Differential growth rates of the cerebellum and posterior fossa assessed by post mortem magnetic resonance imaging of the fetus: Implications for the pathogenesis of the Chiari 2 deformity

Purpose: To evaluate post mortem magnetic resonance imaging of the fetus to provide data on the rate of growth of the cerebellum, bony posterior fossa, supratentorial bony compartment and cerebrum. Material and Methods: Twenty fetuses subsequently shown to have normal brain and spines on autopsy were studied using MRI post mortem. MRI from 20 normal pediatric brain examinations in children aged 6 years or younger were studied for comparison. Post mortern MRI was performed using a high-resolution fast spin echo technique providing T2 weighted images. The area of the cerebellar vermis, posterior fossa, supratentorial skull cavity and cerebral hemispheres was measured in the sagittal plane in all cases. These measurements were compared over the age ranges studied. Results: We have shown that there are differences in the rate of growth and the apparent commencement of growth between the structures under study. The cerebellum appeared to start its significant growth at 16.5 weeks with a rate of 16 mm(2) /week throughout pregnancy, while the cerebral hemisphere appeared to commence significant growth at 13 weeks at a rate of 184 mm 2 /week throughout pregnancy. This is in contrast to the bony posterior fossa, whose growth paralleled the supratentorial bony compartment at all ages studied. Conclusion: We interpret our findings as showing relatively late commencement of cerebellar growth compared to the cerebral hemispheres, whereas the growth of the bony posterior fossa appears to be in advance and independent of cerebellar growth. Our results support the hypothesis that posterior fossa development depends on raised hydrostatic pressure in the CSF containing structures of the posterior fossa. When this mechanism fails, as in some cases of myelomeningocoele, a small posterior fossa is formed which characterizes the Chiari 2 malformation.