Uncal herniation

Uncal herniation is a subtype of transtentorial downward brain herniation, usually related to cerebral mass effect increasing the intracranial pressure.

​Clinical presentation

Abnormal posture and poor GCS. There may be pupillary dilation and loss of light reflex due to direct compression of the oculomotor nerve. 

Pathology

In uncal herniation, the uncus and 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.

Aetiology

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.

Radiographic features

Uncal herniation can be suggested on CT, however MRI is the gold standard. Mass

Features of unilateral descending tentorial herniation include

• medial displacement of the uncle and parahippocampal gyrus of temporal lobe
• medial displacement of temporal horn of lateral ventricle 
• mass effect and obliteration of the suprasellar cistern will be seen. The midbrain is displaced and effaced. 

  MRI

  Unilateral transtentorial herniation:

  • more common (ipsilateral)
  • uncus and medial temporal lobe displaced medially causing effacement of the suprasellar cistern if mildall basal cisterns
  • hippocampus will obliteratewidening of cerebellopontine angle (ipsilateral)
  • asymmetrical inferior midbrain displacement and effacement
  • midbrain haemorrhage on the quadrigeminal cistern in moderate transtentorial herniationsame 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. 

  Complications 

  • extensive brainstem ischemia
  • Duret haemorrhage
  • contralateral midbrain compressed against tentorium, may cause Kernohan phenomenon
  • compression of the ipsilateral posterior cerebral artery will result in ischemia of the visual cortex with resultant homonymous hemianopsia

  Practical points

  • if uncal herniation is diagnosed, the referring physician should be notified immediately, because of its life-threatening nature