Idiopathic pulmonary hemosiderosis: a review of the treatments used during the past 30 years and future directions

Idiopathic pulmonary haemosiderosis

In this paper, we report the case of a boy in early childhood who presented with iron-deficiency anaemia, initially thought to be nutritional, who had a subsequent diagnosis of idiopathic pulmonary haemosiderosis (IPH). This is a slowly progressive and life-threatening disorder and is of paramount importance that this is identified early and treated appropriately. His first chest CT was not typical for IPH, and this appearance should be highlighted (small cystic changes alone initially). He also had focal disease, which allowed us to make the diagnosis using CT-guided biopsy. During his treatment, he experienced an uncommon side effect to a commonly prescribed medication (bradycardia with methylprednisolone). Since starting azathioprine as a steroid-sparing agent, he has been doing well.

A Rare Presentation of Late-Onset Idiopathic Pulmonary Hemosiderosis: A Case Report

Idiopathic pulmonary hemosiderosis (IPH) is a rare cause of diffuse alveolar hemorrhage (DAH). It is associated with a high mortality rate and recurrent episodes of widespread alveolar hemorrhage and most commonly affects children. Here, we present a rare occurrence of late-onset idiopathic pulmonary hemosiderosis in a 74-year-old male. He was admitted for non-resolving pneumonia, hemoptysis, and type 1 respiratory failure, along with sideropenic anemia. Chest imaging showed bilateral upper lobe and right middle lobe alveolar opacities. Infective and autoimmune etiologies of diffuse alveolar hemorrhage were ruled out during the evaluation. Transbronchial lung biopsy showed patchy alveolar hemorrhage and abundant hemosiderin pigment deposition, revealing idiopathic pulmonary hemosiderosis. The patient was successfully treated with oral steroids, followed by complete radiological resolution without clinical relapse at one-year follow-up.

An Adult Case of Idiopathic Pulmonary Hemosiderosis Associated with Pulmonary Fibrosis and Emphysematous Change

A 48-year-old woman was admitted to our hospital with acute respiratory failure. Chest computed tomography showed ground-glass opacity and patchy emphysematous lesions in both lungs. Corticosteroid therapy was effective; however, the disease worsened with the tapering of corticosteroids. Bronchoalveolar lavage revealed hemosiderin-laden macrophages, and video-assisted thoracic surgery showed diffuse interstitial fibrosis with diffuse alveolar hemorrhage (DAH). There was no evidence of vasculitis nor autoimmune diseases. This patient was diagnosed with idiopathic pulmonary hemosiderosis (IPH) that progressed to end-stage pulmonary fibrosis despite treatment. Autopsy demonstrated DAH with pulmonary fibrosis and emphysematous change, suggesting IPH-related pulmonary lesions.

Juvenile idiopathic arthritis presenting as diffuse alveolar hemorrhage at onset: A case series

OBJECTIVE

The etiology of diffuse alveolar hemorrhage (DAH) in childhood is often unknown, and it may be an early manifestation of rheumatic disease. Juvenile idiopathic arthritis (JIA) is one of the most common rheumatic diseases in children, but DAH as an onset manifestation of JIA is rare. This study summarizes the clinical characteristics of patients with JIA presenting as DAH.

METHODS

We retrospectively analyzed the age of onset, clinical manifestations, imaging features, treatments, and prognosis of five cases of JIA presenting as DAH.

RESULTS

Themedian age at DAH onset was 6 months (range, 2 months-3 years). Pallor was the most common manifestation of onset (5/5). Other symptoms included cough (2/5), tachypnea (2/5), hemoptysis (1/5), cyanosis (1/5), and fatigue (1/5). Imaging showed ground-glass opacity (GGO) (5/5), subpleural or intrapulmonary honeycombing (4/5), consolidation (3/5), interlobular septal thickening (2/5), and nodules (1/5). Anticitrullinated protein antibodies (ACPA) and rheumatoid factor (RF) were positive in five children (5/5), and antinuclear antibody (ANA) was positive in four children (4/5). ANA in three children and ACPA/RF in one child were positive before the onset of joint symptoms. The median age at the onset of joint symptoms was 3 years and 9 months (2 years and 6 months-8 years). Joint symptoms were mainly characterized by joint swelling, pain, and difficulty walking, and the most commonly affected joints were the knees, ankles, and wrists. After the diagnosis of DAH, the five patients were treated with glucocorticoids. Alveolar hemorrhage was effectively controlled in three cases, but the other two patients still had anemia and poor improvements in chest imaging. After joint symptoms, the patients were treated with glucocorticoids combined with diclofenac, disease-modifying antirheumatic drugs, and biological agents. Alveolar hemorrhage was in remission, and joint symptoms were relieved in the five cases.

CONCLUSION

DAH can be the first clinical manifestation of JIA, and joint involvement occurs 1-5 years later. Children with DAH who are positive for RF, ACPA, and/or ANA and have GGO accompanied by honeycombing on imaging should be concerned about their joint involvement in future.

Idiopathic pulmonary haemosiderosis misdiagnosed as haemolytic anaemia: a case report

Idiopathic pulmonary haemosiderosis is a rare disease primarily affecting children. The condition is characterized by widespread bleeding from alveolar capillaries, resulting in symptoms such as haemoptysis, shortness of breath and iron deficiency anaemia. However, it is not a specific disease and sometimes can manifest solely as anaemia, which may be easily overlooked and misdiagnosed. The purpose of this case report was to describe a 1-year-old boy who exhibited haemolytic anaemia as the only symptom of idiopathic pulmonary haemosiderosis, with the intention of offering clinical insights into the precise diagnosis and subsequent management of this rare and easily misdiagnosed disease. Clinicians should keep idiopathic pulmonary haemosiderosis in mind when evaluating children with haemolytic anaemia and promptly initiate testing and treatment to prevent misdiagnosis and improve outcomes.

Pulmonary hemosiderosis secondary to mitral stenosis: a case report

Background

Pulmonary hemosiderosis is a rare disease that may be idiopathic or have a secondary etiology, such as mitral stenosis. The disease is a clinical and functional consequence of iron overload in the lungs in the form of hemosiderin. The diagnosis should be considered in patients with miliary nodules on chest tomography.

Case presentation

We report a case of a 30-year-old man with severe mitral stenosis who presented with chest pain and dyspnea. High-resolution chest tomography showed bilateral centrilobular micronodules. The diagnosis of pulmonary hemosiderosis secondary to mitral stenosis was suggested by transbronchial biopsy.

Conclusion

The diagnosis of idiopathic pulmonary hemosiderosis requires the exclusion of other etiologies of alveolar hemorrhage, including infections and vasculitis. One possible etiology is mitral stenosis.

Updates in idiopathic pulmonary hemosiderosis in 2022: A state of the art review

This manuscript reports the recent advances in idiopathic pulmonary hemosiderosis (IPH), a rare cause of diffuse alveolar hemorrhage in children and adults. This narrative review of the literature summarizes different aspects of IPH, including proposed pathogenesis, patient demographics, clinical and radiological characteristics, treatment, and prognosis. Additionally, the association between Celiac Disease (CD) and IPH is carefully evaluated. IPH is a frequently misdiagnosed disease. The delay in the diagnosis of IPH is often significant but fortunately, appears to have decreased in recent years. IPH in adults and children have distinct demographic preferences. The autoantibodies are common in IPH but with a definite difference between the adult and pediatric populations. The definitive diagnosis of IPH requires lung biopsy and careful exclusion of all competing diagnoses, even with lung biopsy showing bland pulmonary hemorrhage. The presence of nonspecific inflammatory cells or lymphoid aggregates may suggest a secondary immunologic phenomenon and needs careful evaluation and follow-up. A substantial number of patients suffer from coexisting CD, also known as Lane-Hamilton syndrome (LHS), and all patients with IPH need to be evaluated for LHS by serology. Although strict gluten free diet can manage the majority of patients with LHS, other patients generally require immunosuppressive therapy. The corticosteroids are the backbone of IPH therapy. Recently utilized experimental treatment options include mesenchymal stem cell transplant, liposteroid and bronchial artery embolization. The immunosuppression should be adjusted to achieve optimal disease control. Patients may progress to end-stage lung disease despite all measures, and lung transplantation may be the only viable option.

Rituximab in Idiopathic Pulmonary Hemosiderosis in Children: A Novel and Less Toxic Treatment Option

Idiopathic pulmonary hemosiderosis (IPH) is a rare, potentially life-threatening chronic disease. Steroids are the cornerstone of treatment, even though toxicity and side-effects are very common. Recently, rituximab (RTX) has been suggested as a treatment option, although evidence for its efficacy and long-term safety is lacking. We describe the disease course of two pediatric patients with IPH that were treated with RTX for over 4 years. Demographics, treatments, and clinical variables such as growth, infections, imaging follow-up by CT, and data from pulmonary function tests were retrospectively described. These are the first two cases described with a long-term follow-up of pediatric IPH patients treated with RTX. RTX was well-tolerated and prevented outbreaks of bleeding. In addition, RTX had a robust steroid-sparing effect resulting in the improvement of growth, pulmonary function, and CT abnormalities.

Adult patients with idiopathic pulmonary hemosiderosis: a comprehensive review of the literature

Idiopathic pulmonary hemosiderosis (IPH) is a rare disease without a known incidence or prevalence in adults. Our knowledge of this entity is limited as there is no prospective or retrospective study with a reasonable number of patients. The objective is to describe the demographics, clinical manifestations, diagnosis, treatment, and prognosis of adult patients with IPH. The Medline and Embase databases were searched from inception to 2021 with appropriate search formulas to identify relevant articles following strict inclusion and exclusion criteria. Statistical analyses were performed for the entire cohort and prespecified subgroups. A total of 84 patients were identified. The majority of patients were males 54/84 (64.3%). The median age was 27 years. The manifesting symptoms were present in the following frequencies: anemia 76/83 (91.6%), dyspnea 71/83 (85.5%), hemoptysis 70/84 (83.3%), cough 22/84 (26.2%), and chest pain 9/84 (10.7%). The classic triad was present in 61/84 (79%) patients. The mean hemoglobin during the initial presentation was 8.4 gm/dL. A total of 16/57 (19.5%) tested positive for autoantibodies. The median delay in the diagnosis of IPH was 1.02 years. Immunosuppressive therapy was prescribed in 49/79 (62%) patients, and recurrence occurred in more than half of the patients 36/66 (54.5%). A total of 63/79 (79.7%) patients were alive during the final follow-up. IPH is more common in young adults with a male predominance. A high index of suspicion is necessary to attain an early diagnosis and possibly reduce the short-term mortality of nearly 20% and long-term complications.

Association between idiopathic pulmonary hemosiderosis and celiac disease in pediatric patients: A scoping review of the literature over the past 50 years

INTRODUCTION

Idiopathic pulmonary hemosiderosis (IPH) is a rare cause of diffuse alveolar hemorrhage, the mechanism of which is currently unknown. Nearly one-third of pediatric patients with IPH test positive for Celiac disease (CD) serology. Several hypothetical mechanisms have been proposed to unify the coexistence of these two entities, also referred to as Lane-Hamilton syndrome (LHS).

METHOD

This manuscript is a scoping review of the medical literature. Medline, Embase, and PubMed Central databases were searched between 1971 and 2021 with appropriate search words to identify all cases of pediatric LHS.

RESULTS

A total of 20 manuscripts with 23 pediatric patients with LHS were identified. The mean age was 11 years, and 13/23 (56.5%) of the children were boys. Hemoptysis was present in 57% of patients during diagnosis. Bronchoscopy with bronchoalveolar lavage demonstrating hemosiderin laden macrophages was the primary mode of diagnostic confirmation. Only three patients underwent lung biopsy. Any significant GI symptom was reported in a minority of patients (22%). Iron deficiency anemia on presentation was described in 83% of children. The majority of patients were malnourished. Serology for CD was positive in all patients, as was the histopathologic analysis of the small bowel biopsy. No patients had any other autoantibody positivity. The introduction of gluten free diet (GFD) was associated with a positive response in 20/23 patients.

CONCLUSION

All pediatric patients with IPH should undergo screening for CD. Low serum ferritin in patients with IPH could be suggestive of coexisting CD. Strict GFD should be tried as the initial therapy.

Puzzling (IRIDA-Like and Hemolytic) Anemia in a Child With Idiopathic Pulmonary Hemosiderosis

Before the diagnosis of idiopathic pulmonary hemosiderosis (IPH), unexplained or puzzling anemia may precede and delay in the diagnosis of pediatric IPH is common. A 5.8 years old female child initiated with iron-refractory iron deficiency anemia-like iron deficiency and hemolytic anemia and at 6.8 years of age IPH was materialized, when the patient showed the triad signs of IPH with hemosiderin-laden alveolar macrophages in gastric aspirate. Although time to the diagnosis was previously reported to be ranged from 16 to 30 months, in our case it took 12 months from the initial anemia to IPH diagnosis.

The Spectrum of Autoantibodies in Adult Patients With Idiopathic Pulmonary Hemosiderosis: A Brief Review of the Literature

While autoimmune antibodies or autoantibodies have been reported sporadically in adult patients with idiopathic pulmonary hemosiderosis (IPH), their true prevalence is unknown. The question as to whether any difference exists between antibody-positive and negative patients has not been explored. The primary objective of this paper was to assess the spectrum of autoantibody testing and its positivity rate. The other objectives included a comparative analysis of demographics, symptom onset, clinical manifestations, and differences in clinical outcomes between antibody-positive (cohort A) and negative (cohort B) patients. To that end, we conducted a retrospective review of the relevant published literature. Multiple databases were searched to retrieve studies published between 1990 and 2022.

A total of 35 studies, involving 38 patients, were identified. Five of these patients had a positive autoantibody. Patients in cohort A were older and more likely to be male. The frequencies of testing for these antibodies were as follows: antineutrophil cytoplasmic antibody (ANCA): 37/38 (97.4%), antinuclear antibody (ANA): 31/38 (81.6%), and anti-glomerular basement membrane antibody (anti-GBM): 30/38 (78.9%); 5/38 (13.2%) patients tested positive for an autoantibody, and two of these patients were positive for ANA, two for antithyroid antibody, and one patient tested positive for ANCA, rheumatoid factor (RF), and granulocyte monocyte-colony stimulating factor (GM-CSF) antibody. There was no difference between the cohorts regarding their clinical presentations, recurrence risks, and survival.

The occurrence of autoantibodies is uncommon in adult IPH patients. This is in contrast with the pediatric IPH patient population, where the prevalence is much higher (26.4% vs. 13.2%), and the antibodies are more diverse. Unlike pediatric patients, adult patients with autoantibodies do not necessarily have worse outcomes.

Proposed Pathogenesis of Diffuse Alveolar Hemorrhage in Idiopathic Pulmonary Hemosiderosis

Idiopathic pulmonary hemosiderosis (IPH) is a rare disease that causes diffuse alveolar hemorrhage (DAH). The latest data suggests an immunologic origin of IPH, and a new name, immune mediated pulmonary hemosiderosis (ImPH), has been proposed. However, the exact immunologic mechanism has remained elusive for nearly eight decades despite extensive research, including detailed histopathologic analysis. Although several hypotheses have been proposed to describe the pathobiology of IPH, none of them explain the clinical and histopathologic findings conclusively. In this manuscript, we have presented a new hypothesis for the pathogenesis of DAH in IPH. We hypothesize that DAH in IPH is not immunocomplex mediated but due to histamine, eosinophilic cationic protein (ECP), and possibly vascular endothelial growth factor (VEGF). These bioactive proteins induce endothelial and alveolar epithelial damage, leading to the peri-capillary and intraalveolar escape of RBCs. The deformability of the RBC likely also plays a role. The supranormal secretion of histamine, ECP and VEGF occurs in genetically predisposed individuals with an aberrant immunologic response. The histamine is released from the basophils and possibly the mast cells in response to cytokines secreted by activated lymphocytes. The lymphocyte activation occurs after exposure to a known (gluten) or unknown antigen. The same lymphocyte-derived cytokines also induce eosinophilic degranulation of ECP and VEGF in the pulmonary circulation. We believe that our hypothesis unifies the observed clinical variabilities and histopathologic findings in IPH, and we hope that would promote future research in the field of IPH.

Comparative Analysis of Adult Patients With Idiopathic Pulmonary Hemosiderosis and Lane-Hamilton Syndrome: A Systematic Review of the Literature in the Period 1971-2022

Idiopathic pulmonary hemosiderosis (IPH) causes diffuse alveolar hemorrhage (DAH) by a yet unknown mechanism. The coexistence of IPH and celiac disease (CD), also known as Lane-Hamilton syndrome (LHS), has been reported in both pediatric and adult patients. The objective of this study was to compare demographics, clinical and radiologic findings, treatment, and outcomes between adult patients with IPH and LHS.

This is a systematic review of the literature. Multiple databases were searched using appropriate formulas to identify relevant articles. A total of 60 studies reporting 65 patients were included in the review. Forty-nine of these patients had IPH and 16 had LHS. The prevalence of anti-CD antibodies among tested patients was 13/22 (59%). The symptom onset and diagnosis of IPH occurred earlier in patients with LHS. The median delay in diagnosis was the same between the two groups (52 weeks). The classic triad was more likely to be present in patients with LHS. Only 20% of patients in the LHS cohort had any significant gastrointestinal (GI) symptoms at the time of IPH diagnosis. A gluten-free diet alone was effective in the majority of patients. Fewer patients in the LHS cohort received systemic corticosteroid than the IPH cohort. The recurrence and mortality in patients with LHS appear to be less than in the IPH cohort. The prevalence of CD is 25% in adult patients with IPH. Patients with LHS may have a milder course than patients without CD. Serologic testing for CD should be performed in all patients diagnosed with IPH.

Differentiation of idiopathic pulmonary hemosiderosis from rheumatologic and autoimmune diseases causing diffuse alveolar hemorrhage: establishing a diagnostic approach

This narrative review provides an overview of diffuse alveolar hemorrhage (DAH) associated with rheumatologic and autoimmune diseases and their differentiation from idiopathic pulmonary hemosiderosis (IPH). Relevant immunologic diseases associated with DAH are discussed, and a diagnostic flowchart is proposed to establish a "definitive" diagnosis of IPH within the spectrum of DAH. IPH is a rare cause of recurrent DAH both in children and adults. In adults, a definitive diagnosis of IPH requires a lung biopsy and histopathologic examination demonstrating intraalveolar hemorrhage, hemosiderin-laden macrophages, and a variable degree of fibrosis in the absence of both capillaritis and cellular inflammation. The presence of small vessel vasculitis points towards immunologic, well-differentiated, or sometimes undifferentiated rheumatologic diseases. However, it is essential to recognize that many rheumatologic diseases may in the initial phase present with DAH without any evidence of capillaritis, thus mimicking IPH. Although not definitely established, it is likely that immunologic processes are involved in IPH, and we, therefore, suggest the consideration of a more suitable term for the disease, e.g., "Immune-mediated Pulmonary Hemosiderosis" to acknowledge the aberrancy in the immune parameters and a positive response to immunosuppressive therapy.

Liposteroid Therapy for Idiopathic Pulmonary Hemosiderosis: A Scoping Review of the Literature

Idiopathic pulmonary hemosiderosis (IPH) is a rare cause of diffuse alveolar hemorrhage (DAH). Glucocorticosteroids (CS) represent the first line therapy for IPH. Although most patients respond to CS, steroid refractoriness is seen in an appreciable minority of patients. This paper reviews and evaluates the efficacy and safety profile of liposomal dexamethasone 21-palmitate (liposteroid) for the treatment of IPH. Medline, Embase and Web of Science biomedical databases were searched between 1980 and 2020 to identify papers describing patients with IPH, who were treated with liposteroid. A total of five articles were identified. Four in the form of case reports and one as a case series. A total of 12 pediatric patients (5 boys, 7 girls) were identified, with a median age of 2.3 years (range 0.5-8.6). Liposteroid therapy in intravenous doses ranging 0.06-0.1 mg/kg body weight appeared to be effective for both remission induction therapy, and maintenance therapy. There was no mortality among patients treated with liposteroid, either in the acute phase or during follow-up. The majority of patients for whom long-term follow-up data were available, were cured or in disease remission. No acute adverse events were reported, and long-term side effects were minimal and tolerable. Liposteroid represents a potential alternative or supplement to conventional CS therapy, as it appears to be more efficacious and associated with fewer side effects. Larger prospective, controlled trials are necessary to be able to define more precisely the therapeutic role of liposteroid in IPH.

Short Review of Liposteroid: A Novel Targeted Glucocorticoid Preparation for Treatment of Autoimmune and Inflammatory Diseases

This paper briefly reviews the safety and efficacy of liposteroid in different inflammatory and non-inflammatory diseases. Corticosteroids (CS) are the first-line therapy in many inflammatory and autoimmune disorders. Although highly efficacious, long-term use of CS is limited due to the occurrence of significant side effects. Liposteroid, which is a liposomal formulation of dexamethasone palmitate, possess more potent anti-inflammatory and immunosuppressive properties compared to dexamethasone sodium phosphate. These two formulations have markedly different lipid solubility, resulting in different pharmacokinetic and pharmacodynamic properties. Liposteroid has been used with success in patients with rheumatoid arthritis, macrophage activation syndrome, and idiopathic pulmonary hemosiderosis. In addition, liposteroid has been used in some non-inflammatory diseases. Moreover, we conceive that liposteroid may have a beneficial effect in patients, who are critically ill due to COVID-19, and suffer from the macrophage activation syndrome.

Diffuse alveolar haemorrhage with suspected idiopathic pulmonary hemosiderosis and decrease in lung diffusing capacity and chronic respiratory failure

Idiopathic pulmonary hemosiderosis (IPH) is a rare disease of unknown aetiology that causes recurrent episodes of diffuse alveolar haemorrhage (DAH). A male patient in his 50s had repeatedly experienced hemoptysis for the past 6 years, along with a decrease in the pulmonary diffusing capacity and chronic respiratory failure. After a 6-year follow-up, the patient experienced sudden exacerbation of hemoptysis and respiratory failure, and he was hospitalised. A CT of the chest revealed diffuse pulmonary infiltrates, whereas the bronchoalveolar lavage revealed hemosiderin-laden macrophages. Thus, the patient was diagnosed with DAH. As all diseases that cause DAH other than IPH were negative, the patient was suspected of IPH. He was treated with a combination of glucocorticoids and azathioprine, and his hemoptysis and chronic respiratory failure improved; however, the decrease in the pulmonary diffusing capacity did not improve. Treating adult-onset IPH with glucocorticoids and azathioprine might not improve pulmonary diffusing capacity.

Unilateral Diffuse Alveolar Hemorrhage Due to Selective Directionality of Mitral Regurgitant Jet in a Patient With Severe Aortic Stenosis

Diffuse alveolar hemorrhage (DAH) in cardiac diseases results from pulmonary capillary stress failure due to pulmonary venous hypertension. The most common cardiac causes of DAH are heart failure and mitral valvular disease. Patients typically manifest with hemoptysis, radiologic chest abnormalities, and anemia. The chest infiltrates are generally bilateral, similar to pulmonary edema. Rarely, the chest infiltrates can be unilateral, mimicking an infectious etiology. We present the case of an 88-year-old female with critical aortic stenosis, who presented with shortness of breath, unilateral right lung infiltrates, and mild leukocytosis. The patient was misdiagnosed with pneumonia as pulmonary edema or DAH was expected to be a bilateral finding on chest imaging. The patient deteriorated and DAH was eventually diagnosed by bronchoscopy.

Autoimmune pulmonary alveolar proteinosis and idiopathic pulmonary haemosiderosis: a dual pathology

Pulmonary alveolar proteinosis (PAP) is a rare pulmonary condition which leads to excessive accumulation of proteinaceous material within the alveoli. Idiopathic pulmonary haemosiderosis (IPH) is another orphan lung disease and results in recurrent alveolar haemorrhage. This case study describes a case of these two rare pathologies occurring together. A man in his 50s presented with a 6-week history of haemoptysis and worsening dyspnoea. A CT scan of the thorax showed multifocal, bilateral ground glass opacification with a wide differential diagnosis. Full autoantibody screen including myositis panel and coeliac screen were negative. Bronchoscopy with bronchoalveolar lavage and tissue from a transbronchial lung cryobiopsy were non-diagnostic. Tissue from a video-assisted thoracoscopic surgery biopsy confirmed a diagnosis of PAP with IPH as a second separate pathology. The association of IPH and PAP has not previously been described. We discuss these conditions and postulate how and if they may be related.

Diffuse Alveolar Hemorrhage in Cardiac Diseases

Diffuse alveolar hemorrhage (DAH) is a rare condition with reported mortality ranging between 20 and 100%. There are many etiologies of DAH. Cardiac diseases are likely underreported causes of DAH. Heart failure and mitral valve diseases are the most common cardiac causes of DAH. The DAH results from pulmonary venous hypertension leading to stress failure of the pulmonary capillaries. There is also a contribution of the bronchial circulation. The Alveolar-capillary membrane or blood-gas barrier is an extremely thin structure that allows rapid and passive diffusion of oxygen from the inhaled air to the pulmonary capillaries while preventing pulmonary edema and DAH with chronic elevation of the transmural hydrostatic pressure. The purpose of this manuscript is to inform the clinician about this rare cause of DAH, which may be overlooked unless specifically sought after. We also discuss the pathophysiologic aspects of DAH and the safety mechanisms in place to prevent such occurrences.