Supratentorial abnormalities in the Chiari II malformation, IV: the too-far-back ventricle.

Objective: The sonographic diagnosis of fetal myelomeningocele (MMC) has improved mainly because the diagnostic focus has shifted from observation of the spinal abnormality to observation of cranial abnormalities. We describe an abnormality in the position of the occipital horn in fetuses with MMC. The occipital horn appears to be too posterior in location when compared to healthy fetuses.

Methods: We searched for all cases in which fetal MMC was sonographically detected from 1999 to 2009. Random controls from normal pregnancies were identified. We then measured the shortest distance of the edge of the occipital horn to the occipital bone in fetuses with MMC compared to healthy fetuses. Only fetuses with MMC who had normal-size ventricles were included.

Results: A total of 91 fetuses with MMC were identified. Twenty-six fetuses had a normal ventricle size. The gestational age range in this cohort was 18 weeks 5 days to 30 weeks 0 days. The comparison group of 39 healthy fetuses all had normal ventricles and had a gestational age range of 18 weeks 3 days to 35 weeks 2 days. After adjusting for gestational age, the statistical analysis showed that fetuses with MMC had significantly shorter measured distances from the posterior edge of the occipital horn to the occipital bone than healthy fetuses (P = .003).

Conclusions: The occipital horn both appears to be and measures closer to the occipital bone in fetuses with MMC compared to healthy fetuses.