The cerebellar tonsil is a rounded lobule on the under surface of each cerebellar hemisphere, continuous medially with the uvula of the cerebellar vermis and superiorly by the flocculonodular lobe. Synonyms include: tonsilla cerebelli, amygdala cerebelli, the latter of which is not to be confused with the cerebral tonsils or amygdala nuclei located deep within the medial temporal lobes of the cerebral cortex. The flocculonodular lobe of the cerebellum, which can also be confused for the cerebellar tonsils, is one of three lobes that make up the overall composition of the cerebellum. The cerebellum consists of three anatomical and functional lobes: anterior lobe, posterior lobe, and flocculonodular lobe.

The cerebellar tonsil is part of the posterior lobe, also known as the neocerebellum, which is responsible for coordinating the voluntary movement of the distal parts of limbs. Elongation of the cerebellar tonsils can, due to pressure, lead to this portion of the cerebellum to slip or be pushed through the foramen magnum of the skull resulting in tonsilla herniation. This is a life-threatening condition as it causes increased pressure on the medulla oblongata which contains respiratory and cardiac control centres. A congenital condition of tonsilla herniation of either one or both tonsils is Chiari malformation. A Type I Chiari malformation is a congenital anomaly of the brain in which the cerebellar tonsils are elongated and pushed down through the opening of the base of the skull, blocking the flow of cerebrospinal fluid as it exits through the medial and lateral apertures of the fourth ventricle. Also called cerebellar tonsillar ectopia, or tonsillar herniation. Although often congenital, Chiari malformation symptoms can also be induced due to physical head trauma, commonly from raised intracranial pressure secondary to a hematoma, or increased dural strain pulling the brain caudally into the foramen magnum.

Head trauma increases risk of cerebellar tonsillar ectopia by a factor of 4. Ectopia may be present but asymptomatic until whiplash causes it to become symptomatic. Cerebellar connections to frontal, prefrontal, and limbic structures underlie important cerebellar contributions to cognition, emotion, and affect. Evidence from anatomical, functional imaging, functional connectivity, and lesion studies all support not only a pronounced role for the cerebellum in normal cognition and emotion, but also possible pathogenic and therapeutic roles in patients with neuropsychiatric disease. The goal of this manuscript is to summarize current research on the role of the cerebellum and cerebellar circuitry in cognitive, emotional, and affective processing and the implications this has for the pathogenesis and treatment of neuropsychiatric disease.  Tonsillar Herniation this is commonly known as coning, when the cerebellar tonsils move downward through the foramen magnum. Tonsillar herniation exerts pressure over the lower brain stem and the upper cervical spinal cord against the narrow foramen magnum. In the postoperative setting, brain stem compression results in unconsciousness, flaccid paralysis, and respiratory and cardiac depression. The Chiari malformation is a common developmental variant causing impaction of the cerebellar tonsils into the spinal canal. Tumours that favour the cerebellar hemispheres include pilocytic astrocytomas in children and hemangioblastomas in adults, although in adults metastases are the most frequent type of tumour in this location. Rare, but specific for this location, are dysplastic gangliocytomas of the cerebellum desmoplastic medulloblastomas, more frequent in adults, also tend to involve the cerebellar hemispheres.

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Managing Editor

Damon Wilson

Imaging in Interventional Radiology