OC2.05: 3D ultrasound evaluation of the fetal optic chiasm: a potential parameter for the differential diagnosis of developmental midline brain anomalies

Abstract

Objective: Visualization of the fetal optic chiasm by ultrasound has not previously been reported. MRI is often used to image the fetal optic chiasm for the differential diagnosis of septo-optic dysplasia and absent cavum septum pellucidum. We describe herein a new sonographic technique to visualize the optic chiasm using multiplanar three-dimensional ultrasound and to evaluate its dimensions during gestation. Methods: This cross-sectional study included 76 singleton fetuses with no congenital or anatomical abnormalities. The optic chiasm was identified as a hypoechoic structure in the shape of an ‘‘X’’, located anterior to the cerebral peduncles and below the circle of Willis. The optic chiasm diameter was measured in the axial plane at its midline. Intraand inter-observer agreement was analyzed by the Bland–Altman plot. Regression analysis was used to determine the relationship between the optic chiasm and gestational age. Results: Seventy-two of 76 (94.7%) normal fetuses had a visible optic chiasm. The mean optic chiasm diameter at 12 and 40 weeks of gestation measured 1 mm and 4.27 mm, respectively. Intraand inter-observer mean differences between paired measurements were 0.007 mm (95% limits of agreement −0.30 to 0.32 mm) and 0.19 mm (95% limits of agreement −0.85 to 1.24 mm), respectively. The optic chiasm diameter and gestational age fit a linear regression model with a multiple R = 0.84, and a R2 = 0.70 (chiasm diameter = 0.129 + 0.119 * gestational age). Conclusions: 1) The fetal optic chiasm can be reproducibly imaged and measured from 12.5 weeks of gestation to term; 2) the diameter of the optic chiasm increases with gestational age; 3) this novel parameter may be useful in the differential diagnosis of septo-optic dysplasia and absent cavum septum pellucidum, which currently requires MRI. OC2.06 ‘‘Intraventricular fused fornices’’, a marker for complex midline anomalies