Brain Herniation Syndromes

Why herniation happens

The skull is a closed box containing brain tissue, blood and cerebrospinal fluid (CSF). If one of these increases, or a mass appears, the others can compensate only a little. Once that reserve is used up, pressure (intracranial pressure, ICP) rises steeply. Rigid structures divide the skull into compartments:

• the falx cerebri, a fold separating the two hemispheres
• the tentorium cerebelli, a shelf separating the upper brain from the cerebellum and brainstem
• the foramen magnum, the opening at the base of the skull where the brainstem meets the spinal cord

High pressure pushes brain tissue across or through these structures.

Types of brain herniation

Causes

• traumatic brain injury with bleeding or swelling
• epidural and subdural hematomas
• intracranial hemorrhage from high blood pressure or aneurysms
• large ischemic strokes causing cerebral edema, typically 2 to 5 days after the stroke
• brain tumors and metastases
• brain abscess
• hydrocephalus, a build-up of CSF
• severe liver failure, low sodium or lack of oxygen causing brain swelling
• a lumbar puncture performed when pressure is very high, which is why doctors scan first when this is a risk

Symptoms and signs

Early signs of raised pressure:

• worsening headache, often worse lying down and in the morning
• nausea and vomiting
• blurred or double vision
• confusion and drowsiness

Signs of herniation:

• a dilated, unreactive pupil, usually on the side of the mass (the third cranial nerve is compressed)
• weakness of the arm and leg
• decreasing level of consciousness
• abnormal posturing (decorticate or decerebrate)
• irregular breathing patterns
• Cushing triad: high blood pressure, slow heart rate and irregular breathing, a late and ominous sign
• seizures
• cardiac or respiratory arrest

Diagnosis

Herniation is a clinical emergency, recognized from examination including the Glasgow Coma Scale and pupils. An urgent CT scan of the head shows the cause, midline shift, compressed fluid spaces and herniation. MRI gives more detail when the patient is stable. ICP may be measured directly with a monitor inserted through the skull.

Emergency treatment

The aim is to reduce pressure immediately and treat the cause.

• Airway and breathing: intubation and controlled ventilation, keeping oxygen normal
• Head position: head of the bed raised to about 30 degrees, head in line to allow venous drainage
• Hyperosmolar therapy: IV mannitol or hypertonic saline to draw fluid out of the brain
• Brief hyperventilation: as a temporary bridge to definitive treatment
• Sedation, fever control, seizure control and maintaining blood pressure
• Steroids for swelling around tumors (not for head injury)
• Surgery: removing a blood clot or tumor, draining CSF with an external ventricular drain, or decompressive craniectomy, in which part of the skull is removed to give the swollen brain room

Outlook

The outcome depends on the cause, how quickly treatment starts and how far herniation has progressed. Herniation that has caused fixed, dilated pupils and loss of brainstem reflexes has a very poor prognosis, and herniation can lead to brain death. Rapid recognition of early warning signs gives the best chance of recovery.

Frequently asked questions

What is brain herniation?

Brain herniation happens when raised pressure inside the skull pushes brain tissue out of its normal position, across rigid folds or through the opening at the base of the skull. It compresses the brainstem and blood vessels and is a life-threatening emergency.

What are the signs of brain herniation?

Warning signs include worsening headache, vomiting, increasing drowsiness, confusion, a dilated pupil on one side, weakness of one side, abnormal posturing and irregular breathing. High blood pressure with a slow heart rate is a late sign.

What are the types of brain herniation?

The main types are subfalcine (under the central fold), uncal and central transtentorial (downward past the tentorium), tonsillar (through the base of the skull), upward herniation and transcalvarial herniation (through a skull defect).

Can you survive brain herniation?

Some people survive with rapid treatment, especially if pressure is relieved before brainstem damage occurs. Survivors may have lasting disability. Once herniation has progressed to fixed pupils and loss of brainstem function, survival is unlikely.

What causes brain herniation?

Common causes are head injuries with bleeding or swelling, brain hemorrhages, large strokes with swelling, brain tumors, abscesses and hydrocephalus. Anything that adds volume inside the skull can raise pressure enough to cause herniation.