Brain herniation occurs when increased pressure inside the skull forces brain tissue to shift from its normal position. This medical emergency requires immediate intervention due to its potential for severe neurological damage or fatality.
The Brain’s Anatomy and Pressure Dynamics
The human skull is a rigid, enclosed space containing the brain, cerebrospinal fluid (CSF), and blood. These components maintain a delicate balance of volume and pressure, known as intracranial pressure (ICP). Normally, the skull’s fixed volume means that an increase in one component, such as brain tissue swelling or excess fluid, must be compensated by a decrease in another to keep ICP stable. If this compensatory mechanism is overwhelmed, ICP can rise significantly. This high pressure can force brain tissue to move from an area of higher pressure to an area of lower pressure, often through the dura mater’s internal partitions or openings.
Types of Brain Herniation
Brain herniation is classified by where the tissue shifts.
Subfalcine Herniation
Subfalcine herniation, also known as cingulate herniation, involves the cingulate gyrus being pushed under the falx cerebri, a fold of dura mater separating the cerebral hemispheres. This can lead to a midline shift, potentially compressing the anterior cerebral artery and causing leg weakness.
Transtentorial Herniation
Transtentorial herniation occurs when brain tissue moves across the tentorium cerebelli, a dural fold separating the cerebrum from the cerebellum. This type includes uncal and central herniation. In uncal herniation, a part of the temporal lobe (the uncus) is squeezed downward through the tentorial notch, compressing the third cranial nerve, which can lead to pupil dilation on the affected side, and potentially the brainstem. Central herniation involves a downward shift of both temporal lobes and the diencephalon through the tentorial notch, which can compress the brainstem and cause severe complications.
Tonsillar Herniation
Tonsillar herniation, or downward cerebellar herniation, happens when the cerebellar tonsils are displaced downward through the foramen magnum, the opening at the base of the skull where the spinal cord connects. This movement can compress the brainstem, specifically the medulla, which controls vital functions like breathing and heart rate.
Transcalvarial Herniation
Transcalvarial herniation, or external herniation, involves brain tissue protruding through a defect in the skull, such as a fracture or a surgical opening.
Causes of Brain Herniation
Brain herniation results from conditions that significantly increase intracranial pressure (ICP). Traumatic brain injury (TBI) is a frequent cause, often leading to bleeding within the skull, such as epidural or subdural hematomas, or widespread brain swelling. These collections of blood or fluid exert mass effect, raising pressure.
Strokes, both ischemic and hemorrhagic, can also cause brain swelling and increased pressure. Brain tumors, whether primary or metastatic, act as space-occupying lesions, directly increasing intracranial volume and pressure. Infections like meningitis or encephalitis can lead to inflammation and swelling of brain tissues, contributing to elevated ICP.
Hydrocephalus, an abnormal accumulation of cerebrospinal fluid (CSF) within the brain’s ventricles, also increases pressure. This buildup can result from an imbalance in CSF production and absorption, or a blockage in its flow. Other causes include brain abscesses and swelling after radiation therapy or due to oxygen deprivation.
Recognizing the Signs
Recognizing the signs of brain herniation is urgent, as prompt medical attention significantly influences outcomes. Symptoms often begin with general indicators of increased intracranial pressure, including severe headache, nausea, and vomiting.
As the condition progresses, a lowered level of consciousness becomes apparent, ranging from drowsiness to coma. Specific neurological changes can also emerge, depending on the compressed brain area. These include changes in pupil size and reactivity to light, such as one or both pupils becoming widely dilated and unresponsive.
Abnormal breathing patterns, an irregular or slow pulse, and high blood pressure are concerning signs, collectively known as Cushing’s reflex, indicating brainstem compression. Patients may also exhibit abnormal posturing, such as decorticate or decerebrate rigidity. Loss of reflexes, seizures, or cardiac arrest can occur in severe cases.
Diagnosis and Medical Management
Diagnosing brain herniation requires rapid assessment to initiate swift treatment. Clinical examination, including a neurological evaluation, helps identify changes in alertness, pupil responses, and motor function. Imaging studies are critical for confirming a diagnosis and determining the type and extent of herniation.
Computed tomography (CT) scans and magnetic resonance imaging (MRI) of the head are commonly used to visualize brain tissue shifts, identify underlying causes like hematomas or tumors, and assess swelling. In some cases, monitoring intracranial pressure directly by placing a device into the brain’s fluid-filled spaces may be necessary to guide treatment.
Medical management focuses on emergency measures to reduce intracranial pressure and address the underlying cause. Treatment often involves elevating the head of the bed to help reduce pressure, administering medications such as osmotic agents like mannitol or hypertonic saline to draw fluid out of the brain, and sometimes using sedatives to calm the patient and reduce metabolic demand. Mechanical ventilation may be employed to control breathing and temporarily reduce carbon dioxide levels. Surgical interventions, such as removing blood clots, draining excess cerebrospinal fluid, or performing a decompressive craniectomy (removing part of the skull), may be necessary to relieve pressure and prevent further damage.