Posterior cerebellomedullary cistern

Posterior cerebellomedullary cistern, also known as the cisterna magna, is the largest among all the subarachnoid cisterns. It is positioned centrally below the cerebellum and behind the medulla oblongata. This space serves as an alternate site for draining cerebrospinal fluid when the lumbar cistern is inaccessible.

The posterior cerebellomedullary cistern expands laterally on both sides and joins with the lateral cerebellomedullary cisterns. Some experts classify these two cisterns as the medial and lateral compartments of the cisterna magna.

Within the posterior cerebellomedullary cistern (or cisterna magna), you can find the fifth segment of the posterior inferior cerebellar artery (PICA), while the third and fourth segments of the PICA are positioned more laterally. Additionally, the cisterna magna contains other important vascular structures such as the vertebral arteries, inferior vermian vein, medial posterior medullary vein, and the vein of the cerebello-medullary fissure. Also, it houses the glossopharyngeal, vagus, and accessory nerves.