abnormal fourth ventricle morphology enlarged fourth ventricle MP:0008534 MPheno.ontology enlarged 4th ventricle enlarged IVth ventricle enlarged rhombencephalic ventricle enlarged ventricle of rhombencephalon enlarged ventriculus quartus increased size of the irregularly shaped cavity in the rhombencephalon, between the medulla oblongata, the pons, and the isthmus in front, and the cerebellum behind; it is continuous with the central canal of the cord below and with the cerebral aqueduct above, and through its lateral and median apertures it communicates with the subarachnoid space enlarged brain ventricles