Tonsillar herniation is a type of cerebral herniation characterized by the inferior descent of the cerebellar tonsils below the foramen magnum. Clinically the presence of tonsillar herniation is often called coning.
The terminology of caudally displaced tonsils is discussed in the article on cerebellar tonsillar ectopia.
It is a secondary sign of markedly raised intracranial pressure. Any intra-axial or extra-axial lesion (e.g. tumor, hemorrhage, 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 centers.
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