Brain herniation occurs when pressure inside the skull (intracranial pressure) increases, and displaces the brain tissues. This is commonly the result of brain swelling (cerebral edema) from a head injury, or may be the result of space-occupying lesions such as primary brain tumor, metastatic brain tumor, or other lesions within the brain. Brain herniation may also occur with bacterial meningitis.

The most common type of brain herniation occurs when a portion of the temporal lobe is displaced (uncal herniation), which compresses cranial nerve III, the midbrain, and the posterior cerebral artery, leading to coma and respiratory arrest (breathing stops).

Another critical type of brain herniation occurs when part of the cerebellum is displaced through the foramen magnum (the area where the spine joins the skull). This compresses the brainstem and causes death from lack of breathing (the respiratory center in the brain stem is destroyed) and circulation. Other areas of the brain may be herniated, but they occur much more rarely than uncal herniation or foramen magnum herniation (also known as tonsillar herniation, since the cerebellar tonsils are what herniate through the foramen magnum).

• Reduced alertness (decreased consciousness) getting worse (progressive)
• Coma or loss of consciousness
• Irregular breathing pattern
• Respiratory arrest (no breathing)
• Irregular pulse
• Sensations of feeling the heart beat (palpitations) with an irregular pattern
• Cardiac arrest (no pulse)
• Eyes, pupils different size

Eye examination may show CN III dysfunction (see cranial mononeuropathy III - compression type), including paralysis of the eye muscles (causing the eyes to deviate) and eyelid drooping, and a dilated, unresponsive pupil of one eye prior to development of progressive coma. Papilledema (swelling of the optic nerve) may be visible on examination of the eyes as a later finding. Other signs of increased intracranial pressure are often present. Signs of obstructive hydrocephalus may precede brain herniation in some cases.

Brain herniation is a medical emergency! The goal of treatment is preservation of life.

Preventing and treating increased intracranial pressure prevents brain herniation. If obstructive hydrocephalus is present, placing an external ventricular drain may prevent brain herniation. Reducing or removing tumors by surgery may reduce intracranial pressure and prevent brain herniation.

Corticosteroids such as dexamethasone can reduce brain swelling (cerebral edema) and intracranial pressure, especially in cases where tumor edema is involved; they may prevent imminent brain herniation. Mannitol or other osmotic diuretics may be given to reduce intracranial pressure.

Deliberate hyperventilation may also be used to reduce intracranial pressure. This often includes placement of a tube in the airway (endotracheal intubation) and mechanically ventilating (forced breathing) at a rapid rate.

Once herniation of the temporal lobe or the cerebellum occurs, death is often inevitable. Herniation of other brain areas has a variable prognosis (probable outcome).

• Permanent and profound neurologic losses
• Death

If decreased alertness or other symptoms develop suddenly, particularly if there is a known head injury or brain lesion, go to the emergency room or call the local emergency number (such as 911). Usually it will be someone other than the patient who will recognize the problem and seek assistance.

Prompt treatment of increased intracranial pressure and related disorders may reduce the risk of brain herniation.