[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Head and neck](https://durgical.com/en/anatomy/region/head-and-neck/) › Cerebellar tonsils [Nervous system](https://durgical.com/en/anatomy/system/nervous-system/) · [Head and neck](https://durgical.com/en/anatomy/region/head-and-neck/) # Cerebellar tonsils The cerebellar tonsils are two rounded lobes on the underside of the [cerebellum](https://durgical.com/en/anatomy/anterior-quadrangular-lobule/), just above the large opening of the skull base, into which they can be pushed. Official name: Tonsil of cerebellum [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=head_neck&select=cerebellar_tonsils&side=r) Opens in the Durgical app, no sign-up needed. ## What it is Two rounded lobules on the underside of the cerebellum, above the foramen magnum. ## What it does Can be pushed down into the foramen magnum (Chiari malformation, herniation). ## Key points - Tonsillar herniation (coning): raised pressure pushes the tonsils through the foramen magnum onto the medulla, with neck stiffness, slowing pulse, rising blood pressure, irregular breathing, then arrest - This is why imaging comes before lumbar puncture when raised pressure or a posterior fossa mass is suspected - Chiari I malformation: low, pointed ('peg-like') tonsils crowd the foramen magnum; occipital headache brought on by coughing or straining, and a syrinx in the cervical cord - Chiari II is different: the vermis and brainstem descend too, with a myelomeningocele and hydrocephalus ## Key facts **Where** Underside of each cerebellar hemisphere, next to the foramen magnum **Blood supply** Posterior inferior cerebellar artery (PICA) **Function** Part of the cerebellum; can herniate through the foramen magnum and press on the medulla (Chiari I: more than 5 mm below) ## Relations **In front** [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/), with the posterior inferior cerebellar artery looping between them **Medial** The other tonsil, separated by the vallecula, where the inferior [vermis](https://durgical.com/en/anatomy/central-lobule/) and the median aperture of the [fourth ventricle](https://durgical.com/en/anatomy/fourth-ventricle/) lie **Below** Rim of the [foramen magnum](https://durgical.com/en/anatomy/foramen-magnum/) **Behind** Cisterna magna and the inner surface of the [occipital bone](https://durgical.com/en/anatomy/occipital-bone/) **Above** Inferior medullary velum and the uvula of the vermis; the tonsil lies outside the fourth ventricle, below its lower roof ## How to find it On a sagittal MRI the tips of the tonsils are compared with a line joining the front and back edges of the foramen magnum (basion to opisthion); tips more than about 5 mm below it are the usual threshold for Chiari I. At surgery they appear beneath the cisterna magna once the posterior fossa dura is opened. ## On imaging Midline sagittal MRI: the tip of each tonsil is compared with a line from the front to the back edge of the foramen magnum. CSF flow studies show whether flow around them is blocked; a cord syrinx is looked for in the same scan. ## Clinical notes - Symptomatic Chiari I is treated by posterior fossa decompression: removing bone from the back of the foramen magnum and the arch of the [atlas](https://durgical.com/en/anatomy/atlas/), often widening the dura - A spinal CSF leak (low pressure) makes the brain sag and the tonsils descend, mimicking Chiari I ## Anatomical variants - Tonsils lying a few millimetres below the foramen magnum without symptoms (tonsillar ectopia) are a common chance finding on MRI, especially in children and young adults, and are not treated by themselves ## Development Chiari I is thought to arise mainly from a small, underdeveloped posterior fossa (occipital bone) that leaves too little room for a normal-sized cerebellum, rather than from the tonsils themselves. ## Related conditions - [Raised intracranial pressure and brain herniation](https://durgical.com/en/conditions/raised-intracranial-pressure-and-brain-herniation/) - [Syringomyelia and Chiari malformation](https://durgical.com/en/conditions/syringomyelia-and-chiari-malformation/) - [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/) ## Test yourself Think of the answer, then tap the question to see it. ## What is tonsillar herniation (coning), and what are its signs? Raised pressure pushes the cerebellar tonsils through the foramen magnum onto the medulla: neck stiffness, slowing pulse, rising blood pressure, irregular breathing, then arrest. ## Why is imaging done before lumbar puncture when raised pressure or a posterior fossa mass is suspected? Because of the risk of tonsillar herniation through the foramen magnum. ## What is Chiari I malformation? Low, pointed cerebellar tonsils (more than 5 mm below the foramen magnum) causing cough-induced occipital headache and a cervical syrinx. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/anatomy/tonsil-of-cerebellum/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** tonsilla cerebelli TA2 5817 **Official Spanish (DeCS)** Amígdalas **English (TA2)** Tonsil of cerebellum **Also called** lobule IX of cerebellar hemisphere · lobule H IX ## Related - [Foramen magnum](https://durgical.com/en/anatomy/foramen-magnum/) Sources: Terminologia Anatomica (TA2); [Neuroanatomy, Cerebellum](https://www.ncbi.nlm.nih.gov/books/NBK538167/); Tratado de anatomía humana (9.ª ed.), tomo II, Libro V, sección 2 (Encéfalo), capítulos I-V: bulbo, protuberancia, cerebelo, cuarto ventrículo, mesencéfalo. ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/tonsil-of-cerebellum/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/tonsil-of-cerebellum/