Uncal herniation

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

Clinical presentation

• pupils and globe clinical features ³
  • initially, an ipsilateral dilated pupil that is unresponsive to light, signifying ipsilateral oculomotor nerve compression
  • may develop into bilaterally blown pupils due to compression of the mesencephalon and its parasympathetic nuclei
  • rarely, an isolated contralateral dilated pupil that is unresponsive to light may develop, signifying contralateral oculomotor nerve compression from midline shift
  • tonic lateral deviation may occur due to unopposed abducens nerve activity
  • ptosis may occur due to oculomotor nerve palsy (not paralysis of Müller's muscle)
  • vertical gaze palsy may occur after compression of the rostral interstitial nucleus of the medial longitudinal fasciculus
• altered mental state ³
  • compression of the reticular activating system of the mesencephalon leads to alteration in conscious state
• motor deficits ³
  • usually contralateral hemiparesis
  • in ~25% ipsilateral hemiparesis due to Kernohan phenomenon

Pathology

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. Compression of the posterior cerebral artery may lead to infarction

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. Masses are typically supratentorial.

Radiographic features

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. 

Complications 

• 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

Practical points

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