It is a secondary sign of significant intra-cranial mass effect. Any intra-axial or extra-axial lesion (e.g. tumour, haemorrhage, stroke, abscess) exerting mass effect on the brain parenchyma can displace the posterior cranial fossa structures inferiorly. In doing so the brainstem is compressed against the clivus thereby altering the vital life-sustaining functions of the pons and medulla, such as the respiratory and cardiac centres.
Non-life threatening tonsillar descent can bee seen in conditions such as Chiari malformations.
Tonsillar herniation is seen on CT and MRI as effacement of the CSF cisterns surrounding the brainstem and as inferior descent of the cerebellar tonsils below the foramen magnum.
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