Atypical Pulmonary Hypertension in the Newborns

The differential diagnosis for atypical PH is broad and hinges on identifying underlying developmental or genetic anomalies. Key considerations include:

• Developmental Lung Diseases (DEVLD): These include conditions associated with high degree of mortality such as alveolar capillary dysplasia (ACD), acinar dysplasia, and congenital alveolar dysplasia.

• Genetic Mutations: Variants in genes such as FOXF1 (associated with ACD), TBX4, and surfactant protein deficiencies (SFTPB, SFTPC, ABCA3) are frequently implicated.

• Structural and Vascular Malformations: Conditions such as pulmonary vein stenosis (PVS), pulmonary veno-occlusive disease (PVOD), and congenital diaphragmatic hernia (CDH) can masquerade as or complicate persistent PH.

• Systemic and Syndromic Conditions: Fetal renal disease, Trisomy 21 (Down Syndrome), and congenital portosystemic shunts are also known causes of an atypical, non-resolving PH course.

Developmental Lung Diseases

• Alveolar capillary dysplasia, often (but not always) occurring with misalignment of pulmonary veins (ACDMPV), is a rare and typically lethal congenital disorder characterized by fulminant pulmonary hypertension and severe respiratory failure shortly after birth . It is most frequently associated with deletions or loss-of-function mutations in the FOXF1 gene on chromosome 16q24.1 (or transcription factors related to FOXF1, as well as rarely mosaicism within the lungs - see articles below as well as here). Pathologically, the disease is defined by a reduction in the number of alveolar capillaries, thickening of the interalveolar septa, and the presence of pulmonary veins running alongside pulmonary arteries within the bronchovascular bundles . Clinically, affected neonates often present with refractory hypoxemia that fails to respond to aggressive therapies such as inhaled nitric oxide (iNO) or extracorporeal membrane oxygenation (ECMO). 

• Acinar dysplasia represents a severe arrest of lung development at the pseudoglandular stage, resulting in a profound lack of distal airspace formation. Infants with this condition typically present on the first day of life with severe respiratory failure, small lung volumes, and a high incidence of air leaks such as pneumothorax. Histologically, the lung tissue shows airway profiles surrounded by abundant mesenchyme with virtually no primitive airspaces or alveoli. This condition has been strongly linked to genetic variants or deletions involving the TBX4 and FGF10 genes, which are essential for normal lung branching and mesenchymal development. 

• Congenital alveolar dysplasia (CAD) is characterized by a developmental arrest at the canalicular stage, leading to a simplified lung structure with a significant deficiency in alveolar growth. Similar to other lethal DEVLDs, it presents in the neonatal period with severe respiratory distress and is frequently accompanied by pulmonary hypertension. Pathological findings include widened alveolar septa and simplified, often cystically dilated, distal airspaces that lack the complexity of normal alveolarization. Research indicates that CAD is often part of a spectrum of disorders caused by disruptions in the TBX4-FGF10-FGFR2 signaling pathway, which is critical for early human lung organogenesis.

Genetic Mutations

• FOXF1 Mutations: FOXF1 encodes a highly conserved transcription factor that is essential for angiogenesis and vasculogenesis in the human lung. Deletions or loss-of-function mutations in this gene on chromosome 16q24.1 are most frequently associated with Alveolar Capillary Dysplasia with Misalignment of Pulmonary Veins (ACDMPV). This condition typically presents as fulminant pulmonary hypertension shortly after birth, characterized by severe respiratory failure and refractory hypoxemia. While most cases are lethal in the neonatal period, research indicates that certain hypomorphic variants can lead to a less severe phenotype or even late-onset pulmonary arterial hypertension in older children and adults.

• TBX4 Variants: Variants in the TBX4 gene are recognized as a major cause of both lethal developmental lung diseases and heritable pulmonary arterial hypertension. TBX4 is a transcription factor critical for lung branching during embryogenesis, and its reduction can lead to a severe arrest in distal lung development, such as acinar dysplasia. Clinically, many infants with TBX4 variants present with a biphasic course, where initial neonatal hypoxic respiratory failure seems to improve, only to be followed by the development of severe, chronic PH later in infancy or childhood. These variants are enriched in pediatric populations and are currently considered the second most common genetic cause of PAH in children after BMPR2.

• Surfactant Protein Deficiencies (SFTPB, SFTPC, ABCA3): Genetic mutations affecting the production and function of pulmonary surfactant, specifically in the SFTPB, SFTPC, and ABCA3 genes, are a well-documented cause of atypical neonatal PH. These mutations lead to diverse forms of interstitial lung disease (ILD), such as pulmonary alveolar proteinosis and chronic pneumonitis of infancy. Affected neonates often present with non-specific diffuse pulmonary opacities on imaging and respiratory failure that is poorly responsive to conventional therapies. Identifying these genetic variants is vital for clinical decision-making, as they often indicate a non-resolving disease course that may ultimately require lung transplantation.

Structural and Vascular Malformations

• Pulmonary Vein Stenosis (PVS): Pulmonary vein stenosis is a serious condition that can act as both a comorbidity and a masquerader of developmental lung diseases. It involves the narrowing of the pulmonary veins, leading to increased post-capillary pressure and secondary PH. PVS is frequently identified in ex-premature infants with bronchopulmonary dysplasia (BPD), occurring in nearly 40% of pediatric PVS cases. Infants may present with recurrent pulmonary edema, increasing oxygen requirements, and clinical instability that fails to improve with standard PH treatments.

• Pulmonary Veno-Occlusive Disease (PVOD): Pulmonary veno-occlusive disease is characterized by the progressive fibrous occlusion of small pulmonary veins, leading to severe, often fatal, PH. A clinical "red flag" for PVOD in neonates is clinical deterioration and the development of pulmonary edema following the administration of inhaled nitric oxide (iNO). This occurs because iNO preferentially dilates the pulmonary arterioles, increasing blood flow into a venous system that is obstructed, thereby forcing fluid into the airspaces. PVOD is strongly associated with biallelic mutations in the EIF2AK4 gene, and lung transplantation is currently the only curative treatment.

• Congenital Diaphragmatic Hernia (CDH): Congenital diaphragmatic hernia is a major cause of neonatal PH characterized by an arrest in both lung parenchymal and vascular development. The presence of abdominal organs in the thoracic cavity during fetal life results in pulmonary hypoplasia, meaning the lungs have fewer and smaller alveoli and blood vessels. This simplified and hypoplastic pulmonary circulation is unable to handle the full cardiac output after birth, leading to elevated pulmonary vascular resistance. While PH in CDH is a common part of the disease course, it is often refractory to therapy and remains a significant cause of mortality in this population.

Systemic and Syndromic Conditions

• Fetal Renal Disease: Conditions that affect renal development in utero, such as fetal renal disease or bilateral renal tubular dysgenesis, are known causes of fetal developmental PPHN. These renal anomalies often result in oligohydramnios (low amniotic fluid), which provides insufficient space for the lungs to grow, leading to secondary pulmonary hypoplasia. Neonates with renal-associated PH typically present with severe respiratory distress at birth that does not follow the typical resolving course of perinatal PPHN and has a very high mortality rate.

• Trisomy 21 (Down Syndrome): Infants with Trisomy 21 are at a significantly higher risk for an atypical, non-resolving PH course due to a combination of factors. They often exhibit an arrest in alveolarization and pulmonary vascular development, predisposing them to persistent PH. While PPHN in Down Syndrome may initially appear transient, these infants have a higher potential for recurrence and the development of chronic pulmonary vascular disease beyond the neonatal period. Furthermore, Trisomy 21 is a common genetic modifier that can increase the severity of PH when other comorbidities, such as congenital heart defects, are present.

• Congenital Portosystemic Shunts: Congenital portosystemic shunts, such as those seen in agenesis of the ductus venosus (ADV), are rare but significant causes of severe pulmonary arterial hypertension. These shunts allow portal blood to bypass the liver and enter the systemic circulation directly, which can lead to vascular remodeling in the lungs. While PH may present as an early complication in newborns, it can also manifest as late-onset severe PAH in older children who were previously stable. Continued surveillance using echocardiography and MRI is often recommended for these patients due to the high mortality associated with shunt-related PAH.

Vascular Shunts and Malformations

• Vein of Galen Malformation (VGM): Vein of Galen malformation is a critical diagnosis to consider in neonates with severe, refractory pulmonary hypertension that presents immediately after birth. This is a rare intracranial arteriovenous malformation that creates a high-flow, low-resistance shunt, forcing a massive volume of blood back to the right side of the heart. This volume overload leads to severe pulmonary hypertension and high-output heart failure. In clinical practice, these infants are often identified through head ultrasound when standard respiratory treatments fail to stabilize their hemodynamics. Unlike typical PPHN, the PH in severe VGM is often "flow-driven" (with a pseudo-coarctation physiology) and will not resolve until the underlying vascular shunt is addressed.

• Pulmonary Capillary Hemangiomatosis (PCH): Pulmonary capillary hemangiomatosis is a rare condition characterized by the patchy, disorganized proliferation of capillaries within the alveolar walls. While it was once considered a separate entity, modern classifications (such as the 6th World Symposium on Pulmonary Hypertension) now group it with Pulmonary Veno-Occlusive Disease (PVOD) due to their often identical genetic substrate—specifically mutations in the EIF2AK4 gene—and indistinguishable clinical presentations. Neonates with PCH/PVOD are at an extremely high risk of developing lethal pulmonary edema if treated with standard vasodilators like inhaled nitric oxide, because these drugs increase blood flow into a capillary bed that cannot effectively drain. Some of these infants may also have PCH reactive to underlying pulmonary arterial disease with maldevelopment or discoorniated development of compensatory pulmonary vessels. Sometimes described as a "vascular cancer" of the lungs.  

Additional Developmental and Interstitial Lung Diseases

• Bronchopulmonary Dysplasia (BPD)-Associated PH: While Bronchopulmonary Dysplasia is a common consequence of preterm birth, it is a cause of multi-factorial PH that can persist well into childhood. It is characterized by an arrest in both alveolar and vascular lung growth, often initiated by prenatal factors like maternal pre-eclampsia. PH in BPD infants is multifactorial, frequently complicated by chronic hypoxia, hypercarbia, and sometimes pulmonary vein stenosis. Because BPD represents a failure of the "lung-vascular unit" to mature, the associated PH requires a multidisciplinary approach focusing on long-term nutritional support and aggressive lung protection rather than just acute vasodilation.

• Pulmonary Interstitial Glycogenesis (PIG) and Filamin A: Other rare forms of childhood interstitial lung disease (chILD) can present as atypical neonatal PH. Pulmonary interstitial glycogenesis is a specific condition where cells within the lung interstitium contain excessive glycogen, leading to respiratory failure; it is often a definitive diagnosis made through lung biopsy. Additionally, mutations in the Filamin A gene can lead to a specific form of lung disease characterized by marked upper lobe hyperexpansion and hyperlucency, which mimics emphysema on CT imaging. These conditions should be addressed in a differential diagnosis for neonates who require oxygen and support disproportionate to their clinical imaging.

Environmental and Maternal Factors

• Prenatal Maladaptation and In-Utero Shunt Closure: Atypical PH can also result from environmental insults that occur while the fetal vasculature is still maturing. Maternal conditions such as pre-eclampsia, chorioamnionitis, or the ingestion of nonsteroidal anti-inflammatory drugs (NSAIDs) can trigger pathological changes in the fetus. Specifically, maternal NSAID use is associated with the premature closure of the ductus arteriosus. When these fetal shunts close too early, the right heart is forced to pump against high resistance before the lungs are even inflated, leading to chronic vascular remodeling and "fetal origins" of severe PH that is present the moment the infant is born.

Systemic and Rare Syndromic Associations

• Extrahepatic Portal Hypertension and Abernethy Syndrome: Systemic conditions outside the heart and lungs can also cause atypical PH in newborns. Pediatric portal hypertension—which may occur secondary to portal vein thrombosis following the placement of an umbilical line—is a frequently overlooked cause. Furthermore, congenital portosystemic shunts, such as Abernethy Syndrome, allow blood to bypass the liver and enter systemic circulation directly, potentially carrying toxic substances that trigger pulmonary vascular remodeling. These patients may have normal liver function tests, making a high index of clinical suspicion and abdominal ultrasound necessary for diagnosis.

• Multiple Congenital Malformation Syndromes: Atypical PH is frequently a component of complex malformation syndromes. In addition to Trisomy 21, clinicians should look for signs of CHARGE syndrome, VACTERL association, Alagille and DiGeorge syndrome. These infants often suffer from PH due to a "multifactorial" combination of factors, including cardiac shunts that progress more rapidly than usual and underlying developmental anomalies in the airway and pulmonary vessels. Addessing these syndromic diagnoses is vital because they often change the prognosis and the goals of care for the infant.

Metabolic and endocrine disorders 

• Metabolic and endocrine disorders represent a rare but important subset of conditions that can lead to atypical pulmonary hypertension (PH) in neonates and children. These are typically classified under Group 5 PH (pulmonary hypertension with unclear and/or multifactorial mechanisms) because their pathogenesis often involves a complex mix of vascular remodeling, mechanical obstruction, and biochemical triggers.

• Glycogen Storage Disease (GSD): Glycogen storage diseases, particularly Types 1 and 2, are well-documented causes of severe pulmonary hypertension. In GSD Type 1, the development of PH is thought to be partly driven by the release of vasoconstrictive amines, such as serotonin, which trigger pathological changes in the pulmonary vessels. Cases of severe PH in children with GSD Type 1 have shown some response to oral vasodilators like sildenafil, though the underlying metabolic instability remains a primary challenge.

• Gaucher Disease: Neonates or infants with Gaucher disease may develop PH through a unique multifactorial process. The primary mechanism involves the plugging of the pulmonary vasculature by abnormal, lipid-laden macrophages (Gaucher cells), which is further complicated by asplenia and progressive vascular remodeling. Notably, enzyme-replacement therapy has been shown to improve pulmonary hemodynamics in some patients by reducing the systemic burden of these abnormal cells.

• Nonketotic Hyperglycinemia: Nonketotic hyperglycinemia is an inherited metabolic disorder that has been specifically linked to the development of pulmonary hypertension in early life. It can manifest as part of a severe clinical picture known as progressive vacuolating glycine leukoencephalopathy, where the metabolic derangement coincides with significant pulmonary vascular disease.

• Mitochondrial and Storage Disorders: Diverse mitochondrial disorders and mitochondrial depletion syndromes have been identified in pediatric patients presenting with "idiopathic" pulmonary hypertension. Additionally, lysosomal storage diseases such as Mucopolysaccharidosis (including Hunter syndrome) are known to cause atypical, non-resolving PH courses in children, often requiring long-term surveillance.

• Metabolic-Related Thromboembolic PH: Some metabolic conditions do not affect the lung vessels directly but instead create a pro-thrombotic state. For example, methylmalonic acidemia and homocystinuria can lead to PH by predisposing the infant to chronic pulmonary thromboemboli, which eventually obstruct the pulmonary circulation.

• Treatment-Induced PH (Diazoxide): A critical consideration for your website is the risk of drug-induced PH when treating metabolic conditions. Diazoxide, a common medication used to manage hyperinsulinemic hypoglycemia in neonates, has been identified as a specific trigger for severe pulmonary hypertension. This side effect is a major "red flag" in the NICU; infants treated with diazoxide for metabolic issues must be closely monitored for the development of respiratory distress and "out of proportion" PH.

• Systemic and Syndromic Metabolic Issues: Metabolic complications are often seen as part of larger syndromes that present with atypical PH. For instance, infants with omphalocele or Beckwith-Wiedemann syndrome frequently suffer from hyperinsulinism alongside structural lung hypoplasia, creating a highly complex, atypical clinical course that requires interdisciplinary management. Integrating metabolic screening into the workup of any infant whose PH persists beyond the standard 7-to-10-day window is essential for identifying these rare but potentially treatable underlying causes.

Thyroid anomalies

Thyroid function disorders are recognized as significant contributors to atypical and multifactorial pulmonary hypertension (PH) in pediatric populations. These conditions are typically classified under Panama Category 10 (Pediatric pulmonary vascular disease associated with other system disorders) or as Group 5 PH due to their unclear or multifactorial mechanisms. The following are the key ways thyroid conditions intersect with pulmonary hypertension:

• Brain-Lung-Thyroid Syndrome: A critical genetic link exists between the thyroid and the lungs via mutations in the NKX2-1 gene (also known as Thyroid Transcription Factor-1 or TTF-1). This transcription factor is essential for the regulation and development of both the thyroid gland and the pulmonary surfactant system. Mutations in NKX2-1 lead to a clinical triad known as Brain-Lung-Thyroid Syndrome, characterized by congenital hypothyroidism, movement disorders (chorea), and severe interstitial lung disease that often presents as atypical neonatal PH. Because NKX2-1 regulates surfactant proteins, these infants often suffer from surfactant protein abnormalities that lead to non-resolving respiratory failure.

• Both hypothyroidism and hyperthyroidism are listed as recognized endocrine causes of pediatric PH. Specifically:

• Hyperthyroidism: This condition can lead to what is termed "unclassified PH," where an elevation in mean pulmonary artery pressure (mPAP) is primarily driven by elevated pulmonary blood flow rather than high vascular resistance alone.

• Hashimoto’s Thyroiditis: While more common in adults, this autoimmune thyroid condition has been linked with pulmonary veno-occlusive disease (PVOD), a rare and severe form of PH that involves progressive occlusion of the small pulmonary veins.

• Because of the strong association between thyroid dysfunction and severe pulmonary vascular disease, standard diagnostic protocols for PH now emphasize endocrine screening. For any child or adult with a new diagnosis of pulmonary arterial hypertension or other forms of severe PH, it is recommended that laboratory screening include at least thyroid-stimulating hormone (TSH) to identify underlying function disorders that may be complicating the clinical course. This is often done in the cases of atypical PH of the newborn or chronic PH not otherwise explained or having a complex course. 

• Other Thyroid-Related Risk Factors: In the context of Group 4 PH (Chronic Thromboembolic Pulmonary Hypertension or CTEPH), high-dose thyroid hormone replacement has been identified as a specific clinical risk factor for the development of the disease. This highlights the need for clinicians to maintain a high index of suspicion when managing patients on chronic thyroid therapy who present with progressive shortness of breath or exercise limitation.

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Important References and Articles

• ACD Association: https://acdassociation.org/ 

• TBX4life: https://tbx4.org/