Introduction: We support the normal standards for two sonographic (US) signs of the posterior cranial fossa, namely the magnitude of the cisterna magna and the height of the fourth ventricle fastigium, in preterm and fullterm infants.
Materials and methods: The cisterna magna was measured as the distance between the inferior aspect of the cerebella vermis and the inner aspect of the occiput. The height of the fourth ventricle fastigium was calculated as the line drawn from the fastigium to the inner aspect of the occiput, close to the opisthion.
Results: The cisterna magna was .46 +/- .13 cm in preterm infants and .45 +/- .1 cm in fullterm infants (the difference was not significant), whereas the height of the fourth ventricle fastigium was 1.7 +/- .26 cm in preterm and 1.9 +/- .16 cm in fullterm infants (the difference was significant, p < .005).
Conclusions: The finding of a cisterna magna of equal size in preterm and fullterm infants is in agreement with the concept that subarachnoid and cisternal spaces of the posterior fossa are still large at birth and start to decrease in size later in life. Our results suggest that a cisterna magna smaller than .2 cm in height and/or a fastigium closer than 1.2 cm to the basiocciput in preterm and closer than 1.6 cm in fullterm infants indicate, in a proper clinical context, an abnormally low cerebellum position (Arnold-Chiari malformation). On the other hand, cisterna magna values exceeding .72 in preterm and .65 cm in fullterm infants, as well as fastigium values exceeding 2.22 cm in both groups may suggest the presence of the Dandy-Walker complex or of an arachnoid cyst.