cerebellar tonsillar herniation
Downward displacement of cerebellar tonsils through the foramen magnum, potentially causing brainstem compression.
Full Definition
Cerebellar tonsillar herniation occurs when the cerebellar tonsils are displaced inferiorly through the foramen magnum, often due to increased intracranial pressure or structural abnormalities such as Chiari malformation. This condition can result in compression of the medulla oblongata and upper cervical spinal cord, leading to potentially life-threatening symptoms including respiratory compromise, altered consciousness, and cranial nerve dysfunction. On sagittal MRI, tonsillar herniation is identified when the cerebellar tonsils extend more than 5mm below the foramen magnum, though clinical significance depends on associated symptoms and degree of displacement.
Usage
Usage note: Specify the degree of herniation in millimeters; distinguish from normal variants.
In Context
- "Mild cerebellar tonsillar herniation of 8mm was identified, consistent with Chiari I malformation." — MRI brain report
- "Emergency decompression was indicated due to acute cerebellar tonsillar herniation following posterior fossa hemorrhage." — Neurosurgical consultation