Abernethy malformation (Type II) presenting in a 6-day-old boy with Noonan syndrome: a case report.

Background: Abernethy malformation (AM) is a rare vascular anomaly characterized by the diversion of splanchnic venous blood directly into the systemic circulation, bypassing the liver. We present the clinical features, diagnostic workup, and follow-up of a 6-day-old Chinese male with type II AM combined with Noonan syndrome (NS).

Case Presentation: The patient was prenatally suspected of having AM based on ultrasonographic findings, which were postnatally confirmed through enhanced computed tomography (CT) and magnetic resonance (MR) imaging. Due to dysmorphic facial features, whole-exome sequencing (WES) was performed, identifying a heterozygous c.848G > A (p. Arg283Gln) variant in the LZTR1 gene (NM_006767.4), consistent with NS. During follow-up, the patient exhibited progressive elevation of liver enzymes and hyperammonemia, prompting laparoscopic portosystemic shunt ligation at six months of age. Postoperatively, the patient demonstrated rapid biochemical normalization and sustained clinical improvement.

Conclusions: In patients with RASopathies, clinicians should maintain a high index of suspicion for AM and NS. Comprehensive vascular evaluation, particularly imaging screening of the portal venous system, is essential to avoid missed diagnoses and ensure timely intervention, thereby improving patient outcomes. A multidisciplinary approach that integrates genetic testing, advanced imaging, and surgical expertise is essential for optimizing outcomes in these complex cases.