Posterior fossa and vermian morphometry in the characterization of fetal cerebellar abnormalities: a prospective three-dimensional ultrasound study

Objectives: To assess whether, by applying a predefined set of morphometric measurements to the posterior fossa and the cerebellar vermis, it is possible to detect and quantify the following parameters, which represent key features of abnormalities of the vermis and posterior fossa: 1) upward rotation of the vermis; 2) upward displacement of the tentorium; 3) underdevelopment of the vermis. Also to assess, in a small subset of cases, the correlation between ultrasound and magnetic resonance imaging (MRI) in the measurement of these three parameters.

Methods: This was a prospective observational trial. The study population included 51 normal fetuses and 20 abnormal fetuses with Dandy-Walker malformation (n = 5), Dandy-Walker variant (n = 8) or megacisterna magna (n = 7). Three-dimensional (3D) ultrasound volumes of the fetal head were acquired from the posterior fontanelle or the posterior part of the sagittal suture and processed offline with dedicated software. The following parameters were analyzed: tentorovermian angle; tentoroclivus angle; clivovermian angle; vermian diameter/biparietal diameter x 100 ratio (VD/BPD ratio). In seven cases, MRI performed on the same day as sonography was available for comparison. Non-parametric statistics were used to assess differences in means and correlations.

Results: The tentoroclivus angle was increased only in fetuses with Dandy-Walker malformation (P < 0.001). Upward rotation of the vermis was demonstrated both by the tentorovermian and the clivovermian angles in fetuses with Dandy-Walker variant (P < 0.001), while in those with Dandy-Walker malformation the difference was not statistically significant, probably due to the small number of cases. Finally, the VD/BPD ratio demonstrated that the degree of vermian hypoplasia was higher in Dandy-Walker malformation compared with Dandy-Walker variant (P < 0.001).

Conclusions: Using 3D ultrasound, we have demonstrated in the fetus the key elements characterizing most anomalies of the posterior fossa, namely the upward displacement of the tentorium, the counterclockwise rotation and the significant hypoplasia of the cerebellar vermis. In addition, our observations in a small number of cases indicate that there is a good correlation between MRI- and 3D ultrasound-derived morphometric measurements of the vermis.