Uncal Herniation

What is Uncal herniation?

Uncal herniation occurs when the uncus, the anteromedial part of the temporal lobe, is displaced downward through the tentorial notch. The tentorial notch is the opening in the tentorium through which the brainstem passes. This movement compresses the midbrain and is a critical example of brain herniation, where rising intracranial pressure (ICP) displaces brain tissue.

Brain herniations include several patterns of tissue displacement, and uncal herniation is a subtype of descending transtentorial herniation. This puts pressure on the brainstem, making the pattern particularly prone to injuring the third cranial nerve. Any form of cerebral hernia is a medical emergency because it can compress the brainstem, cranial nerves, and major vessels, leading to irreversible injury or death.

What causes Uncal herniation?

Rapid expansion of a mass within the skull drives the medial temporal lobe downward through the tentorial notch. This explains why an acute subdural hematoma often results in significant uncal herniation. Among the various types of brain herniation, uncal herniation is typically associated with unilateral supratentorial mass effect. Cerebral herniation begins when rising intracranial pressure overwhelms compensatory mechanisms and displaces brain tissue across rigid intracranial openings.

Major risk factors include traumatic bleeding, neoplasms, abscesses, and large strokes accompanied by significant swelling. Clinicians classify the different types of herniation by the direction and location of tissue displacement, but all clinically significant forms require urgent recognition.

What are the signs and symptoms associated with Uncal herniation?

The earliest cerebral herniation signs often include an ipsilateral dilated pupil from oculomotor nerve compression, with impaired eye movement that can produce a classic down-and-out position. As herniation progresses, brainstem compression can produce weakness on the opposite side of the body. Rising pressure eventually triggers a triad of symptoms known as the Cushing reflex: high blood pressure, a slow heart rate, and irregular breathing.

Herniation signs, such as fixed, non-reactive pupils and motor deficits, demand immediate intervention. Identifying early signs of brain herniation, especially anisocoria or a declining level of consciousness, is vital because the window for reversal is narrow.

How is Uncal herniation diagnosed?

A noncontrast CT scan of the head is the preferred tool to quickly reveal mass effect and the crowding of the suprasellar cisterns. CT may show medial displacement of the uncus, effacement of the suprasellar or basal cisterns, and mass effect on the midbrain. If the patient is stable, an MRI can provide a more detailed characterization of the underlying lesion.

A lumbar puncture is contraindicated when increased intracranial pressure or mass effect is suspected, because removing spinal fluid can accelerate the downward shift of a cerebral hernia. Instead, clinicians rely on continuous neurological monitoring of pupil size and motor strength to track the patient’s status while awaiting neurosurgical intervention.

How is Uncal herniation treated?

Initial treatment focuses on stabilizing the patient by securing the airway and carefully managing blood pressure. Hyperosmolar therapy, using mannitol or hypertonic saline, helps lower intracranial pressure. Sedation and brief periods of hyperventilation may also be used to temporarily reduce intracranial pressure while definitive treatment is arranged.

Definitive uncal herniation treatment depends on the cause and may include hematoma evacuation, resection of a mass lesion, ventricular drainage, or decompressive craniectomy. These procedures reduce mass effect, lower ICP, and relieve compression of the brainstem and surrounding neurovascular structures. In some cases, a drain may be placed in the ventricles to relieve pressure caused by trapped fluid. The goal is always to preserve cerebral perfusion while preventing further swelling.

What are the most important facts to know about Uncal herniation?

• Uncal herniation is a life-threatening displacement of the medial temporal lobe through the tentorial notch, usually caused by severe supratentorial mass effect.
• Key cerebral herniation signs include a dilated pupil on the side of the injury and progressive weakness on the opposite side.
• Noncontrast head CT is the preferred emergent imaging test to identify mass effect, herniation, and possible surgical causes.
• Uncal herniation treatment may require airway support, hyperosmolar therapy to lower ICP, and urgent neurosurgical intervention to relieve the underlying mass effect.