Characterization of a Novel MRI Sign of the Optic Chiasm in Patients With Optic Nerve Hypoplasia

Purpose: To characterize a novel magnetic resonance imaging (MRI) finding affecting the optic chiasm in patients with optic nerve hypoplasia (ONH). Methods: This was a retrospective case-control study including 78 patients diagnosed as having ONH at a single institution, with an average age at MRI of 2.66 years. The control group included 70 patients of similar demographics without a diagnosis of ONH or gross abnormalities on MRI. MRI images obtained on a Siemens 3T Prisma or Siemens 1.5T Aera (Siemens Healthineers) were examined for all patients by two independent graders for presence of an optic chiasm deviation termed the Y-sign. Main outcome measures were presence of the Y-sign on MRI imaging in the coronal view. Results: The Y-sign was identified in 55.1% of patients with ONH and 0% of control patients. Odds of the Y-sign were significantly higher in patients younger than 1 year (odds ratio [OR]: 3.58, 95% CI: 1.4 to 9.14; P = .008) and in patients with bilateral ONH (OR: 2.82, 95% CI: 1.08 to 7.33; P = .034). No association was found between the Y-sign and visual acuity or endocrine abnormalities ( P = .11 and P = .78, respectively). However, there was a moderate effect size noted between the Y-sign and absence of the septum pellucidum ( P = .052). Conclusions: The authors present a novel MRI finding seen in more than half of patients with ONH that appears to be highly specific and mildly sensitive for the condition. Results indicate that the Y-sign may be a sign for absence of the septum pellucidum, but other clinical outcomes require further investigation.