Brain herniation is a serious medical condition where brain tissue shifts from its normal position inside the skull. This displacement occurs due to very high pressure within the skull, a medical emergency requiring immediate attention. The skull is a rigid structure, and any increase in its contents, such as brain swelling or a mass, elevates the pressure inside. When the brain, cerebrospinal fluid, and blood within the skull exceed their fixed volume, brain tissue can be forced into areas where it does not belong. This displacement can compress vital brain regions, leading to severe neurological dysfunction and potentially life-threatening outcomes.
Defining Brain Herniation
Brain herniation describes the abnormal displacement of brain tissue from its usual compartment within the skull to an adjacent space. This elevated pressure, known as intracranial pressure (ICP), can force brain tissue through natural openings or across the tough membranes that divide brain regions. The displaced brain tissue becomes compressed, leading to damage or disruption of its function. The severity of brain herniation depends on the amount of tissue displaced and the specific brain structures affected.
Underlying Causes
Brain herniation results from conditions that significantly increase intracranial pressure. Common causes include traumatic brain injury, which can lead to brain swelling or hematomas. Brain tumors, strokes (ischemic or hemorrhagic), and infections like meningitis or encephalitis also contribute to pressure buildup. Hydrocephalus, an excess accumulation of cerebrospinal fluid, can directly increase pressure. Other factors, such as brain abscesses or radiation therapy, can also cause swelling that precedes herniation.
Recognizing Specific Herniation Types
Brain herniation manifests in several distinct types, each defined by the specific brain region that shifts and the direction of its displacement. The signs and symptoms observed depend directly on which parts of the brain are compressed.
Cingulate Herniation
Also known as subfalcine herniation, this is the most common type. It occurs when the cingulate gyrus, located in the innermost part of the frontal lobe, is pushed under the falx cerebri, a membrane separating the two brain hemispheres. While this type may not directly compress the brainstem, it can interfere with blood vessels, potentially leading to leg weakness due to reduced blood flow to specific brain areas.
Uncal Herniation
Also called descending transtentorial herniation, this involves the uncus, a part of the temporal lobe, being forced downward and inward through a natural opening called the tentorial notch. This displacement can compress the oculomotor nerve, leading to an enlarged pupil on the same side as the herniation that may not react to light. As the condition progresses, it can affect consciousness and cause motor weakness on the opposite side of the body.
Central Herniation
This occurs when there is widespread swelling or pressure, causing the diencephalon and central parts of the temporal lobes to shift downward through the tentorial notch. This type often presents with a progressive decline in consciousness, changes in breathing patterns, and specific motor responses such as abnormal posturing. The compression can affect brainstem functions that control vital signs.
Tonsillar Herniation
This involves the cerebellar tonsils, located at the base of the brain, moving downward through the foramen magnum, the large opening at the bottom of the skull where the spinal cord connects. This movement can compress the brainstem, which regulates breathing and heart rate. Symptoms can include severe headache, neck stiffness, and irregularities in heart rate and breathing.
Upward Transtentorial Herniation
This happens when a mass in the lower back part of the brain pushes the cerebellum and brainstem upward through the tentorial notch. This can result in symptoms such as nausea, vomiting, headache, and difficulties with coordination. It may also lead to specific eye movement abnormalities.
Transcalvarial Herniation
This describes brain tissue protruding through a defect in the skull, often resulting from a severe head injury or a previous surgical procedure. The symptoms associated with this type are generally related to the underlying cause of the skull defect and the extent of the brain tissue involved.
Diagnosis and Immediate Intervention
Diagnosing brain herniation requires rapid assessment in a medical setting. Healthcare professionals typically utilize imaging tests like computed tomography (CT) scans or magnetic resonance imaging (MRI) to visualize the brain and identify any shifts in tissue or underlying causes such as swelling or bleeding. These scans help pinpoint the type and severity of herniation.
Once diagnosed, immediate interventions focus on reducing intracranial pressure to prevent further brain damage. Medical strategies include administering osmotic agents such as mannitol or hypertonic saline, which help draw fluid out of swollen brain tissue. Controlled hyperventilation can also be used to temporarily reduce pressure by constricting blood vessels in the brain.
Sedation and medications to control seizures may be used to stabilize the patient and reduce brain activity, thereby lowering metabolic demand and pressure. In some cases, a drain may be placed into the brain to remove excess cerebrospinal fluid. Surgical interventions are often necessary, such as evacuating a blood clot or tumor that is causing the pressure. A decompressive craniectomy, where a portion of the skull is temporarily removed, might be performed to provide more space for the swollen brain, often as a life-saving measure.