Definition

An encapsulated fluid filled space within the choroid plexus of the developing fetal cerebral ventricle.

Explanation

The choroid plexus is a bundle of blood vessels with can be clearly seen within the cerebral ventricles. The blood vessels appear echogenic, but cysts can form within these developing blood vessels. Choroid plexus cysts (CPCs) can be unilateral or bilateral, but are generally best seen on ultrasound in the far field – the side of the head furthest away from the scan probe.

Technique for measurement/assessment

A CPC is seen in the standard view for the cerebral ventricles. A transverse section including both choroids should be obtained, and the image showing the CPC most clearly should be measured and assessed. The CPC should be imaged in 3 planes and seen to lie within the developing choroid. The largest diameter of the CPC should be measured. It is common for CPCs to be bilateral, but the cysts lying in the near field may be more difficult to image. There should be no change in the appearance of the lateral ventricles, or other structures in the fetal brain. The ultrasound assessment should be completed, looking in particular for the other soft markers: nuchal pad, echogenic bowel, echogenic foci, short femur length and renal pyelectasis.

Implications of the finding

Choroid plexus cysts are associated more with trisomy 18 (Edward’s syndrome) than trisomy 21 (1)(Down’s syndrome). CPCs are most common at 16 weeks gestation, when about 2% of fetuses will demonstrate this finding, and almost always disappear by 26 weeks (2). Trisomy 18 has a high chance (80%) of demonstrating other ultrasound features. Hydrocephalus, holoprosencephaly, midline facial cleft, cardiac anomalies, diaphragmatic hernia, exomphalos, clenched hands with overlapping fingers, talipes and early onset growth failure should be sought (3). The fetal hands in particular have been identified as an important feature to distinguish a case of trisomy 18 (4) and therefore normal, open hands should be seen in conjunction with normal anatomy and growth for a diagnosis of an isolated CPC to be made.

Trisomy 18 is associated with advanced maternal age and a low HCG on serum screening (in contrast to trisomy 21 where the serum HCG is high). Therefore women of >35 years of age with an isolated CPC should be considered high risk for trisomy 18 even if serum screening gives a low risk for trisomy 21 (5).

If there are no other problems or risk factors there is no need for further action. The cysts usually disappear, and although possibly reassuring in terms of the growth of the fetus, there is no evidence that repeating ultrasound scans later in the pregnancy affects the outcome. If a further scan is to be undertaken it is logical to do this in the third trimester (28 to 34 weeks) to identify growth failure which is commonly associated with trisomy 18. There seems little to be gained by repeating scans earlier than this unless the anatomical structural survey, or markers checklist remains incomplete.

Image 6- Choroid plexus cyst

References

1. Fitzsimmons J, Wilson D, Pascoe-Mason J, Shaw CM, Cyr DR, Mack LA. Choroid plexus cysts in fetuses with trisomy 18. Obstetrics and Gynecology. 1989; 73: 257-260, Abstract

2. Morocos CL, Platt LD, Carlson DE, Gregory KD, Greene NH, Korst LM. The isolated choroids plexus cyst. Obstet Gynecol 1998; 92: 232-236, Abstract