First trimester supratentorial and infratentorial abnormalities in fetuses with open spina bifida.

OBJECTIVES

To examine whether supratentorial, infratentorial, and calvarial manifestations of open spina bifida (OSB) occur in the first-trimester.

METHODS

Supratentorial (midbrain curvature, thalamic height), infratentorial (fourth ventricle diameter, brain-stem diameter [BSD], ratio of BSD to BSD-occipital bone diameter [BSOB]), and calvarial (frontomaxillary facial [FMF] angle) parameters were measured in stored midsagittal facial images at 110-136 weeks of 500 normal and 10 open spina-bifida fetuses by 3 blinded examiners.

RESULTS

OSB fetuses displayed a larger midbrain curvature (93.26 ± 6.97 versus 97.79 ± 10.04, p < .05), decreased FMF angle (84.10 ± 5.44 versus 78.46 ± 5.82, p < .001), smaller fourth ventricle diameter (0.25 ± 0.04 mm versus 0.18 ± 0.05 mm, p < .001), smaller BSD (0.29 ± 0.04 mm versus 0.23 ± 0.04 mm; p < .001), and increased BSD to BSOB ratio (0.64 ± 0.71 versus 0.73 ± 0.93, p < .03) than normal controls, respectively. Thalamic height was not significantly altered. Detection rates of OSB were highest for fourth ventricle diameter and FMF angle (60% and 40%, respectively, at 95% specificity). BSD, BSD to BSOB ratio, midbrain curvature yielded sensitivities of 30% with 95% specificity. Two-dimensional discrimination for pairs of measures combining fourth ventricle diameter with FMF angle increased sensitivity to 90% with specificity of 90.7%.

CONCLUSIONS

Our findings suggest that supratentorial, infratentorial, and calvarial changes consistent with the Chiari-II malformation are already established in first-trimester fetuses with OSB.