Two-dimensional visualization and measurement of the fetal optic chiasm: improving counseling for antenatal diagnosis of agenesis of the septum pellucidum.

Objective: To develop an objective method for visualizing and measuring the fetal optic chiasm (OC) using transvaginal two-dimensional (2D) ultrasound in the coronal plane and to report measurements in fetuses with agenesis of the septum pellucidum (SP).

Methods: This was a prospective cross-sectional study of 115 morphologically normal fetuses in low-risk pregnancies, between 21 and 30 weeks' gestation. The OC was measured in a coronal plane at the level of the third ventricle and was seen as a horizontally aligned dumbbell-shaped structure of moderate echogenicity. In addition, OC measurements from eight fetuses with agenesis of the SP and complete follow-up were compared with the reference range.

Results: OC measurements were obtained in 110/115 normal fetuses and showed that OC increases linearly with gestational age. Our method of measurement demonstrated good intraobserver repeatability and excellent interobserver reproducibility. Among the eight fetuses with agenesis of the SP, five had normal OC measurements and five had normal vision postnatally. Pregnancy continued to term in all cases and the follow-up period varied from 6 months to 7 years.

Conclusion: Our study demonstrates that it is possible to visualize and measure the OC directly on a 2D ultrasound coronal plane. In fetuses with agenesis of the SP, the morphology and width of the OC visual pathway could prove a relevant tool for assessing its development. It would also help in the difficult task of providing antenatal counseling when faced with the diagnosis of agenesis of the SP. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.