"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 >3 mm below foramen magnum level (McRae line)."
"It is a secondary sign of markedly raised intracranial pressure. The skull is a rigid structure and an increase in volume of the contents is matched by a corresponding decrease in CSF volume, intravascular blood volume and/or brain herniation; see Monro-Kellie hypothesis. Common causes of raised intracranial pressure are tumour, haemorrhage, oedema, stroke or abscess. Tonsillar descent is often a preterminal event as the brainstem is compressed against the clivus, affecting the vital life-sustaining functions of the pons and medulla, such as the respiratory and cardiac centres."
"It is a secondary sign of markedly raised intracranial pressure. The skull is a rigid structure and an increase in volume of the contents is matched by a corresponding decrease in CSF volume, intravascular blood volume and/or brain herniation; see Monro-Kellie hypothesis. Common causes of raised intracranial pressure are tumour, haemorrhage, oedema, stroke or abscess. Tonsillar descent is often a preterminal event as the brainstem is compressed against the clivus, affecting the vital life-sustaining functions of the pons and medulla, such as the respiratory and cardiac centres."