Prenatal ultrasound screening in early pregnancy involves an assessment of the nuchal region of the fetus. Although a small hypoechoic space in the posterior fetal neck is a normal finding in all first trimester fetuses, excessive enlargement is associated with an increased risk of Down syndrome, as well as other fetal abnormalities. Excessive enlargement may be related to a cystic hygroma or to mesenchymal edema (termed increased nuchal translucency).
The diagnosis and clinical significance of first trimester cystic hygroma and increased nuchal translucency will be reviewed here. A detailed discussion of first trimester screening for Down syndrome and other aneuploidies is provided separately. (See "First trimester combined test and integrated tests for screening for Down syndrome and trisomy 18".)
ANATOMY AND PATHOGENESIS OF NUCHAL ABNORMALITIES
Cystic hygroma — Cystic hygroma is a congenital malformation of the lymphatic system in which obstruction between the lymphatic and venous pathways in the fetal neck leads to lymph accumulation in the jugular lymphatic sacs of the nuchal region. Cystic hygromas can be subclassified as septated or nonseptated (simple). In the first trimester, the overall prevalence of septated and nonseptated cystic hygromas is about 1 in 100 fetuses, and the prevalence of septated cystic hygromas is 1 in 285 fetuses [1,2].
Nuchal translucency — Nuchal translucency is the hypoechoic region located between the skin and soft tissues behind the cervical spine. This hypoechoic space is presumed to represent mesenchymal edema and is often associated with distended jugular lymphatics [3,4]. A small, but measurable, amount of nuchal fluid can be identified in virtually all fetuses between the 10th and 14th week of gestation, and is considered a normal finding if below a defined threshold (see 'Prenatal diagnosis' below). On the other hand, an increased nuchal translucency is associated with a wide range of abnormalities, which suggests that there are different pathophysiologic pathways that lead to the common end-point of increased nuchal fluid. Possible etiologies include:
- Altered dermal collagen composition (eg, Down syndrome)
- Abnormal nuchal lymphogenesis (eg, Turner syndrome)
- Hemodynamic alterations and cardiac dysfunction (eg, heart defects)
- Abnormal endothelial cell differentiation