Diagnosis & Imaging
McRae's line (basion-opisthion line)
McRae's line is a reference line across the foramen magnum used to measure how far the cerebellar tonsils descend in Chiari malformation.
In short: McRae's line (basion-opisthion line)
- It is the line from the basion to the opisthion across the foramen magnum.
- It defines the level of the foramen magnum.
- Tonsillar descent is measured below this line.
- It is the standard reference for diagnosing Chiari I.
What it is
McRae's line (the basion-opisthion line) is a reference line drawn across the foramen magnum, from the basion (front edge) to the opisthion (back edge). It defines the level of the foramen magnum and is used to measure how far the cerebellar tonsils descend in Chiari malformation.
Understanding McRae's line (basion-opisthion line)
To determine the degree of tonsillar descent — the key measurement in Chiari I — radiologists draw McRae's line across the foramen magnum and measure how far the cerebellar tonsils extend below it. Descent beyond a threshold (commonly cited as 5 mm in adults and 3 mm in children) is used to define Chiari I, though this is interpreted alongside symptoms and other findings rather than as an absolute rule. McRae's line is the standard reference for this measurement, providing a consistent, defined level of the foramen magnum. Understanding McRae's line clarifies the reference used to measure tonsillar descent and how the central Chiari I measurement is made. It anchors the most basic radiologic assessment in Chiari.
Why it matters
Understanding McRae's line helps families see how the standard reference across the foramen magnum is used to measure tonsillar descent — the central measurement in diagnosing Chiari I.
Common questions
What is McRae's line?
The line from the basion to the opisthion across the foramen magnum.
What is it used for?
Measuring how far the cerebellar tonsils descend.
Why does it matter?
It is the standard reference for diagnosing Chiari I.
Bottom line
McRae's line is the reference across the foramen magnum used to measure tonsillar descent, the central measurement in diagnosing Chiari I.
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