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Posterior Cranial Fossa

fossa cranii posterior

The posterior cranial fossa is the lowest and largest of the three cranial fossae, bounded anteriorly by the dorsum sellae and petrous ridges, laterally by the sigmoid sinuses and temporal and occipital bones, and posteriorly by the occipital bone including the internal occipital protuberance. It contains the cerebellum, brainstem (pons and medulla), and cranial nerves V through XII, and is traversed by the basilar artery. The foramen magnum opens at its floor.

Region: Head & Skull

Clinical Relevance

Clinical Notes

The posterior fossa is the site of cerebellar and brainstem tumours (medulloblastoma, ependymoma, haemangioblastoma, acoustic neuroma), vascular malformations, and Chiari malformations. Its rigid bony boundaries mean that small posterior fossa masses produce early hydrocephalus from aqueductal obstruction and tonsillar herniation through the foramen magnum (upward or downward transtentorial herniation). Posterior fossa surgery (suboccipital craniotomy) is the workhorse approach for these lesions, requiring careful understanding of the foramen magnum, the sigmoid sinuses, and the cerebellar tonsils.

Pathology

Common Injuries & Conditions

Posterior Fossa Tonsillar Herniation from Mass Effect

Posterior fossa mass (tumour or haematoma) produces downward herniation of the cerebellar tonsils through the foramen magnum, compressing the medulla and producing respiratory depression, bradycardia, and death; emergency surgical decompression of the posterior fossa or urgent ventriculostomy for obstructive hydrocephalus are life-saving interventions.