Figure 3.

A large supratentorial lesion may cause uncal herniation, where the hippocampal gyrus is forced into the posterior fossa, resulting in compression of the midbrain and the ipsilateral third nerve. Uncal herniation may also cause Kernohan's notch phenomenon, where the contralateral corticospinal tract becomes compressed against the tentorium cerebelli as it passes through the midbrain. A large supratentorial mass may also result in midline shift and subfalcine herniation, which may cause a stroke in the ipsilateral anterior cerebral artery distribution. Finally, downward herniation can cause a pontine hemorrhage and herniation of the cerebellar tonsils, resulting in dysfunction of the medulla.