Dimensions of the optic chiasm: quantitative ultrasound comparison between fetuses with anophthalmia/microphthalmia and normal fetuses.

Background The precise pathogenesis of anophthalmia/microphthalmia remains unknown. Prenatal observation of the optic chiasm in fetuses with this malformation would assist in understanding the embryonic development of the condition. The present study aimed to establish the normal fetal size ranges of decussation of the optic chiasm, optic nerves, and optic tracts in the axial plane using two-dimensional transabdominal ultrasound throughout gestation and to compare these ranges to the corresponding values in fetuses with anophthalmia/microphthalmia. Methods In total, 310 normal fetuses and 16 fetuses with anophthalmia/microphthalmia were included in this study. The widths of the decussation of the optic chiasm, optic nerves, and optic tracts of normal fetuses at 19-40 weeks' gestation were measured in the axial plane by two-dimensional transabdominal ultrasound. The same widths were retrospectively measured in the axial plane using three-dimensional ultrasound in fetuses with anophthalmia/microphthalmia and compared to the results from the normal fetuses. Results The decussation, optic nerves, and optic tracts of 310 normal fetuses were measured. The normal widths of the decussation of the optic chiasm, optic nerves, and optic tracts increased linearly with gestational age. The interobserver and intraobserver reproducibility was excellent for the decussation but relatively low for the optic nerves and optic tracts. The optic nerve width of fetuses with anophthalmia/microphthalmia was significantly smaller than that of normal fetuses (P<0.001), but the widths of the decussation (P=0.061) and optic tracts (P=0.053) were not significantly different between the two groups. Conclusions The normal ranges of the decussation of the optic chiasm, optic nerves, and optic tracts established in this study can provide a quantitative basis for prenatal evaluation of the optic pathway. Fetal anophthalmia/microphthalmia may be associated with optic nerve hypoplasia.