The neonatal cisterna magna: ultrasonic evaluation.

The normal topography of the region of the cisterna magna can be delineated by cranial ultrasound in neonates. Evaluation of this region requires ultrasonic imaging of the echodense occipital bone and the inferior vermis, as well as approximation of the plane of the foramen magnum. It also requires imaging of the anechoic cisterna magna and fastigium of the fourth ventricle. In order to appreciate subtle alterations in size and/or position of these structures, standardized measurements for the midsagittal height of the cisterna magna and the distance of the fastigium of the fourth ventricle from the plane of the foramen magnum were established. Normal values established by measurement in 59 patients were cisterna magna 4.52 +/- 1.29 mm and fastigium of the fourth ventricle 16.05 +/- 3.03 mm. These dimensions were demonstrated to be abnormally increased in posterior fossa arachnoid cysts, Dandy-Walker syndrome, and hydrocephalus.