Brain herniation is the displacement of brain tissue from one intracranial compartment to another. It is the most severe and final complication of dangerously high intracranial pressure (ICP).

To understand why herniation occurs, it is essential to appreciate the Monro-Kellie doctrine, a fundamental principle of neuro-critical care. This doctrine states that the skull is a rigid, closed box with a fixed internal volume. This box is completely filled with three components:

1. Brain tissue
2. Blood (within the blood vessels)
3. Cerebrospinal fluid (CSF), the clear fluid that bathes the brain and spinal cord

In a healthy person, these three components exist in a state of delicate equilibrium. If the volume of one component increases, the volume of one or both of the others must decrease to keep the overall pressure inside the box stable. For example, the body can temporarily compensate for a small amount of swelling by shunting some CSF out of the skull.

Brain herniation occurs when these compensatory mechanisms are overwhelmed. When a new mass (like a blood clot) or widespread swelling develops, the pressure inside the rigid skull skyrockets. With nowhere else to go, the only “soft” component, the brain tissue itself, is forced to move, squeezing through any available opening in the stiff dural membranes that separate the brain into compartments, or downward through the opening at the base of the skull.

Clinically, I’ve seen that brain herniation often presents suddenly, patients can go from confusion to coma within minutes. It’s one of the few true neurological emergencies where every second counts.

Brain herniation is caused by uncontrolled, elevated intracranial pressure. The underlying causes are any medical conditions that can create a “mass effect” or cause widespread swelling within the fixed volume of the skull.

The most common causes include:

• Traumatic Brain Injury (TBI): This is a leading cause. A severe blow to the head can cause several types of problems that increase ICP.
  • Hematomas: A collection of blood from a ruptured vessel. This can be an epidural hematoma (between the skull and the dura), a subdural hematoma (under the dura), or an intracerebral hematoma (within the brain tissue).
  • Cerebral Edema: Widespread swelling of the brain tissue itself as a response to the injury.
• Hemorrhagic Stroke: Bleeding within the brain from a ruptured blood vessel, often caused by uncontrolled high blood pressure.
• Large Ischemic Stroke: A major stroke caused by a blocked artery can lead to the death of a large area of brain tissue, which then swells significantly.
• Brain Tumors: A large primary brain tumor or a metastatic tumor from another part of the body can grow and exert pressure on the surrounding brain.
• Intracranial Infections: A large brain abscess (a collection of pus) or severe inflammation from meningitis or encephalitis can cause massive swelling.
• Hydrocephalus: A blockage in the normal circulation of cerebrospinal fluid, causing it to build up and increase pressure.

In my experience, it’s the rapid increase in pressure that’s most dangerous. A slow-growing tumor might be asymptomatic for months, but a sudden bleed can herniate the brain in under an hour.

Brain herniation is not a disease you “get,” but rather the end-stage complication of one of the catastrophic brain events listed above. Thus, hernia risk factors are risk factors for these underlying conditions. This includes risks for TBI (e.g., motor vehicle accidents, falls, not wearing a helmet), risks for stroke (e.g., hypertension, diabetes, smoking), or having a known brain tumor or other neurological condition.

I’ve often seen patients deteriorate due to delayed recognition, the early symptoms of rising ICP may be vague, like headache or vomiting. But by the time herniation sets in, the situation becomes far more critical.

Neurosurgeons classify brain herniation syndromes based on the direction in which the brain tissue is being pushed and the anatomical structures involved. While there are several types, the most common include:

• Subfalcine Herniation: The most common type, where the cingulate gyrus on one side of the brain is pushed under a stiff dural fold called the falx cerebri to the other side.
• Transtentorial (Uncal) Herniation: This is a classic and highly dangerous type. The innermost part of the temporal lobe, the uncus, is squeezed downward through an opening called the tentorial notch. This directly compresses the brainstem and the third cranial nerve, which controls pupil constriction.
• Central Herniation: A downward shift of the entire brainstem.
• Tonsillar Herniation: The lowest part of the cerebellum, the cerebellar tonsils, are forced down through the foramen magnum, the large opening at the base of the skull. This directly compresses the lower brainstem (the medulla), which contains the centers that control breathing and heart rate.

Symptoms vary depending on the type and location of herniation, but most cases present with signs of increased ICP and progressive neurological decline.

As the intracranial pressure rises to critical levels, a classic set of signs known as Cushing’s Triad may appear. This triad is a late and ominous sign indicating severe pressure on the brainstem. It includes:

1. High Blood Pressure (Hypertension), with a widening pulse pressure (a growing difference between the systolic and diastolic numbers).
2. An Irregular and Slowed Breathing Pattern.
3. A Slow Heart Rate (Bradycardia).

Other key neurological signs that point to a specific herniation syndrome include:

• A Unilateral Dilated Pupil: One pupil becomes large, “fixed,” and does not react to light. This “blown pupil” is the hallmark sign of an uncal herniation compressing the third cranial nerve.
• Loss of Consciousness: A rapid decline in the patient’s level of consciousness, progressing from lethargy and confusion to stupor and finally coma.
• Abnormal Posturing: As the brainstem is compressed, the patient may exhibit involuntary, reflexive motor posturing. This can be decorticate posturing (abnormal flexion of the arms) or, as the damage descends, decerebrate posturing (abnormal extension of the arms and legs).
• Paralysis: Weakness or paralysis on one side of the body.
• Respiratory Arrest: As the respiratory center in the brainstem is compressed, the patient will stop breathing, followed by cardiac arrest and death.

In my clinical experience, a sudden blown pupil in a previously conscious patient is a major red flag, it often signals uncal herniation. Time is critical for decompression in such cases.

A diagnosis of brain herniation is made urgently based on clinical signs and neuroimaging. It is a time-critical diagnosis.

• Neurological Examination: An emergency room physician or neurosurgeon will perform a rapid neurological assessment, checking the patient’s Glasgow Coma Scale (GCS) score, pupil size and reactivity, and looking for abnormal posturing. The presence of a blown pupil in a comatose patient is often enough to make a presumptive diagnosis and trigger an emergency intervention.
• Neuroimaging: An urgent CT scan of the head is the primary imaging tool. It is fast and can immediately show the underlying cause, such as a large hematoma or swelling. It can visualize the physical shifting and compression of the brain structures. An MRI provides more detail but is usually too slow for this type of emergency.

In cases I’ve seen, a head CT is the first and most essential tool, it can confirm herniation in minutes, guiding the surgical team’s next steps.

Brain herniation is a medical emergency. Treatment focuses on reducing intracranial pressure immediately and addressing the underlying cause.

1. Immediate Medical Management

While the operating room is being prepared, a team in the ICU or emergency room will perform several life-saving interventions to temporarily lower the ICP.

• Intubation and Hyperventilation: The patient is placed on a breathing machine (ventilator). Temporarily increasing the breathing rate can lower carbon dioxide levels in the blood, which causes cerebral blood vessels to constrict and can briefly reduce ICP.
• Osmotic Therapy: Highly concentrated intravenous medications, such as mannitol or hypertonic saline, are given. These drugs work by drawing excess water out of the brain tissue and into the bloodstream, thereby reducing brain swelling.

2. Emergency Neurosurgery

Surgical intervention is the definitive treatment to relieve the pressure.

• Evacuation of Mass: If the herniation is caused by a blood clot (hematoma), the primary procedure is an emergency craniotomy to open the skull and surgically evacuate the clot.
• Decompressive Craniectomy: If the cause is widespread brain swelling, a neurosurgeon may perform a decompressive craniectomy. This involves removing a large piece of the skull bone and leaving it off for days or weeks. This creates an opening that allows the swollen brain to expand outward, relieving the deadly internal pressure. The bone flap is stored and can be replaced weeks or months later after the swelling has resolved.
• CSF Drainage: A catheter can be placed into one of the brain’s ventricles (an external ventricular drain, or EVD) to drain excess cerebrospinal fluid.

The prognosis for a patient with brain herniation is extremely poor and depends on the severity and duration of the compression. Even with successful surgery, many patients who survive are left with severe, permanent neurological disabilities.

In my experience, timing is everything, patients who receive decompression within the golden hour can sometimes recover fully, while delays often lead to irreversible damage or death.

Brain herniation represents the most severe and final stage of uncontrolled pressure inside the skull. It is a catastrophic event where the brain is physically crushed and squeezed, leading to the rapid destruction of the vital structures that control our most basic life functions. It is caused by life-threatening conditions like severe head injuries, brain hemorrhages, and large tumors. The sudden appearance of a fixed and dilated pupil or abnormal posturing in a deteriorating patient is a dire sign that requires immediate and aggressive neurosurgical intervention. This devastating condition underscores the critical importance of measures that prevent its primary causes, such as wearing helmets to prevent head injuries and controlling blood pressure to prevent strokes and highlights the absolute urgency of seeking emergency care for any major head trauma or sudden, severe neurological symptoms.

American Association of Neurological Surgeons (AANS). (n.d.). Traumatic Brain Injury. Retrieved from https://www.aans.org/Patients/Neurosurgical-Conditions-and-Treatments/Traumatic-Brain-Injury

National Institute of Neurological Disorders and Stroke (NINDS). (2023). Brain and Spinal Tumors. Retrieved from https://www.ninds.nih.gov/health-information/disorders/brain-and-spinal-tumors

The Merck Manual Professional Version. (2023). Brain Herniation. Retrieved from https://www.merckmanuals.com/professional/neurologic-disorders/coma-and-impaired-consciousness/brain-herniation