentations and typical findings associated with each of the seven IIPs, depicted in descending order of prevalence. Though we have presented the IIPs as distinct entities, many of them can overlap in presentation and histopathologic manifestations. Diagnosis is further complicated in severe and/or end-stage presentations, as the terminal histological findings of all the IIPs is ultimately pulmonary fibrosis. A more robust discussion highlighting the differences in the two most common IIPs, IPF and nonspecific interstitial pneumonia (NSIP), appears later in this text.
EVALUATION AND DIAGNOSIS OF ILD
A complete clinical history is a key component in the evaluation of ILD. Particular attention should be paid to signs and symptoms of collagen vascular disease (dry eyes, dry mouth, arthralgias, and myalgias) and potential occupational and environmental exposures (employees in ship yards, sandblasters, agricultural workers). Classically, the physical examination will reveal clubbing and bilateral crackles typically described as “sounding like Velcro.” Patients with severe disease may also display elements of air trapping with audible “squeaks” and wheezing. Objective evaluation of functional status and oxygen saturation with a 6-minute walk test with pulse oximetry monitoring is an important prognostic tool in the evaluation of ILD.
Table 42.1 A COMPARISON OF THE IDIOPATHIC INTERSTITIAL PNEUMONIAS (IIPS)
Presented in this table is a summation of the clinical aspects of the seven known IIPs. Diseases are presented in descending order of commonality. Of note, perhaps with the exception of infectious etiologies that can cause organizing pneumonia, the histopathological diagnosis of all these diseases can only occur in the absence of active infectious agents. In general, but not uniformly, the diagnosis of one type of IIP includes the exclusion of other types of IIP.
Diagnostic evaluation of patients with a suspected ILD should include a complete set of PFTs. PFTs typically show a restrictive pattern, with symmetric decreases in forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) on spirometry. A diminished total lung capacity (TLC) on lung volume assessment confirms restrictive physiology, and a decreased diffusion capacity of carbon monoxide (Dlco) reflects destroyed lung architecture. Given that approximately 20% of Americans are either active or former smokers, it is important to realize that coexisting obstructive lung diseases can “offset” findings of a restrictive disease on PFTs. The diagnosis of combined pulmonary fibrosis and emphysema (CPFE) may portend an increased mortality risk on patients with pulmonary fibrosis. In such circumstances, attention should be drawn to the Dlco, which will demonstrate reduced values (generally <50% of predicted) profoundly out of proportion to other PFT findings (i.e., normal vital capacity).
Chest roentgenograms (CXRs) can demonstrate reticular and nodular patterns, though in less detail than computed tomography (CT); CXRs are notoriously insensitive and nonspecific. Therefore, patients suspected of having an IIP should have a high-resolution CT (HRCT) scan with prone and supine images ordered. HRCT provides thin (1 mm thickness) high-resolution images of the lung parenchyma. Radiocontrast is generally not needed in the evaluation of ILD unless there is a desire to simultaneously evaluate for thrombotic disease or mediastinal lymphadenopathy. As some IIPs (particularly IPF) can arise in the lung bases and may be mistaken for atelectasis on supine films, specialized prone protocols are recommended for evaluating these patients.
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