Anatomy: Posterior Fossa, Hindbrain & Craniocervical Junction
Opisthion
The opisthion is a bony landmark at the back edge of the foramen magnum, paired with the basion to measure tonsillar descent in Chiari malformation.
In short: Opisthion
- It is the midpoint of the back edge of the foramen magnum.
- With the basion, it defines the line across the foramen magnum.
- This basion-opisthion line measures tonsillar descent.
- It is a key reference for diagnosing Chiari I.
What it is
The opisthion is a bony landmark at the midpoint of the posterior (back) edge of the foramen magnum. Together with the basion (at the front edge), it defines the line across the foramen magnum used to measure tonsillar descent in Chiari malformation.
Understanding Opisthion
The opisthion pairs with the basion to form the basion-opisthion line (McRae's line), which spans the foramen magnum from front to back. Radiologists measure how far the cerebellar tonsils extend below this line to determine the degree of tonsillar descent — the central measurement in diagnosing Chiari malformation type I (with thresholds commonly cited as 5 mm in adults and 3 mm in children). The opisthion is therefore an essential reference point for this key measurement. Understanding the opisthion, alongside the basion, clarifies how the standard line across the foramen magnum is defined and how tonsillar descent is measured against it.
Why it matters
Understanding the opisthion helps families see how, together with the basion, it defines the standard line across the foramen magnum used to measure tonsillar descent — the key measurement in diagnosing Chiari I.
Common questions
What is the opisthion?
The midpoint of the back edge of the foramen magnum.
What is it paired with?
The basion, to form the line across the foramen magnum.
Why does it matter?
The basion-opisthion line measures tonsillar descent for diagnosing Chiari I.
Bottom line
The opisthion is a bony landmark at the back of the foramen magnum that, with the basion, defines the line used to measure tonsillar descent in Chiari.
Browse all 265 Chiari malformation definitions
Read the Chiari malformation patient guides
Not sure which page you need? Put words to it with the symptom navigator