A case of cystic hygroma with confined placental mosaicism of tetraploidy leading to noonan syndrome

Objective To report the first case of cystic hygroma with confined placental mosaicism (CPM) of tetraploidy leading to Noonan syndrome. Case report At 12 weeks of gestation, fetal cystic hygroma was identified, and chorionic villus sampling (CVS) revealed tetraploid mosaicism of 46,XX[20]/92,XXXX[2]. The infant was born at 36 weeks of gestation with low-set ears and ventricular septal defects. Evaluation of the chromosomal mosaicism revealed CPM of tetraploidy. At 7 months of age, growth delay (−2 standard deviation) and left ptosis were noted, raising the suspicion of Noonan syndrome. Genetic testing revealed a heterozygous pathogenic KRAS variant (NM_004985.5:c.65A > G, p.Gln22Arg). Conclusion When mosaicism is detected by prenatal CVS, it is crucial to perform continuous phenotypic evaluation and detailed genetic testing after birth, even if the infant exhibits related symptoms.