Uncal Herniation
In uncal herniation, a common subtype of transtentorial herniation, the innermost part of the temporal lobe, the uncus, can be squeezed so much that it moves towards the tentorium and puts pressure on the brainstem, most notably the midbrain. The tentorium is a structure within the skull formed by the meningeal layer of the dura mater. Tissue may be stripped from the cerebral cortex in a process called decortication.
The uncus can squeeze the third cranial nerve, which may affect the parasympathetic input to the eye on the side of the affected nerve, causing the pupil of the affected eye to dilate and fail to constrict in response to light as it should. Pupillary dilation often precedes the somatic motor effects of cranial nerve III compression, which present as deviation of the eye to a "down and out" position due to loss of innervation to all ocular motility muscles except for the lateral rectus (innervated by cranial nerve VI) and the superior oblique (innervated by cranial nerve IV). The symptoms occur in this order because the parasympathetic fibers surround the motor fibers of CNIII and are hence compressed first.
Compression of the ipsilateral posterior cerebral artery will result in ischemia of the ipsilateral primary visual cortex and contralateral visual field deficits in both eyes (contralateral homonymous hemianopsia).
Another important finding is a false localizing sign, the so called Kernohan's notch, which results from compression of the contralateral cerebral crus containing descending corticospinal and some corticobulbar tract fibers. This leads to ipsilateral (same side as mass) hemiparesis (as these tracts are above their decussation where they are compressed). Since the corticospinal tract predominately innervates flexor muscles, extension of the leg may also be seen. With increasing pressure and progression of the hernia there will be distortion of the brainstem leading to Duret hemorrhages (tearing of small vessels in the parenchyma) in the median and paramedian zones of the mesencephalon and pons. The rupture of these vessels leads to linear or flamed shaped hemorrhages. The disrupted brainstem can lead to decorticate posture, respiratory center depression and death. Other possibilities resulting from brain stem distortion include lethargy, slow heart rate, and pupil dilation. Uncal herniation may advance to central herniation.
A complication of an uncal herniation is a Duret hemorrhage. This results in the midbrain and pons being compressed, possibly causing damage to the reticular formation. If untreated, death will ensue.
Read more about this topic: Brain Herniation, Classification