Expansion
Subfalcine, uncal, central and tonsillar
Expansion
- Subfalcine (cingulate): cingulate gyrus under the falx. May compress the anterior cerebral artery, causing leg weakness
- Uncal (transtentorial): medial temporal lobe over the tentorial edge. Compresses the oculomotor nerve, giving an ipsilateral fixed dilated pupil and then a third nerve palsy, plus the posterior cerebral artery, causing occipital infarction
- Central: downward displacement of the diencephalon, causing progressive deterioration in consciousness and Duret haemorrhages in the brainstem
- Tonsillar (coning): cerebellar tonsils through the foramen magnum, compressing the medulla. Causes respiratory and cardiac arrest
Kernohan notch phenomenon is the classic false localising sign: the contralateral cerebral peduncle is compressed against the tentorium, so the hemiparesis appears ipsilateral to the lesion, misleading the unwary.
Another false localising sign is a sixth nerve palsy from raised intracranial pressure, due to its long intracranial course.
The practical point is that these syndromes are the mechanism by which raised intracranial pressure kills, and they explain why lumbar puncture is contraindicated when there is a mass lesion with raised pressure.