cerebellar tonsillar ectopia

Full Definition

Cerebellar tonsillar ectopia refers to the abnormal descent of the cerebellar tonsils through the foramen magnum. This is typically measured on sagittal T1 or T2-weighted MRI images, with descent greater than 5mm below the basion-opisthion line considered abnormal in adults. When associated with clinical symptoms such as headache, neck pain, or neurological deficits, it may represent Chiari I malformation. The condition can also be acquired secondary to increased intracranial pressure or mass effect.

Usage

Usage note: Always provide measurement in millimeters; distinguish from symptomatic Chiari malformation.

In Context

• "The cerebellar tonsillar ectopia measured 8mm below the foramen magnum on sagittal imaging." — MRI measurement report
• "Mild cerebellar tonsillar ectopia was noted but appeared asymptomatic." — Incidental finding report