(941) Frequency of additional counseling with detailed first-trimester posterior fossa assessment at 11 to 13-6/7 weeks

Detailed neurosonography is recommended with nuchal translucency assessment between 11 and 13 6/7 weeks gestation.  Our goal was to determine the potential counseling impact of adding calculations to identify posterior fossa syndromes such as Dandy-Walker malformation (DWM), vermian hypoplasia (VH), persistent Blake's pouch cyst (BPC), and open spina bifida (OSB) to standard nuchal translucency exams.

Study Design:

We retrospectively reviewed ultrasounds from 4811 first trimester nuchal translucency assessments within a single maternal-fetal medicine practice from 2020 to 2022.  Images were reviewed for the presence of the cisterna magna, intracranial translucency, and the brainstem.  The brainstem to brainstem occipital bone ratio (BS\BSOB) was calculated and the location of the fourth ventricle choroid plexus complex was identified.

Results:

There were 18 posterior fossa abnormalities for an incidence of 1 in 268 exams.  These included 11 Dandy-Walker malformations, 4 vermian hypoplasia, 2 Blake's pouch cyst, and 1 open spina bifida.  There were 93 patients (1/51 exams) with an inconclusive or suspicious assessment which then returned normal on subsequent imaging.  Most of these were technically poor evaluations in which the posterior fossa could not be cleared.

Conclusion:

Adding detailed posterior fossa first-trimester assessment at the time of the standard nuchal translucency exam identifies posterior fossa abnormalities requiring immediate counseling in approximately 1 in 268 patients.  For patients with abnormalities, detailed genetic counseling, a recommendation for amniocentesis or chorionic villous sampling, and follow-up imaging in 2 to 3 weeks were the practical consequences. An additional 1 in 51 exams will be inconclusive, requiring counseling for detailed sonography and a more frequent follow-up than the traditional 20-week anatomic survey.