The aqueduct of Sylvius: a sonographic landmark for neural tube defects in the first trimester.

OBJECTIVES To describe a new first-trimester sonographic landmark, posterior displacement of the midbrain and aqueduct of Sylvius, which may be useful in early screening for neural tube defects. METHODS This was a prospective study of 457 normal fetuses at 11 + 0 to 13 + 6 weeks' gestation. We measured the distance from the posterior border of the aqueduct of Sylvius to the anterior border of the occiput (AOS-to-occiput distance) in the axial plane and created a reference range. In the nine fetuses with abnormal midbrain position identified in the first trimester and with neural tube defect subsequently confirmed, we analyzed ultrasound images to determine the position of the aqueduct of Sylvius. RESULTS The lower limit of normal AOS-to-occiput distance (mean minus 2 SD) ranged from 1.7 mm at a crown-rump length (CRL) of 45 mm to 3.7 mm at a CRL of 84 mm. In the nine cases with abnormal position of the midbrain and confirmed neural tube defect, juxtaposition of the midbrain to the occiput was the clue to diagnosis of the spinal abnormality. In all nine cases, the AOS-to-occiput distance was below the established normal range. CONCLUSIONS Examination of the midbrain in an axial plane may prove a reliable marker for the first-trimester diagnosis of neural tube defects. In contrast to recently reported subtle changes in the mid-sagittal view of the posterior cranial fossa, axial imaging of the midbrain reveals striking displacement of this structure, with virtual juxtaposition to the occiput, in fetuses with confirmed open spina bifida. This anatomical distortion of the midbrain can be quantified by measurement of the AOS-to-occiput distance.