Sonographic anatomy of the developing cerebellum: normal embryology can resemble pathology.

Objective: The aims of this study were to evaluate sonographically observable fetal cerebellar development by use of MR image and anatomic correlations to determine what aspects of this development can be visualized by sonography and to define the normal sonographic appearance of cerebellar development to assist in the differentiation of normal development from pathologic development during prenatal sonography.

Materials And Methods: The posterior fossae of 19 normal, whole, fixed fetal specimens of 11-21 weeks' gestational age were imaged by sonography and MR imaging. Images were correlated with anatomic atlases and anatomic sections.

Results: By 13-14 weeks, both sonography and MR imaging demonstrated the vermis between the hemispheres rostrally, but the caudal vermis was undeveloped. Although visualized on MR images, the caudal fourth ventricular roof was not yet appreciated on sonograms, giving the false impression of communication between the fourth ventricle and the cisterna magna. Separation of these CSF spaces was not appreciated at all levels sonographically until 16 weeks.

Conclusions: The sonographic appearance of normal cerebellar development can resemble pathology early in the second trimester. Caution is warranted in making an early diagnosis of fetal cerebellar dysgenesis.