The clivus–supraocciput angle: a useful measurement to evaluate the shape and size of the fetal posterior fossa and to diagnose Chiari II malformation

Objective To obtain a nomogram of the clivus–supraocciput angle as a basis for the diagnosis of Chiari II malformation in fetuses with ventriculomegaly. Methods A cross-sectional study was undertaken on 310 normal pregnant women of 16–34 weeks' gestation. A mid-sagittal section of the fetal skull was obtained and the angle between the clivus and the supraocciput was measured. Forty-four fetuses with ventriculomegaly due to various causes (13 Chiari II malformation, 12 dysgenesis of the corpus callosum, 7 aqueductal stenosis, 6 borderline ventriculomegaly, 3 Dandy–Walker malformation, 2 porencephaly, 1 schizencephaly) were also included in the study and the values of the angle found in the pathological cases were compared with those found in the normal population. Results The clivus–supraocciput angle did not change during gestation and was almost constant with an average value of 79.3 ± 6°. All cases of Chiari II malformation showed a value below the 5th centile of our nomogram. Conclusions The evaluation of the posterior fossa and particularly the measurement of the clivus–supraocciput angle is a useful parameter to differentiate the various causes of fetal ventriculomegaly and particularly to recognize Chiari II malformation. Copyright © 2001 International Society of Ultrasound in Obstetrics and Gynecology