In the present study, FHP was the most frequent cause of a probable UIP pattern on HRCT. Considering only the patients without a history of environmental exposure, IPF was observed in less than 50% of the sample, with several ILDs diagnosed in the remaining subjects.
In 2011, the ATS proposed a tomographic classification for UIP in three categories.(1) In 2017, the Fleischner Society suggested splitting the possible group from ATS 2011 guidelines into the probable and indeterminate groups.(8) Probable UIP was characterized by the presence of a reticular pattern with peripheral traction bronchiectasis or bronchiolectasis in the absence of honeycombing. A diagnosis of IPF could confidently be made in a patient with a typical clinical context of IPF, with an HRCT pattern of probable UIP. (8)This statement was based on two papers: one had a recognized selection bias(3), and the other included patients with an undefined location of honeycombing on HRCT as a probable UIP pattern.(4)
Similar to the Fleischner Society, in 2018, the ATS/ERS/JRS/ALAT suggested a pattern of “probable UIP”, but the panel emphasized that the decision to perform SLB should be made in the context of an MDD by experienced clinicians.(9)
Criteria for the diagnosis of IPF have been discussed for a long time. Criteria for the diagnosis of FHP have been suggested more recently, but many differences persist.(24–26) Relevant environmental exposure, suggestive HRCT findings, and increased lymphocytes in bronchoalveolar lavage are important for diagnosis but are not helpful in many cases. Histopathologic findings can be typical, but these are absent in many cases of FHP.
In Brazil, HP is more common than IPF as a cause of ILD (24 vs 10%), as shown in a recent multicentre study.(7) However, 53% of all patients with ILD had potential exposure to HP, raising the question of whether other diagnoses can be excluded simply by the presence of environmental antigens. In the present study, only one patient without apparent exposure had a diagnosis of FHP, established by typical findings on biopsy. In contrast, in many studies, the causal agent for HP has not been identified.(25) Indeed, the prevalence of HP can be underestimated. In a study from Spain, almost half of the patients diagnosed with IPF on the basis of the 2011 criteria were subsequently diagnosed with FHP.(27)
Hypersensitivity pneumonitis can present with several histopathologic patterns.(18) In FHP, a typical pattern is characterized by chronic fibrosing pneumonia, airway-centred fibrosis and poorly formed non-necrotizing granulomas. Fibrosing interstitial pneumonia is characterized by architectural distortion; FF and subpleural honeycombing can be present. In airway-centred fibrosis or bronchiolocentric fibrosis, there is also extensive peribronchiolar metaplasia and, in many cases, bridging fibrosis. In FHP, the presence of granulomas and giant cells is uncommon. Another possible presentation for FHP is a pattern of nonspecific interstitial pneumonia.
In the present study, six patients were diagnosed with ILD ascribed to GERD. In these patients, the histological pattern was BF, there was no environmental exposure to HP, and GERD was substantiated. In our opinion, the role of GERD as an aetiological factor of fibrosing ILD, not UIP, has been neglected. (14)
In two patients in the present series with BF, FHP and ILD ascribed to GERD were possible causes. In our opinion, these patients cannot be discriminated by biopsy. In the other two patients, a possible cause for BF was not determined.
Patients with nonspecific interstitial pneumonia were not observed in the present series, probably due to a high frequency of findings inconsistent with UIP on HRCT.
In the present study, five patients had familial ILD. On SLB, three displayed BF with FF and/or microscopic honeycombing, one had a typical PH pattern, and one had an isolated UIP pattern. In a study of 30 patients with familial ILD, diagnostic features of UIP were observed in less than 50% of the samples, but FF was observed in 87%, and multifocal bronchiolocentric fibrosis was observed in 37%.(20) Familial interstitial lung disease presents in many different ways on HRCT and in lung biopsies.(12, 28) Genetic factors can predispose patients to several ILDs, and diverse pathologic expressions can be found in the same family.(12, 29) The indications for SLB in familial ILD remain controversial. (30, 31)
The likelihood of a histopathological UIP pattern in patients with a probable UIP pattern on HRCT remains undefined.
In a recent paper from Japan, the prevalence of a histopathological UIP pattern was 83% (90 of 109) in patients with a probable UIP pattern on HRCT, but the diagnosis of IPF after the MDD was made in only 66%. (30) The median survival time was 72.1 months for patients with the probable UIP pattern, very similar to that found in our series. This survival time is longer than that found in patients with IPF and suggests that a heterogeneous number of diagnoses are included in the group with a probable UIP pattern. In this study, different clinical diagnoses were not the objective and were not explored. (32)
In the present study, the degree of dyspnoea, the presence of honeycombing not in the lower lobes irrespective of diagnosis, and the presence of FF on SLB were predictive of poor survival. The presence of honeycombing on HRCT in an ILD other than IPF is a predictor of a poor prognosis, but in most cases, honeycombing is present in the lower lobes. (33) Even with a limited sample, we found that honeycombing outside of the lower lobes was associated with poor survival, irrespective of the final diagnosis.
Fibroblastic foci are a major histological feature of UIP in SLB but can be present in other conditions. It has been recognized for a long time that the presence and extent of FF predicts poor survival in patients with IPF as well as in those with FHP. (15, 19, 34, 35)
In the INPULSIS trial, which evaluated the efficacy and safety of nintedanib in the treatment of IPF, subjects enrolled with the possible UIP pattern and traction bronchiectasis showed similar disease progression and treatment responsiveness to subjects enrolled with the IPF HRCT pattern. (36) This was used as an argument supporting these cases as IPF.
The INBUILD trial was a randomized, double-blind, multicentre, parallel group trial performed in 663 patients with a progressive fibrosing ILD other than IPF.(37, 38) Chronic hypersensitivity pneumonitis was diagnosed in 173 patients (26%). Participants were randomly assigned to receive 150 mg nintedanib twice daily or placebo for at least 52 weeks. Nintedanib reduced the rate of ILD progression, as measured by a decline in the FVC, irrespective of the underlying ILD diagnosis. In patients with a UIP-like fibrotic pattern, the adjusted rate of decline in the FVC over the 52-week period was more conspicuous. (38)
These studies raise the question of whether the diagnosis of ILD fibrosing really matters. In FHP, if the antigen is not identified, the prognosis is worse. (39) Otherwise, identification and antigen avoidance, even in FHP, can result in a prolonged survival time. (40) A subset of patients with FHP experience a progressive disease course, even after antigen avoidance, and these patients can be treated with pharmacologic agents, including antifibrotic drugs if necessary.
In patients with fibrotic ILD of indeterminate aetiology, we now indicate transbronchial lung cryobiopsy before entertaining SLB. (41)
Some authors suggest that older age, male se and smoking could increase the positive predictive value for IPF diagnostics in patients with a possible UIP pattern on HRCT.(2) In the present study, these data were not helpful.
Several limitations to our study should be noted. The sample size was relatively small. This was a retrospective study, and selection bias should be considered. All the patients were reviewed by two expert pathologists in ILD, but the concordance was not evaluated. HRCT in expiration was not performed in all patients, so air trapping was not evaluated; however, this finding has a lower predicted value for separating FHP from IPF in comparison to the mosaic pattern.(42) Treatment was not standardized, making it difficult to evaluate its effect on survival.