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Uncal herniation (a form of transtentorial herniation) is the downward displacement of the medial temporal lobe (uncus and parahippocampal gyrus) through the tentorial notch, compressing midbrain structures.

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It is a life-threatening complication of raised intracranial pressure (ICP).

Etiopathology

Causes:

Supratentorial mass lesions (epidural/subdural hematoma, intracerebral hematoma, tumor, abscess).
Generalized cerebral edema (trauma, hypoxia, encephalitis).

Mechanism:

The temporal lobe is forced medially and downward against the tentorial edge.
This leads to compression of midbrain, cranial nerves, and vessels, causing secondary ischemic injury and hemorrhages (e.g., Duret hemorrhages in the brainstem).

Pathophysiological Consequences

| Compression of CN III (Oculomotor nerve) | • Ipsilateral fixed, dilated pupil (early sign). • Ptosis, impaired extraocular movements. | | --- | --- | | Compression of ipsilateral cerebral peduncle | • Contralateral hemiparesis (corticospinal tract involvement). • Sometimes paradoxical ipsilateral hemiparesis (Kernohan’s notch phenomenon) when the opposite peduncle is compressed against the tentorial edge. | | Brainstem compression | • Altered consciousness (reticular activating system involvement). • Abnormal respiration patterns. | | Vascular compromise | • Posterior cerebral artery compression → occipital lobe infarction (contralateral homonymous hemianopia). • Basilar artery perforator stretching → Duret hemorrhages (midbrain/pons). |

Clinical Features

Early	Ipsilateral pupillary dilation, impaired adduction (CN III palsy).
Progressive	Hemiparesis, hemianopia, decerebrate posturing.
Late	Coma, abnormal respiration, hemodynamic instability, death.

Radiology

Modality	Imaging features
CT	• Medial temporal lobe (uncus) displaced across tentorial edge.
• Effacement of ipsilateral basal cisterns (especially suprasellar and perimesencephalic cisterns).
• Midline shift, compression of lateral ventricle, contralateral hydrocephalus due to aqueduct obstruction.
• Small hemorrhages in brainstem (Duret hemorrhages) may be seen as hyperdense foci in pons/midbrain.
MR	• More sensitive for cisternal effacement, CN III compression, and brainstem changes.
• Susceptibility-weighted imaging (SWI) may show brainstem hemorrhages.
• Perfusion imaging can demonstrate vascular compromise.

https://case.edu/med/neurology/NR/herniation01 axial.htm