Cerebellar tonsillar ectopia

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. 

Terminology

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). 

Suggested usage

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
  • tonsils only slightly below the base of skull (<5 mm for simplicity ⁷)
  • a better term than benign tonsillar ectopia as not all cases with protrusion of less than 5 mm are asymptomatic nor are cases with greater than 5 mm protrusion 'malignant' 
• 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
  • caudal displacement of tonsils is secondary to another defined pathological process (e.g. intracranial hypertension or craniospinal hypotension)
• tonsillar herniation
  • best reserved for acute displacement usually seen in the setting of massive raised intracranial pressure (e.g. extradural hemorrhage)