Foramen magnum !

The foramen magnum is the large opening in the base of the skull where the brainstem becomes the spinal cord and the vertebral arteries enter.

See in 3D →

Opens in the Durgical app, no sign-up needed.

What it is

The large opening in the base of the skull.

What it does

The brainstem becomes the spinal cord here; the vertebral arteries enter.

Key facts

Where: Occipital bone, floor of the posterior cranial fossa
Function: Transmits the medulla-cord junction, vertebral arteries, spinal roots of XI and meninges; cerebellar tonsils herniate here

Relations

Contents: Lower end of the medulla oblongata becoming the spinal cord, with its meninges; the vertebral arteries and the anterior and posterior spinal arteries; the spinal roots of the accessory nerve (XI) rising into the skull
In front: Basion, the front rim, continuing up as the clivus; the apical ligament of the dens and the tectorial membrane attach here, with the dens of the axis just below
Lateral: Occipital condyles, which articulate with the atlas; the hypoglossal canal pierces the bone just above the front of each condyle
Behind: Opisthion, the back rim, and the squamous part of the occipital bone, with the suboccipital muscles outside
Above: Cerebellar tonsils on each side of the medulla, with the cisterna magna behind it
Below: Atlas (C1); behind, the posterior atlanto-occipital membrane, which the vertebral arteries pierce before turning up through the opening

How to find it

It cannot be felt. On a dry skull it is the large oval opening in the floor of the posterior cranial fossa, between the occipital condyles. In the living it lies deep to the top of the back of the neck, just above the posterior arch of the atlas; midline sagittal MRI shows it from the basion in front to the opisthion behind.

On imaging

Sagittal MRI shows where the cerebellar tonsils sit relative to the basion–opisthion line, a crowded medulla and any syrinx; cine CSF-flow MRI shows blocked flow in Chiari malformation. CT with reconstructions shows basilar invagination (the dens tip above lines such as Chamberlain's), condyle fractures and the small opening of achondroplasia.

Clinical notes

Foramen magnum decompression (removing bone at the back of the opening, often with the arch of C1, and widening the dura) is the usual operation for a symptomatic Chiari malformation
Tumours at the opening, most often meningiomas, give slow and confusing signs: neck pain, then weakness and numbness that may move from one limb to the next
Lumbar puncture in a person with a large brain mass can set off tonsillar herniation, which is why imaging comes first when a mass is suspected
In children with achondroplasia the opening is screened with MRI, and surgical decompression is considered when the cord junction is compressed

Development

Four parts of the occipital bone ring the opening (the basilar part in front, a condylar part on each side and the squama behind) and fuse in early childhood; anomalies of this ring underlie congenital basilar invagination.

Test yourself

Think of the answer, then tap the question to see it.

What is tonsillar herniation through the foramen magnum, and why is it fatal?

Raised pressure forces the cerebellar tonsils down through the opening onto the medulla oblongata, whose breathing and circulation centres fail.

What defines Chiari I malformation at the foramen magnum, and what headache is typical?

Cerebellar tonsils below the opening (commonly 5 mm or more); headache brought on by coughing or straining, sometimes with a syrinx.

Which structures pass through the foramen magnum?

The medulla–spinal cord junction with its meninges, both vertebral arteries, the anterior spinal artery, the posterior spinal arteries and the spinal roots of XI.

Why does imaging come before lumbar puncture when a large brain mass is suspected?

Lumbar puncture can set off tonsillar herniation through the foramen magnum.

What is the usual operation for a symptomatic Chiari malformation at the foramen magnum?

Foramen magnum decompression: removing bone at the back of the opening, often with the arch of C1, and widening the dura.

Add to my cards (5)

Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): foramen magnum TA2 553
Official Spanish (DeCS): Foramen Magno DeCS D005539
English (TA2): Foramen magnum
Also called: foramen occipitale magnum · Agujero occipital

Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Head and Neck: Foramen Magnum; Anatomy, Head and Neck, Skull Foramen.

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.