Syndrome of transtentorial herniation: is vertical displacement necessary?

MRI from a comatose patient with a massive acute subdural haematoma showed most of the features of transtentorial herniation described in the classic pathology literature. In addition to encroachment on the perimesencephalic cisterns, infarction in the anterior and posterior cerebral artery territories, ischaemic change in the lower diencephalon, and ventricular enlargement were visualised. Despite the clinical syndrome and these secondary changes due to compression, there was only approximately 2 mm of downward displacement of the upper brainstem compared with 13 mm horizontal displacement. Although tissue shifts adjacent to the tentorial aperture cause brainstem and vascular compression, these changes may occur with minimal downward herniation.