Normal position of the Cerebellar Tonsils in relation to the anteroposterior diameter of the foramen magnum in individuals of differnet age groups

The cerebellar tonsils are part of the nervous system which is the chief controlling and coordinating system of the body. Their functional role is not yet known but are believed to coordinate skilled and learned movements. The cerebellar tonsils are prone to herniation through the foramen magnum in conditions causing increased intracranial pressure. If increased pressure compresses important brain structures, it can lead to serious, permanent problems or even death. Determining the normal position of cerebellar tonsils and its relationship with the Anteroposterior diameter of the foramen magnum in the Zambian population could provide guidance on considerations which need to be taken into account when making a possible diagnosis of individuals with cerebellar tonsillar herniation as well as in formulation of treatment objectives.
The objective of the study was to determine retrospectively the relationship between the normal position of the cerebellar tonsils and the anteroposterior diameter of the foramen magnum in individuals of different age groups.
METHODS: A cross sectional study design was used which analysed, retrospectively, patient digital records of MRI scans of the brain taken at the Cancer Diseases Hospital in Lusaka from the year 2012 to 2014. Computer software tools, which included rulers, were used to measure lengths of the anteroposterior diameter of the foramen magnum by drawing a line from the Basion to the Opisthion (Basion-Opisthion reference line) and the position of the tonsils by drawing a perpendicular line from the inferior most point of the tonsils to the Basion-Opisthion reference line. Data analysis was performed using the Stata version 12 statistical software package.
From a sample of 127, 67 (52.8%) were female and 60 (47.2%) were male. The median age was 28 years (Inter Quartile Range 1 - 75). The average anteroposterior diameter of the foramen magnum and cerebellar tonsillar position were 36.5mm (range; 23.4mm - 47.6mm) and 0.5mm (range; -14.5mm – 13.1mm) respectively. Trends in the means of the position of the cerebellar tonsils showed that the tonsils were below the foramen magnum from ages 1-4 years ascending to a position above the foramen magnum by age 5years and reaching their highest point between 12-17 years then gradually descending again. Trends in the means of the AP diameter of the foramen magnum showed an increase in the size of the foramen magnum up to the beginning of adolescence at which point it remains almost constant. One unit (1mm) increase in the diameter of the foramen magnum resulted into a significant lowering of 0.13mm in the position of the cerebellar tonsils.
The study showed an association between cerebellar tonsillar position and the anteroposterior diameter of the foramen magnum with 1mm increase in the AP diameter of the foramen magnum causing a descent of 0.13mm of the cerebellar tonsils.