abnormal fourth ventricle morphology dilated fourth ventricle MP:0000829 MPheno.ontology dilated 4th ventricle dilated IVth ventricle dilated rhombencephalic ventricle dilated ventricle of rhombencephalon dilated ventriculus quartus fourth ventricle dilation the luminal space of the fourth ventricle is increased in volume or area, usually with an increase in contained fluid, in 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 dilated brain ventricles