Brain herniation is a life-threatening medical emergency that occurs when brain tissue is physically squeezed and displaced from its normal anatomical compartment inside the skull. This abnormal shifting happens when severe swelling or a mass causes a dangerous pressure buildup within the rigid confines of the head. The displacement can compress vital structures, leading to catastrophic consequences like respiratory arrest and death if not treated immediately.
The Role of Intracranial Pressure
The human skull is a fixed, bony structure that contains three main components: brain tissue, cerebrospinal fluid (CSF), and blood. Since the skull cannot expand after infancy, the total volume of these three elements must remain relatively constant. An increase in the volume of any one component must be compensated for by a decrease in one or both of the others.
When the body’s natural compensatory mechanisms are overwhelmed, the pressure inside the skull, known as intracranial pressure (ICP), begins to rise dramatically. Normal ICP ranges between 7 and 15 mm Hg, but pressure above 20 mm Hg is considered elevated and hazardous. This excessive pressure forces the soft brain tissue to move across the rigid internal membranes and openings within the skull, defining brain herniation.
Specific Types of Brain Displacement
The location of the brain’s displacement determines the specific type of herniation, each leading to distinct patterns of injury and symptoms.
Uncal Herniation
Uncal herniation, also known as descending transtentorial herniation, is a common type where a part of the temporal lobe, called the uncus, is pushed downward. This movement forces the tissue through the tentorial notch, often compressing the third cranial nerve, which controls eye movement and pupil constriction.
Tonsillar Herniation
Tonsillar herniation is considered one of the most dangerous types. Pressure in the lower part of the brain forces the cerebellar tonsils downward through the foramen magnum, the opening at the base of the skull. This action severely compresses the lower brainstem, which regulates breathing and heart rate.
Cingulate Herniation
Cingulate, or subfalcine, herniation is the most common type and involves the innermost part of the frontal lobe shifting sideways under the falx cerebri, the membrane separating the two hemispheres. While often less immediately life-threatening, cingulate herniation can still interfere with blood flow.
Triggers and Warning Signs
Brain herniation is an end-stage complication caused by underlying conditions that lead to severe, uncontrolled ICP. The most frequent triggers include severe traumatic brain injury, which causes widespread brain swelling, and large strokes (ischemic or hemorrhagic). Other causes involve brain tumors, which act as expanding masses, and hydrocephalus, a buildup of excess cerebrospinal fluid.
The clinical presentation of rising ICP progresses from general symptoms to specific, ominous signs. Early warning signs can be vague, such as a severe, persistent headache, nausea, and vomiting. As pressure becomes more severe, patients show an altered mental status, progressing from drowsiness to confusion and ultimately to coma.
The progression to true herniation is signaled by late-stage signs that indicate severe brainstem compression. One sign is a fixed and dilated pupil, resulting from compression of the third cranial nerve. Another indicator is Cushing’s Triad, which includes high blood pressure with a widening pulse pressure, a slowed heart rate (bradycardia), and an irregular breathing pattern. These findings require immediate emergency medical intervention.
Diagnosis and Immediate Interventions
When brain herniation is suspected based on a patient’s deteriorating neurological exam, immediate diagnosis and intervention are essential. The primary diagnostic tool is a rapid computed tomography (CT) scan of the head. This imaging is fast and can quickly identify the cause of the pressure, such as a large blood clot or tumor, and visualize the physical shift and compression of the brain structures.
The immediate goal of treatment is to rapidly reduce the dangerous ICP. Medications called osmotic agents, such as mannitol or hypertonic saline solution, are administered to draw excess fluid out of the brain tissue. Doctors may also use mechanical ventilation to manage the patient’s breathing, temporarily lowering carbon dioxide levels to cause blood vessel constriction in the brain, which helps lower ICP.
Surgical Interventions
In many cases, surgical interventions are necessary to relieve the pressure directly. A neurosurgeon may insert an external ventricular drain (EVD) to remove excess CSF, providing immediate decompression. For mass lesions like hematomas or tumors, surgery to remove the offending mass is the definitive treatment. A procedure called a decompressive craniectomy may also be performed, which involves removing a section of the skull bone to allow the swollen brain tissue to expand outward, reducing pressure on vital brainstem structures.