The role of the optic chiasm in distinguishing isolated agenesis of the septum pellucidum from septo-optic dysplasia

Agenesis of the septum pellucidum (ASP) refers to the incomplete formation of the thin membrane that separates the brain's lateral ventricles. It may occur on its own or alongside other brain abnormalities, often associated with conditions such as holoprosencephaly or corpus callosal agenesis. While isolated ASP generally carries a good prognosis.

We present a case of isolated ASP that required a complex management approach. A 27-year-old (G2P1) pregnant woman underwent a second trimester anomaly scan, which showed collapsed anterior horns and the absence of the cavum septum pellucidum. However, the corpus callosum, A. pericallosum, and optic chiasm were fully visualised and reduced the possibility of septo-optic dysplasia (SOD), that characterised by optic nerve hypoplasia, pituitary dysfunction, and midline brain malformations, including ASP. First trimester screening indicated low risks for T21, T18, and T13, though incomplete nasal bone ossification was noted. NIPT results were low risk, and the 16-week ultrasound showed no abnormalities. The family chose to continue the pregnancy, and further evaluations – including amniocentesis and Chromosomal Microarray Analysis. MRI confirmed the absence of the septi pellucidi but did not provide details regarding the optic chiasm. The pregnancy progressed without complications, and the baby was delivered at 38+1 weeks, weighing 2935 g, with an APGAR score of 9/9. At six months, the child exhibited normal hearing, age-appropriate vision, and MRI findings confirmed isolated ASP.

A thorough understanding of fetal brain anatomy, paired with advanced ultrasound techniques, is crucial for detecting subtle structures and identifying associated anomalies. In fetuses with ASP, a detailed antenatal evaluation of the optic tract, optic nerve, and optic chiasm is essential for distinguishing SOD.