The foramen magnum is the large opening at the base of the occipital bone through which the medulla oblongata continues as the spinal cord, accompanied by the vertebral arteries, the anterior and posterior spinal arteries, the spinal roots of the accessory nerve (CN XI), and the tectorial membrane and alar ligaments. Its anterior border is the basion (basiocciput) and its posterior border is the opisthion (posterior rim). The normal distance from the basion to the tip of the odontoid (BAI) and from the occipital condyles to the odontoid is used to assess atlanto-occipital relationships.
The foramen magnum is the critical anatomical reference for craniocervical junction pathology. Chiari malformation involves downward herniation of the cerebellar tonsils through the foramen magnum below the McRae line (basion-opisthion), producing syringomyelia and neurological symptoms managed by foramen magnum decompression. Basilar invagination elevates the odontoid above Chamberlain line (hard palate to opisthion). Foramen magnum tumours (meningiomas, neuromas) produce a characteristic constellation of lower cranial nerve and cervical cord findings.
Downward displacement of the cerebellar tonsils more than 5 mm below the foramen magnum basion-opisthion line produces occipital headache worsened by Valsalva, syringomyelia, and progressive myelopathy, treated by posterior fossa decompression with foramen magnum craniectomy and C1 laminectomy.
Meningiomas at the foramen magnum margin produce progressive lower cranial nerve palsies, myelopathy, and contralateral hemiparesis through a characteristic caudal-to-rostral progression, requiring microsurgical excision with preservation of the vertebral arteries and lower cranial nerve rootlets.
