Page 1 of 6 Critical review Gross and neurosurgical anatomy of the cerebellar tonsil Clinical Anatomy IN Mavridis1* Abstract anomalies such as arterial Discussion Introduction hypoplasticity, makes this structure The author has referenced one of its The primary purpose of this review potentially vulnerable to vascular own studies in this review. This was to focus on the gross anatomy, accidents (mainly infarcts) of the referenced study has been conducted in including morphometric inferior cerebellar arteries. A accordance with the Declaration of characteristics and arterial thorough understanding of the Helsinki (1964) and the protocols of vasculature, of the human cerebellar regional tonsillar anatomy, as well as these studies have been approved by tonsil. We also aimed to investigate its relations with the branches of the the relevant ethics committees related the neurosurgical anatomy of this posterior inferior cerebellar artery is to the institution in which they were clinically important structure of the of paramount significance for performed. human brain. neurosurgical interventions in this Discussion area. Gross anatomy The cerebellar tonsil is a rounded Morphology, locations and relations lobule on the undersurface of each Introduction The cerebellar tonsil is a rounded cerebellar hemisphere, continuous The human cerebellar tonsil is famous lobule on the undersurface of each medially with the uvula of the for its herniation through the foramen cerebellar hemisphere, continuous cerebellar vermis and superiorly with magnum either in congenital or in medially with the uvula of the the flocculonodular lobe. Arterial acquired conditions. Synonyms of the cerebellar vermis and superiorly with branches entering the tonsil originate ‘cerebellar tonsil’ include ‘tonsilla the flocculonodular lobe2. The not only from the posterior but from cerebella’, ‘ventral paraflocculus’, rostromedial margin of the tonsils the anterior inferior cerebellar artery ‘cerebellar amygdala (= almond- borders the tapering edges of the as well, often anastomosing with shaped)’ and ‘amygdala cerebelli’1,2. uvula3. This ovoid structure is attached those from the posterior. The main The tonsil belongs to the posterior to the cerebellum along its clinical conditions able to affect the cerebellar lobe1. The uvula, the lower superolateral border by a white matter position of the cerebellar tonsil half of the diamond-shaped formation bundle called the tonsillar peduncle. include raised intracranial pressure, of the vermian surface, projects The remaining tonsillar surfaces are myelomeningocele, Chiari downward between the tonsils, thus free surfaces3. malformations, posterior fossa mimicking the situation in the preparation, read approvedand the final manuscript. hypoplasia and Idiopathic scoliosis. 3 oropharynx . The inferior (caudal) pole and posterior t The main neurosurgical approaches surface of the cerebellar tonsil face the related to the region of the cerebellar The primary purpose of this review cisterna magna. The lateral surface of tonsil include the was to focus on the gross anatomy, each tonsil is covered by, but is transcerebellomedullary fissure including morphometric separated from the biventral lobule by approach, the tonsillouveal fissure characteristics and arterial a narrow cleft3 (the tonsillobiventral3 approach (telovelar approach), the vasculature, of the human cerebellar or postpyramidal fissure1), except lateral approach to the fourth tonsil. It was also aimed to investigate superiorly at the level of the tonsillar ventricle, the supra- and subtonsillar the neurosurgical anatomy of this peduncle3. The medial, anterior, and Conflict of interests: None declared. approaches, the median and far- clinically important structure of the superior surfaces all face other neural lateral suboccipital approaches. human brain. structures, but are separated from them Conclusion by narrow fissures. The position of the cerebellar tonsil The existing literature regarding the can be affected in some congenital and gross and clinical anatomy of the The anterior surface of each tonsil faces acquired disorders including Chiari human cerebellar tonsil was reviewed and is separated from the posterior malformations and raised intracranial with emphasis on its neurosurgical surface of the medulla by the pressure. The high incidence of anatomy. Functional and imaging cerebellomedullary fissure. The medial tonsillar arterial supply of single anatomy data are also presented. surfaces of the tonsils face each other origin, partially explained by vascular Furthermore, neuroanatomical across a narrow cleft, the vallecula comments on the tonsillar cerebelli, which leads into the fourth *Corresponding author morphometry and vasculature are ventricle. The superior (rostral) pole is Email: [email protected] All authors contributedAll to conception design,and manuscrip All authors abide by the Association Medical for Ethics (AME) ethical of rules disclosure. provided. separated from the surrounding Competing interests: None declared. 1 Department of Neurology, ‘K.A.T.-N.R.C.’ structures by a posterior extension of General Hospital of Attica, Athens, Greece the cerebellomedullary fissure, called Licensee OAPL (UK) 2014. Creative Commons Attribution License (CC-BY) FOR CITATION PURPOSES: Mavridis IN. Gross and neurosurgical anatomy of the cerebellar tonsil. OA Anatomy 2014 Apr 09;2(1):8. Page 2 of 6 Critical review the telovelotonsillar cleft. The hemispheric groups. PICA branches part of the ipsilateral cerebellar tonsil5. superior extension of this cleft over supplying the cortical surface of the The anterior inferior cerebellar artery the superior pole of the tonsil is called tonsil are usually emerging from its (AICA), usually originating from the the supratonsillar cleft3. The ventral lateral trunk (when present)4. The left basilar artery as a single vessel, aspect of the superior pole of each PICA usually courses around the bifurcates into a rostral and a caudal tonsil faces the three structures (tela lower pole of the tonsil while the right trunk5. The caudal trunk often enters choroidea, inferior medullary velum PICA descends well below the tonsil to the lateral portion of the and nodule) forming the lower half of the level of the foramen magnum cerebellomedullary fissure just below the roof of the fourth ventricle. The before ascending along the medial the lateral recess before turning posterior aspect of the superior pole tonsillar surface. The PICAs ascend laterally to supply the inferior part of faces the uvula medially and the between the tonsils and medulla to the petrosal surface. The distal biventral lobule laterally3. reach the interval between the tonsil branches of the caudal trunk often and uvula and to supply the anastomose with the PICA. The AICA The tonsil is the most prominent suboccipital surface3. gives rise to perforating arteries to the structure blocking access to the caudal brainstem, choroidal branches to the part of the fourth ventricle3. The The cortical PICA segment begins lateral segment of the choroid plexus 5 medial segments of the choroid plexus where the trunks and branches leave and nerve-related arteries . of the fourth ventricle stretch from the the groove between the vermis level of the nodule anterior to the medially and the tonsil and the Arterial branches entering the tonsil tonsils to the level of the foramen of hemisphere laterally, and includes the originate not only from PICA but from Magendie. Each medial segment is terminal cortical branches. The AICA as well, often anastomosing with subdivided into a rostral or nodular bifurcation of the PICA, into a medial those from PICA. The number of part and a caudal or tonsillar part. The and lateral trunk, often occurs near cortical branches entering the tonsil tonsillar parts are anterior to the the origin of this segment. The cortical ranges, to the author’s gross anatomical tonsils and extend inferiorly through branches radiate outward from the experience, from two to seven the foramen of Magendie3. superior and lateral borders of the regardless of their origin. The PICA tonsil to the remainder of the vermis supplies averagely 2.4 (0-7) while AICA The tonsillobiventral fissure is, to the and hemisphere5. The lateral trunk 2.1 (0-7) tonsillar branches. Although author’s anatomical experience, divides into a larger hemispheric PICA dominates the supply of cortical deeper and richer in vessels than the trunk that gives off multiple branches tonsillar branches, AICA seems to other nearby located sulci. The to the hemisphere and smaller contribute substantially (46.7%) in this maximum tonsillar dimension (24 tonsillar branches that supply the supply. Interestingly, in half of mm) reaches almost the half of the posterior and inferior surfaces of the specimens the cortical tonsillar cerebellar hemisphere’s height (50 tonsil. This division of the lateral branches seem to have a single origin mm). Tonsillar folia can be either trunk into tonsillar and hemispheric (either PICA or AICA). Vascular preparation, read andapproved the final manuscript. major or minor and the latter can be branches may occur at various sites in anomalies (such as arterial t occasionally incompletely formed. The relation to the tonsil, but is most hypoplasticity) could only partially mean number of tonsillar sulci is 24 commonly located near the posterior explain this phenomenon. (17-32) and respective number of margin of the medial surface of the major folia is 23 (16-31). tonsil5. Functional data The