Fibrosing interstitial lung diseases (ILDs) have significant morbidity and poor overall survival as the investigation of earlier stages of fibrosing ILDs may identify modifiable risk factors and elucidate its pathogenesis. Interstitial lung disease (ILD) constitutes a group of chronic respiratory illnesses that are characterized by repetitive injury to the lung parenchyma that may progress to fibrosis. Fibrosing ILDs, which include idiopathic pulmonary fibrosis (IPF) and other ILD types, can lead to chronic respiratory failure, and poor survival, is the most common indication for lung transplantation. While antifibrotics slow disease progression and may reduce mortality, they carry side effects and are frequently poorly tolerated by patients. Therefore, there is a critical need to identify novel therapeutic targets. Patients often present with a significant burden of fibrosis upon diagnosis as there is interest in identifying these individuals earlier in their disease course (i.e., "subclinical disease") where targeted treatments and modification of risk factors may curb their progression to fulminant fibrosing ILD.

Hiatal hernia and its promotion of gastroesophageal reflux have been implicated as a causal risk factor in pulmonary fibrosis. One potential modifiable risk factor is hiatal hernia and its promotion of gastroesophageal reflux (GER). Gastric materials in the lung induce parenchymal injury and fibrosis in pre-clinical models as GER may be a vehicle of repetitive lung injury that is then followed by aberrant wound healing and eventual fibrosis. Hiatal hernia, which is the protrusion of stomach contents in the thoracic cavity, can promote GER and is more prevalent in adults with pulmonary fibrosis compared with healthy controls and other chronic lung diseases. Further evidence comes from studies in which patients with IPF and other fibrosing ILDs (many of whom had a hiatal hernia) demonstrated elevated levels of pepsin in their bronchoalveolar lavage fluid (BALF), a significant component of reflux and measurable biomarker, compared with healthy controls and is associated with a higher risk of exacerbation. Much of these studies have been case-control and performed in adults with clinically-diagnosed fibrosing ILD which limits causal inferences. It has been hypothesized that fibrosis itself may promote GER due to impaired esophageal sphincter function due to reduced elasticity and more negative intrathoracic pressures. Whether hiatal hernia contributes to subclinical repetitive injury to the lung among adults without clinically-diagnosed ILD is a significant knowledge gap.

Overactivation of the mononuclear phagocyte immune system may contribute to developing fibrosing ILDs and their progression. Single-cell RNA sequencing (scRNA-seq) has accelerated our understanding of fibrosing ILD by identifying biological pathways related to certain cellular populations. Using scRNA-seq, our group and others have demonstrated that overactivation of the mononuclear phagocyte system, a key component of the innate immune system, may be critical in the pathogenesis of IPF with higher populations of monocytes found in the lungs and blood of patients with fibrosis. Higher levels of the absolute monocyte count in the blood are associated with disease progression and worse survival among adults with IPF. The investigators have extended these findings to earlier stages of ILD as higher monocyte counts are associated with more ILA and its progression on CT and a lower forced vital capacity. Notably, adults with ILA have more activated monocytes than those without ILA, suggesting a smoldering activation of innate immunity may prime an individual to developing ILD in combination with other risk factors (i.e., smoking, genetic variants, and hiatal hernia).

Radiologically-detected hiatal hernia is associated with a greater burden of CT interstitial lung abnormalities and worse survival among community-dwelling adults. In a population cohort of U.S. community-dwelling adults, investigators found that the presence of hiatal hernia on CT was associated with more HAAs and their progression over time and more ILA among younger adults. Notably, elevated HAAs on CT were associated with worse survival among those with a hiatal hernia compared with those without a hernia. Our findings suggest that hiatal hernia and its promotion of GER may contribute to subclinical injury and remodeling detected by imaging biomarkers. Whether subclinical abnormalities are detected in the lungs of adults with hiatal hernia remains a major knowledge gap that this study will address by examining BALF contents of adults with hiatal hernia.