Interhemispheric fissure distortion in fetal open spina bifida: an under-recognized prenatal ultrasound finding.

objectivesTo describe, characterize and determine the frequency of interhemispheric fissure (IHF) distortion on ultrasound in fetuses with open spina bifida (OSB), evaluate its association with other intracranial findings and biometric parameters and assess its evolution after prenatal closure of the spinal defect.

methodsThis retrospective cohort study included fetuses with suspected OSB referred to a single tertiary fetal center (Ontario Fetal Centre, Toronto, ON, Canada) between June 2017 and May 2025. Inclusion required prenatal diagnosis of OSB and at least one ultrasound examination with adequate axial transventricular and transthalamic planes of the fetal brain for evaluation of the IHF. IHF distortion was defined as deviation from the expected linear echogenic midline appearance. Biometric measurements were converted to gestational age-adjusted Z-scores. Associations between IHF distortion and clinical and imaging variables were assessed using univariable and multivariable logistic regression. Postoperative imaging was reviewed in fetuses that underwent prenatal spinal defect closure to evaluate changes in IHF appearance.

resultsAmong 133 fetuses with confirmed OSB, IHF distortion was identified in 54 (40.6%). Interobserver agreement was excellent (Cohen's kappa coefficient = 0.83). Fetuses with IHF distortion had smaller transcerebellar diameter (TCD) Z-scores compared with those without distortion (-4.80 ± 2.11 vs -3.91 ± 2.54; P = 0.047). On multivariable logistic regression, later gestational age at OSB diagnosis (adjusted odds ratio (aOR), 1.43 (95% CI, 1.08-1.94); P = 0.01) and smaller TCD Z-score (aOR, 0.89 (95% CI, 0.76-0.99); P = 0.04) were independently associated with IHF distortion. Among fetuses that underwent prenatal closure and had postoperative imaging available, resolution of IHF distortion paralleled improvement in hindbrain herniation, whereas persistence of IHF distortion occurred only in cases with minimal or no improvement in hindbrain herniation.

conclusionsIHF distortion is a frequent and reproducible prenatal ultrasound finding in fetuses with OSB. Its association with a smaller TCD and its apparent resolution following prenatal closure of OSB in cases that demonstrated improvement in hindbrain herniation suggest that IHF distortion may represent a secondary mechanical manifestation within the Chiari II spectrum. © 2026 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.