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# 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.

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## 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 points

- Tonsillar herniation (coning): raised pressure above forces the cerebellar tonsils down through it onto the medulla oblongata, whose breathing and circulation centres fail; it can be fatal within minutes
- Chiari I malformation: the tonsils sit below the opening (commonly taught cut-off: 5 mm or more), with headache brought on by coughing or straining and sometimes a syrinx in the cord
- Basilar invagination: the tip of the dens rises into the opening and presses on the brainstem; it is congenital or acquired (rheumatoid arthritis, Paget disease, softened bone)
- Achondroplasia narrows it; in infants this can compress the junction of the brainstem and spinal cord, causing breathing pauses in sleep, weakness or sudden death

## 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](https://durgical.com/en/anatomy/medulla-oblongata/) becoming the [spinal cord](https://durgical.com/en/anatomy/spinal-cord/), with its meninges; the vertebral arteries and the anterior and [posterior spinal arteries](https://durgical.com/en/anatomy/posterior-spinal-arteries/); the spinal roots of the [accessory nerve (XI)](https://durgical.com/en/anatomy/accessory-nerve/) 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](https://durgical.com/en/anatomy/axis/) just below

**Lateral**
Occipital condyles, which articulate with the [atlas](https://durgical.com/en/anatomy/atlas/); the [hypoglossal canal](https://durgical.com/en/anatomy/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](https://durgical.com/en/anatomy/occipital-bone/), with the [suboccipital muscles](https://durgical.com/en/anatomy/rectus-posterior-major-capitis-muscle/) outside

**Above**
[Cerebellar tonsils](https://durgical.com/en/anatomy/tonsil-of-cerebellum/) 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.

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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

## Related

- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)
- [Cerebellar tonsils](https://durgical.com/en/anatomy/tonsil-of-cerebellum/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=5673&filter=ths_termall&q=Foramen%20Magnum); [Anatomy, Head and Neck: Foramen Magnum](https://www.ncbi.nlm.nih.gov/books/NBK526041/); [Anatomy, Head and Neck, Skull Foramen](https://www.ncbi.nlm.nih.gov/books/NBK546621/).

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