Supratentorial abnormalities in the Chiari II malformation, I: the ventricular "point"

The sonographic diagnosis of fetal myelomeningocele has improved dramatically over the past 20 years, mainly because the diagnostic focus has shifted from observation of the spinal abnormality to observation of cranial abnormalities. We describe an abnormality in the shape of the occipital horn that has received scant attention in the sonographic literature. The abnormality is a misshapen occipital horn that is "pointed" rather than rounded. This study analyzed the frequency of this observation and technical aspects that improve its visualization.

From a database of obstetric sonograms, we searched for all cases in which open fetal myelomeningocele was sonographically detected. The search covered the period from 1999 to 2007. We then retrospectively reviewed the fetal intracranial findings in each case, with special attention to the contour of the occipital horn of the lateral ventricle.

A total of 89 fetuses were identified. The mean and median gestational ages were 22 weeks 4 days and 22 weeks 2 days, respectively. Of the 89 cases, 62 (70%) had an occipital horn with a pointed shape. This abnormality was better seen in cases judged to be well visualized and in cases in which digital video clips were available for review. The abnormality was more frequently seen in fetuses at less than 24 weeks' gestation and in fetuses without ventriculomegaly. Finally, it was seen as commonly among fetuses with mild posterior fossa deformations as in those with more severe distortions.

The occipital point is a common supratentorial feature of the Chiari II malformation. It is seen more commonly in fetuses at less than 24 weeks and in fetuses with normal-sized ventricles. As well, it is seen as commonly among fetuses with mild posterior fossa deformations as in those with more severe distortions. Therefore, its usefulness is enhanced as an observation in the armamentarium of cranial findings for detection of fetal myelomeningocele.