Core Terms & Classification
Foramen magnum
The foramen magnum is the large opening at the base of the skull through which the brainstem and spinal cord connect, the site of herniation in Chiari malformation.
In short: Foramen magnum
- It is the large opening at the base of the skull.
- The brainstem, spinal cord, and CSF pass through it.
- Chiari malformation involves herniation through this opening.
- Its size and the crowding within it are central to Chiari.
What it is
The foramen magnum (Latin for "great hole") is the large opening at the base of the skull through which the brainstem connects to the spinal cord and through which cerebrospinal fluid flows between the head and spine. It is the anatomical site of herniation in Chiari malformation.
Understanding Foramen magnum
Through the foramen magnum pass the lower brainstem and upper spinal cord, blood vessels, and CSF. In Chiari malformation, hindbrain structures (the cerebellar tonsils, and in some types the brainstem) extend down through this opening, crowding it and impeding CSF flow. The position of the cerebellar tonsils is measured relative to a line across the foramen magnum (basion to opisthion). Because the foramen magnum is where the herniation occurs and where CSF flow can be obstructed, it is central to understanding Chiari malformation and its effects, and surgery (posterior fossa decompression) aims to relieve crowding at and around this opening.
Why it matters
Understanding the foramen magnum helps families see exactly where Chiari herniation and CSF flow obstruction occur, why tonsil position is measured relative to it, and what surgery aims to relieve.
Common questions
What is the foramen magnum?
The large opening at the base of the skull where the brainstem meets the spinal cord.
What passes through it?
The brainstem, spinal cord, blood vessels, and CSF.
Why is it central to Chiari?
It is where herniation occurs and CSF flow can be obstructed.
Bottom line
The foramen magnum is the large opening at the skull's base where Chiari herniation and CSF flow obstruction occur, central to the condition.
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