Anatomy: Posterior Fossa, Hindbrain & Craniocervical Junction
Obex
The obex is a landmark at the lower end of the fourth ventricle whose downward position helps define the Chiari 1.5 variant.
In short: Obex
- It is a landmark at the bottom of the fourth ventricle.
- Its position marks the lower brainstem.
- Its descent below the foramen magnum helps define Chiari 1.5.
- It is used to distinguish Chiari 1.5 from type I.
What it is
The obex is an anatomical landmark at the lower end (the caudal tip) of the fourth ventricle, where it narrows toward the central canal of the spinal cord. Its position marks the lower brainstem, and its downward descent is a defining feature of the Chiari 1.5 variant.
Understanding Obex
Because the obex sits at the junction of the fourth ventricle and the spinal cord's central canal, its position indicates how low the lower brainstem sits. In typical Chiari malformation type I, the obex remains in its normal position even though the tonsils descend; in Chiari 1.5, the obex (and thus the lower brainstem) descends below the foramen magnum along with the tonsils. Radiologists measure the obex's position to distinguish Chiari 1.5 from type I — a distinction that matters because Chiari 1.5 can be more severe and may require more extensive surgery. Understanding the obex helps explain how the brainstem's descent is assessed and why it changes the classification and management.
Why it matters
Understanding the obex helps families see how the brainstem's downward position is measured, and why its descent (defining Chiari 1.5) signals a potentially more severe condition needing different management.
Common questions
What is the obex?
A landmark at the lower end of the fourth ventricle.
Why is it important?
Its position indicates how low the lower brainstem sits.
How does it relate to Chiari 1.5?
Its descent below the foramen magnum helps define Chiari 1.5.
Bottom line
The obex is a landmark at the bottom of the fourth ventricle whose descent below the foramen magnum helps define the Chiari 1.5 variant.
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