Severe Early-Onset Pulmonary Hypertension in a Six-Month-Old With Down Syndrome and Isolated Secundum Atrial Septal Defect.

Infants with Down syndrome (trisomy 21) commonly present with congenital heart defects and immune dysregulation, significantly increasing the risk of early-onset pulmonary arterial hypertension (PAH). Although secundum atrial septal defects (ASDs) are often considered hemodynamically mild in non-syndromic children, they can progress aggressively in the presence of trisomy 21. We describe a six-month-old male infant with karyotype-confirmed trisomy 21 who developed severe PAH secondary to a rapidly enlarging secundum ASD - a highly atypical presentation for an isolated lesion. The infant presented with fever, respiratory distress, vomiting, and diarrhea, alongside a clinical history of neonatal sepsis, recurrent infections, failure to thrive (weight below the 5th percentile), and subclinical hypothyroidism (TSH 8.12 μIU/mL). Echocardiography revealed that the ASD had enlarged from 6 mm at five months to 10 mm, creating a substantial left-to-right shunt (Qp:Qs >1.5:1). Management with IV ceftriaxone, sildenafil (2 mg twice daily), supplemental oxygen, and nutritional support stabilized the infant within five days (SpO₂ 93-94% on room air). He was discharged for deferred surgical ASD closure, highlighting the value of early pulmonary vasodilator therapy as a bridge to definitive repair. This case underscores the markedly increased susceptibility of infants with Down syndrome to severe PAH, even in the setting of a seemingly hemodynamically insignificant ASD. Early cardiac evaluation, prompt intervention, and multidisciplinary management are crucial to preventing irreversible pulmonary vascular disease in this high-risk population.