Platelets and Ductus Arteriosus Closure in Neonates

Background: comprehensive review of the role of platelets on ductus arteriosus closure in neonates

Closure of the ductus arteriosus is a complex process and a key step in the postnatal transition of the circulatory system. Patent ductus arteriosus (failure to close) is a congenital heart defect that typically creates a left to right shunt, increasing pulmonary blood flow.

Several neonatal complications are associated with a hemodynamically significant patent ductus arteriosus, including:

• Edema or hemorrhage
• Necrotizing enterocolitis
• Renal impairment
• Bronchopulmonary dysplasia

The process of closure begins in the second trimester with anatomic changes in the ductus arteriosus. After an infant is born, local oxygen tension increases, and prostaglandin levels decrease causing the ductus arteriosus to contract (functional closure).

Beyond inherent changes within the vessel, literature has shown ductus arteriosus closure also depends on interactions between vascular components and circulating blood cells, which includes platelets.

This paper reviewed mechanistic data on the role of platelets in patent ductus arteriosus closure and potential clinical implication in preterm infants. It was co-authored by neonatology physician scientists Cassidy Delaney, MD, from Children’s Hospital Colorado and Hannes Sallmon, MD, from Charité University Medical Center Berlin.

Evidence from murine models

Echtler et al. published milestone findings in 2010 identifying platelets as playing an important role in ductal closure in murine models:

• Developmentally regulated platelet-endothelial interactions contribute to normal prenatal to postnatal cardiovascular transition
• Platelets mobilize to the ductal endothelium, immediately forming a platelet plug after birth
  • Models lacking transcription factor NFE or ITGA2B integrin (impaired platelet biogenesis or function, respectively) develop patent ductus arteriosus with a high shunt
• Antibody administration against GPVI associated with patent ductus arteriosus with a high shunt and functional platelet impairment
  • GPVI- (“collagen-receptor”) and GPIIb/IIIa-mediated pathways (“receptor for fibrinogen and von Willebrand factor”) contribute to platelet-triggered ductus arteriosus closure

Important considerations:

• Murine ductus arteriosus more morphologically like human preterm ductus arteriosus
• There is controversy around clinical significance of these findings

Thrombocytopenia and patent ductus arteriosus closure in preterm infants

With the incidence of thrombocytopenia in extremely low birth weight infants as high as 70%, multiple studies have investigated a possible role of low platelet counts in delayed ductal closure in preterm infants.

• Several past studies had conflicting results on a potential link
• Recently updated meta-analysis of 7,638 infants from 31 studies demonstrated significant association between thrombocytopenia within first days of life and patent ductus arteriosus/patent ductus arteriosus with a high shunt

Platelet count and pharmacologic therapy

• A meta-analysis of 1,087 infants from 8 studies showed pharmacologic ductus arteriosus closure potentially affected by platelet counts in preterm neonates; other individual investigations had conflicting results
• Research by Dr. Sallmon’s team found pre-pharmacological therapy platelet counts not predictive of treatment success; low platelet counts during ibuprofen treatment associated with treatment failure

Together, findings from these retrospective studies suggest platelets moderately but significantly affect spontaneous and pharmacological patent ductus arteriosus closure in preterm infants.

Platelet transfusions to accelerate patent ductus arteriosus closure

The potential application of platelet transfusions to speed up patent ductus arteriosus closure in preterm infants has been investigated once, using a randomized control trial of 22 patients.

• Liberal platelet-transfusion thresholds compared to restrictive thresholds in patients who received standard pharmacological therapy with cyclooxygenase inhibitors
• No significant acceleration in time to closure
  • 72-hour median for both groups
• 41% of liberal transfusion group developed intraventricular hemorrhage compared to 4.5% in restrictive transfusion group
  • Hemodynamic alterations may contribute to higher rates in transfused infants

Recent study by Margraf et al. demonstrated developmental maturation of platelet function in murine models:

• Observed rapid platelet aggregate formation when adult platelets where transfused into the fetal circulation
• Retrospective analysis of human preterm infants found no effect of platelet transfusions on patent ductus arteriosus

The formation of platelet plugs in adults are adversely affected by cyclooxygenase inhibitors. Comparable adverse effects may be found from minor alterations in platelet function in infants with thrombocytopenia.

Platelet function and ductal closure

Additional risk factors for closure failure in preterm infants, all associated with impaired platelet function, include:

• Immaturity
• Inflammation
• Preeclampsia

Extremely premature infants have a morphologically immature ductus arteriosus, increasing their risk for failed postnatal closure, suggesting in these infants:

• Platelets play larger role in ductus arteriosus
• Greater incidence of patent ductus arteriosus may be due in part to developmentally impaired platelet function

Numerous groups have studied surrogate markers of platelet activation and potential link with spontaneous and pharmacological patent ductus arteriosus closure.

• Studies on platelet indices and closure yielded inconsistent results
• Several observations indicated link between impaired platelet function and failed closure in neonates:
  • Lower platelet counts and higher C-reactive protein levels associated with patent ductus arteriosus
    • C-reactive protein only independent predictive favor
  • Significantly longer collagen-adenosine di-phosphate closure times in infants with patent ductus arteriosus compared to those without
    • Lower levels of platelet derived growth factor found in very immature infants with failure of spontaneous ductus arteriosus closure
    • Recent proteomics-based analysis found link with patent ductus arteriosus and differentially expressed plasma proteins potentially involved in platelet-endothelial interactions

These findings have prompted discussion of using platelet-rich plasma as a possible experimental treatment approach for patent ductus arteriosus with a high shunt. Platelets are known to function differently in infants and adults.

• Neonatal platelets exhibit hyporesponsive functional pattern in response to platelet agonists and have many molecular differences in multiple signaling pathways:
• Impaired intracellular signaling impacts ADP and thromboxane
• Significantly reduced protein levels of GPVI and C-type lectin-like receptor impairs collagen-signaling
• Healthy term infants have shorter bleeding and closure times in response to collagen/epinephrine and collagen/ADP compared to adults
• Compared to healthy term infants, preterm infants:
• Have longer bleeding and closure times
• More likely to have greater impaired platelet function
• Show further reduced surface expression of P-selectin

Developmental differences between preterm and term platelets and how they may impact platelet-mediated processes have not been well-studied. One study found immature platelets demonstrated different functional properties than older platelets during the first week after birth.

• Preterm infants with patent ductus arteriosus had lower counts of older platelets than those without.
• Newly-released platelets are not independently associated with patent ductus arteriosus

Multifaceted roles of neonatal platelets beyond hemostasis

Platelets play a vital role beyond hemostasis, serving as key mediators in immune regulation, vascular inflammation, and wound healing. 

• Resting platelets are maintained by endothelial-derived mediators like nitric oxide and prostacyclin.
• Endothelial dysfunction leads to decreased mediator release, causing platelet activation, adhesion, and release of various growth factors and mediators.
• Murine models deficient in alpha granule protein storage exhibit susceptibility to infection, weakened thromboinflammation, and impaired wound healing.
• A postmortem analysis of the human neonatal ductus arteriosus led by Dr. Sallmon found up to 46% of samples had thrombi in ductal lumen or wall of ductus arteriosus.
• A recent study by Echtler et al. of neonatal murine models found platelets quickly adhere to the ductus arteriosus’ endothelium after birth, crucial for vascular remodeling and ductal closure.
• Mechanisms behind platelet recruitment and ductal arteriosus remodeling remain unclear.
• Inflammatory conditions induce platelet-endothelial adhesion, promoting vascular changes.
  • Findings from other conditions suggests platelets may directly mediate processes in ductus arteriosus closure, remodeling
• Platelet activation, influenced by factors like transforming growth factor 1 and vascular endothelial growth factor, is involved in ductal remodeling.
• The inflammatory response post-ductal constriction is crucial for ductus arteriosus closure, involving monocytes/macrophages and their adhesion.
• Platelet activation triggers interactions that worsen vascular inflammation.
• Platelet-erythrocyte interactions in preterm infants with patent ductus arteriosus remain unexplored.

Conclusion and future research on platelets and potential role in neonatal disease

Despite important findings, the role of platelets in human ductal closure remains controversial. Together, evidence from multiple published studies indicate:

• Low platelet counts in preterm infants are linked to:
  • Ductus arteriosus patency
  • Potential pharmacological treatment failure
• Impaired platelet function may contribute to patent ductus arteriosus development in preterm infants
  • Platelet transfusions to facilitate ductal closure is not recommended for preterm thrombocytopenic infants

More research is needed to:

• Determine role of platelets during normal development and their potential role in neonatal disease
• Better understand complex interaction of circulating blood cells and vessel wall and conditions linked to developmental vascular changes

Future research should also recognize and consider developmental distinctions between neonates and adults in megakaryopoiesis/thrombopoiesis and platelet function –– and  the potential impact to the biological roles of platelets in preterm infants.