Cisterna magna width at 11-13 weeks in the detection of posterior fossa anomalies.

OBJECTIVE

To construct reference ranges for cisterna magna (CM) width at 11-13 weeks' gestation in healthy fetuses and determine whether open spina bifida and posterior fossa anomalies could be diagnosed in the first trimester.

METHODS

This was a retrospective study. CM width reference ranges were constructed based on the measurements obtained from 80 healthy fetuses with normal postnatal outcome undergoing routine first-trimester ultrasound at 11-13 weeks, using the Lambda-Mu-Sigma method. CM was measured in the fetal mid-sagittal view, as routinely used for nuchal translucency assessment. In addition, first-trimester ultrasound images in 11 fetuses with open spina bifida or posterior fossa anomalies, most of which were diagnosed later in pregnancy, were retrospectively reviewed, and CM measurements were compared against reference ranges.

RESULTS

CM width was noted to increase with gestational age in normal fetuses. The anomalies in the 11 fetuses we reviewed were: open spina bifida (n = 5), megacisterna magna (n = 3), Blake's pouch cyst (n = 2) and posterior fossa arachnoid cyst (n = 1). All fetuses with open spina bifida had a CM width below the 5(th) percentile. In the two fetuses with Blake's pouch cyst and in two with megacisterna magna, CM width was above the 95(th) percentile. In one of the fetuses with a megacisterna magna and the one with an arachnoid cyst, CM width was within normal range.

CONCLUSION

We have constructed reference ranges for CM width at 11-13 weeks using the mid-sagittal view. It appears that first-trimester CM width can be used as a marker for the early detection of open spina bifida. However, our findings need to be confirmed in prospective large series.