Uncal herniation is a subtype of transtentorial downward brain herniation, usually related to cerebral mass effect increasing the intracranial pressure.
In uncal herniation, the uncus and the adjacent part of the temporal lobe glide downward across the tentorial incisura compressing the brainstem and the posterior cerebral arteries in the ambient cistern. Uncal herniation may be unilateral or bilateral 1,2.
Uncal herniation occurs secondary to large mass effect (that can occur from traumatic or non-traumatic haemorrhage, malignancy, etc.) that will lead to increased intracranial pressure and herniation.
Uncal herniation can be suggested on CT, however, MRI is the gold standard.
Features of unilateral descending tentorial herniation include:
- medial displacement of the uncus and parahippocampal gyrus of the temporal lobe
- medial displacement of the temporal horn of the lateral ventricle
- mass effect and obliteration of the suprasellar cistern (ipsilateral)
- effacement of all basal cisterns
- widening of cerebellopontine angle (ipsilateral)
- asymmetrical inferior midbrain displacement and effacement
- midbrain haemorrhage on the same side
- inferomedial displacement of posterior communicating and posterior cerebral arteries
Bilateral transtentorial herniation:
- occurs due to extensive mass effect or severe trauma, less common
- both temporal lobes herniated into tentorial incisura
- complete obliteration of suprasellar cistern
- midbrain effaced and displaced inferiorly
Treatment and prognosis
Uncal herniation carries a bad prognosis due to the direct compression of the vital midbrain centres. They often require emergency neurosurgical decompression.
- extensive brainstem ischemia
- Duret haemorrhage
- contralateral midbrain compressed against the tentorium may cause Kernohan phenomenon
- compression of the ipsilateral posterior cerebral artery will result in ischemia of the visual cortex with resultant homonymous hemianopsia
- if uncal herniation is diagnosed, the referring physician should be notified immediately, because of its life-threatening nature
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- 2. Harnsberger HR, Osborn AG, Ross J et-al. Diagnostic and Surgical Imaging Anatomy. Lippincott Williams & Wilkins. (2006) ISBN:1931884293. Read it at Google Books - Find it at Amazon
- 3. Brant WE, Helms C. Fundamentals of Diagnostic Radiology. Lippincott Williams & Wilkins. (2012) ISBN:1608319113. Read it at Google Books - Find it at Amazon