Abstract
Background: Down syndrome is one of the most common chromosomal abnormalities occurring in approximately 1 in 700 live births. The majority of cases are attributed to the presence of an additional copy of chromosome 21, resulting in a total chromosome count of 47, as opposed to the typical 46. Distinctive facial characteristics commonly associated with this condition include but are not limited to, upslanting palpebral fissures, epicanthal folds, protruding tongue, and brachycephaly. Other clinical manifestations encompass hypotonia, intellectual disability, and congenital heart defects, among others.
Case Presentation: In this article, we present the case of a premature neonate delivered at 32 weeks of gestation via emergency cesarean section due to absent diastolic flow. The patient’s prenatal history was significant for intrauterine growth restriction. Following birth, the patient displayed very subtle dysmorphic features, notably upslanting palpebral fissures, without additional features suggestive of Down syndrome (DS). Chromosomal analysis revealed an isodicentric chromosome 21 (46, XX idic(21)(q22.3). Array comparative genomic hybridization revealed a concurrent duplication of the majority of chromosome 21 [21p11.2q22.3(7761419_41294939)] and a 4.5 Mb deletion of the long arm of chromosome 21, specifically 21q22.3(41295017_46677460).
Conclusion: Our study highlights a unique etiology of Down syndrome that is associated with a milder pheno-type. However, its limitations include the rarity of the case, which restricts the availability of comparative data, and potential influencing factors like prematurity and low birth weight.

Key words: Down syndrome, trisomy 21, isodicentric chromosome 21.