Enlarged cisterna magna Large cisterna magna Increase in size of the cisterna magna, one of three principal openings in the subarachnoid space between the arachnoid and pia mater, located between the cerebellum and the dorsal surface of the medulla oblongata. HP:0002280 Mega cisterna magna UMLS:C1853377 Cerebrospinal fluid produced in the fourth ventricle drains into the cisterna magna via the lateral apertures and median aperture. Enlarged posterior fossa