Macrocerebellum in Achondroplasia: A Further CNS Manifestation of FGFR3 Mutations?

T1WIs of an 11-year-old child with achondroplasia (A–C) and the same T1WIs with superimposed cerebellum (yellow) and brainstem (cyan) segmentations (D–F). T1WIs from a healthy 11-year-old child (G–I) are provided for comparison. Extension of the cerebellar hemispheres inferior to the vermis is seen on the midsagittal plane (arrow in A); extension of the superomedial portions of the hemispheres above the tentorium cerebelli is seen on the coronal plane (arrow in B); and wrapping of the cerebellar hemispheres around the brainstem with extension around the lateral aspects of the brainstem at the level of the middle cerebellar peduncles is seen on the axial plane (arrows in C). Also seen on the midsagittal plane (A) are effacement of the retro- and infravermian subarachnoid CSF spaces, mass effect on the brainstem (which is anteriorly bowed [asterisk] with narrowing of the prepontine cistern and partial effacement of the fourth ventricle), and shortening of the midbrain with distortion of the tectal plate (open arrow). Note the normal morphology of the midbrain and tectal plate in G (open arrow).