The cerebellar tonsils are two rounded lobules on the underside of the cerebellar hemispheres, sitting on either side of the medulla just above the foramen magnum. They matter clinically because they can be pushed down through the foramen magnum, either from birth (Chiari malformation) or by raised intracranial pressure (tonsillar herniation).
| Location | Inferior medial surface of each cerebellar hemisphere, beside the medulla and above the foramen magnum, next to the cisterna magna |
|---|---|
| Parts |
|
| Connections |
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| Blood Supply | Posterior inferior cerebellar artery (tonsillar loop) |
Form part of the posterior lobe of the cerebellum and contribute to its coordination functions.
In Chiari I malformation the tonsils sit more than about 5 mm below the foramen magnum, which can cause cough-induced occipital headache, syringomyelia and lower cranial nerve problems. In acute raised intracranial pressure, tonsillar herniation compresses the medulla and can cause sudden respiratory arrest. Lumbar puncture is avoided when a posterior fossa mass is suspected because it may precipitate herniation.
Downward displacement of the cerebellar tonsils below the foramen magnum causing headache worse with coughing, neck pain and sometimes syringomyelia.
Forced descent of the tonsils through the foramen magnum from raised pressure, compressing the medulla and threatening breathing and life.
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