Cerebellar tonsillar ectopia denotes an inferior location of the cerebellar tonsils below the margins of the foramen magnum. It, therefore, encompasses both minor asymptomatic tonsilar ectopia and Chiari I malformations.
Use of the term cerebellar tonsillar ectopia is not uniform. It is sometimes used synonymously with tonsillar descent or low-lying tonsils, although many authors impart specific and distinct meaning to each term, in particular using the latter to refer to benign tonsillar ectopia 6.
Many authors use cerebellar tonsillar ectopia merely as a catch-all descriptive term for low-lying tonsils irrespective of the cause and thus including a spectrum from acquired tonsillar ectopia to changes in intracranial pressure 3-5. Others seem to limit the use of the term for cases of congenital tonsillar ectopia 1,2.
Although the former broader usage seems more common, it is worth trying to be as precise as possible to avoid confusion and to use acquired tonsillar ectopia when downward descent of the tonsils is thought to be secondary to another well-defined cause (e.g. intracranial hypertension or craniospinal hypotension).
Based on the above observations the following meanings are suggested for each term to avoid too much confusion.
- cerebellar tonsillar ectopia
- umbrella term denoting all cases in which the cerebellar tonsils are below the base of skull
- includes congenital and acquired etiologies
- includes all degrees of severity
- includes asymptomatic and symptomatic cases
- low-lying tonsils
Chiari I malformation
- tonsils more significantly caudally displaced (>5 mm below base of skull, some authors use other numbers)
- often associated with symptoms or other structural changes (e.g. syrinx)
- thought to be congenital
- acquired tonsillar ectopia
- best reserved for acute displacement usually seen in the setting of massive raised intracranial pressure (e.g. extradural hemorrhage)
- 1. McKinney A.M. (2017) Cerebellar Tonsillar Ectopia. In: Atlas of Normal Imaging Variations of the Brain, Skull, and Craniocervical Vasculature. Springer, Cham DOI: 10.1007/978-3-319-39790-0_2
- 2. A J Barkovich, F J Wippold, J L Sherman, C M Citrin. Significance of cerebellar tonsillar position on MR. (1986) American Journal of Neuroradiology. 7 (5): 795. Pubmed
- 3. Arnett BC. Tonsillar ectopia and headaches. (2004) Neurologic clinics. 22 (1): 229-36. doi:10.1016/S0733-8619(03)00101-4 - Pubmed
- 4. Maximilian F Reiser, Wolfhard Semmler, Hedvig Hricak. Magnetic Resonance Tomography. (2007) ISBN: 9783540293552
- 5. Aiken AH, Hoots JA, Saindane AM et-al. Incidence of cerebellar tonsillar ectopia in idiopathic intracranial hypertension: a mimic of the Chiari I malformation. AJNR Am J Neuroradiol. 2012;33 (10): 1901-6. doi:10.3174/ajnr.A3068 - Pubmed citation
- 6. Jeffrey Leonard. Chiari Malformation, An Issue of Neurosurgery Clinics of North America,. (2016) ISBN: 9780323400930
- 7. Elster AD, Chen MY. Chiari I malformations: clinical and radiologic reappraisal. Radiology. 1992;183 (2): 347-53. Radiology (abstract) - Pubmed citation