Open fourth ventricle prior to 20 weeks' gestation: a benign finding?

Area of Science:

• Prenatal diagnosis
• Fetal medicine
• Neuroimaging

Background:

• Fetal posterior fossa abnormalities require early diagnosis for timely intervention.
• Standard ultrasound methods may have limitations in detecting subtle abnormalities.
• The brainstem-vermis (BV) angle is a potential marker for posterior fossa development.

Purpose of the Study:

• To assess the utility of the brainstem-vermis (BV) angle in diagnosing fetal posterior fossa abnormalities.
• To correlate BV angle measurements with specific posterior fossa malformations.
• To evaluate the BV angle in early gestation (15-18 weeks).

Main Methods:

• Retrospective analysis of 3D ultrasound volumes from fetuses aged 15-18 weeks.
• Categorization into normal posterior fossa (controls) and abnormal cases (Dandy-Walker malformation, Blake's pouch cyst).
• Measurement of the BV angle and assessment of fourth ventricle appearance (open/closed).

Main Results:

• In normal fetuses, 90.3% had a closed fourth ventricle and BV angle <20°.
• Approximately 9.7% of normal fetuses showed an open fourth ventricle with a BV angle between 20°-37°.
• Fetuses with Dandy-Walker malformation had a significantly increased BV angle (>45°), while Blake's pouch cyst cases showed overlapping ranges with normal fetuses with open ventricles.

Conclusions:

• An open fourth ventricle is observed in about 10% of normal fetuses at 15-18 weeks.
• The BV angle is a valuable diagnostic tool, especially when the fourth ventricle appears open.
• A BV angle ≥45° is highly predictive of severe posterior fossa malformation.