BACKGROUND AND PURPOSE:

• The diagnosis of fetal complete agenesis of the corpus callosum is often based on the absence of the cavum septi pellucidi (CSP)
  • Additional findings include teardrop-shaped lateral ventricles
• Partial agenesis of the corpus callosum (pACC) remains a challenge
• Karl et al. (Ultrasound in Obstetrics and Gynecology, 2017) sought to:
  • devise a method to measure the area of the CSP and the length-to-width ratio (CSP ratio)
  • compare these data between fetuses with pACC and normal fetuses

METHODS:

• Retrospective case-control study (20-34 weeks gestation)
• CSP were measured in axial plane of the fetal head and the length-to-width CSP ratio calculated
• Corpus callosum is visualized either
  • Midsagittal plane on 2D ultrasound with color Doppler demonstration of the pericallosal artery (direct visualization) or
  • 3D volume with multiplanar reconstruction (indirect visualization)
• pACC defined as interrupted or short corpus callosum, with anteroposterior length < 5th centile
  • Patient offered transvaginal fetal neurosonography, fetal MRI, diagnostic invasive procedure, neuropediatric consultation
• Controls obtained from the normal population with scans that had direct 2D visualization

RESULTS:

• 323 normal fetuses and 20 pACC fetuses were measured between 20 and 34 weeks of gestation
• In the normal population, as BPD increased so did CSP length and width, while the CSP ratio decreased
• In pACC fetuses
  • 85% (n=17) had CSP length < 5^th  centile CSP ratio
  • Superior predictive performance with CSP ratio
    • 95% (19/20) had CSP ratio < 5^th centile
    • Using empirical cut-off of < 1.5, 80% of pACC fetuses were below this threshold
  • Analysis of Z-scores showed that fetuses with pACC had a significantly smaller CSP ratio
  • No false positives with low CSP ratio

CONCLUSION:

• A normal CSP is rectangular shaped during the second half of gestation
• Most pACC fetuses have not only abnormal size, but abnormal shape of CSP
• pACC CSP are usually wider and more square or circular in shape, with a decreased CSP ratio
• This ratio can be used to identify fetuses at high risk of pACC

Learn More – Primary Sources:

Cavum septi pellucidi (CSP) ratio: A Marker for Partial Agenesis of the Fetal Corpus Callosum

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