Is It Idiopathic Pulmonary Fibrosis or Not?

Mary Salvatore, MD; Genta Ishikawa, MD; Maria Padilla, MD

Disclosures

J Am Board Fam Med. 2018;31(1):151-162. 

In This Article

Mimicker No. 2: CHP

Clinical

CHP is an interstitial lung disease in genetically predisposed individuals caused by an exaggerated immune response to chronic inhalation of a variety of antigens in the environment (fungal, bacterial, protozoal, and animal proteins, or low-molecular-weight chemical compounds, etc.). With long-term inflammation, CHP with progressive fibrosis and bronchiolitis obliterans may develop and fibrosis is often characterized by honeycombing so that in late chronic stages, histopathology may be similar to UIP or fibrotic NSIP pattern. Therefore, CHP is an important mimicker of UIP/IPF.[34–39] CHP frequently occurs in the elderly female population. However, the accurate prevalence or incidence of CHP is difficult to evaluate given that the disease is oftentimes unrecognized or misdiagnosed and exposure conditions vary in intensity of exposure (usually low) and from place to place, and country to country. The onset of disease is insidious with gradually increasing dyspnea on exertion, dry cough, fatigue, and weight loss. Given that patients seldom relate their symptoms to the environmental exposure and the onset of respiratory symptoms is gradual, physicians often misdiagnose the disease for another interstitial lung disease such as UIP/IPF. Diagnosis should be suspected in every patient with insidious respiratory symptoms A careful history regarding the occupational/domestic environment and hobbies is crucial (ie, bird keeping, hay feeding, feather duvet and pillows at home, air conditioning, contaminated ventilators in the buildings, and formation of mold on room walls or in brake fluid oils, or within wind instruments). Proof of sensitization (serum precipitins) and demonstration of a consistent pattern of this ILD on HRCT (discussed in the following "Radiology" section) support the accurate diagnosis. In contrast, pathologic diagnosis is not often required, but when obtained may demonstrate features associated with this disease even in cases with a UIP pattern (bonchocentric inflammation, lymphohystiocitic cell and poorly formed granulomas). Invasive lung biopsy may be avoided with careful history taking and radiologic evaluation by HRCT.

The prognosis of CHP varies among patients and depends on the duration of exposure to the inhaled antigen. Despite a favorable prognosis in the acute and subacute forms, CHP may become a progressive fibrotic lung disorder that results in respiratory failure even after avoiding the presumed antigen and the institution of therapy. In addition to avoidance to further antigen exposure, steroid therapy is usually recommended if patients show progressive functional impairment and immunosuppressants can be added as steroid-sparing agents.[35]

Radiology

Hypersensitivity pneumonitis can be divided into acute, subacute, and chronic forms. The chronic form is most likely to mimic UIP radiographically. CHP is frequently misdiagnosed as IPF, which is particularly distressing because early recognition of disease and removal from antigen allows for cure.[43] CHP is different from UIP radiographically mainly because instead of being peripheral, it is an airway-centered disease, which makes sense given the way in which it is acquired. Second, CHP is upper-lobe predominant and unlike UIP, which is lower-lobe predominant (Figure 6). Third, CHP frequently demonstrates air trapping on expiratory CT images.[41] Air trapping is infrequently associated with a UIP pattern and when present a diagnosis of rheumatoid arthritis should be considered.[42]

Figure 6.

Chronic hypersensitivity pneumonitis (CHP) is upper-lobe predominant, airway centered, and frequently has air trapping. The most useful feature radiographically is its airway-centered distribution seen on this image.

Differentiation Pearl

  • CHP is heterogeneous like UIP but its bronchovascular distribution makes it uniquely different from UIP. Air trapping is also important in making the correct diagnosis of CHP and is rarely seen in association with UIP.

  • Emphysema is frequently seen in association with UIP but rarely with CHP.

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