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.