Herniation Syndromes

Herniation occurs when the brain is subjected to pressure gradients that cause portions of it to flow from one intracranial compartment to another. Although the brain has substantial elasticity, the arteries and veins responsible for its blood supply are relatively fixed in space, producing a risk that brain shifts will cause the moving portions to lose their blood supply. In the case of lateral herniation, the hernia itself may compress or distort vessels, similarly disrupting blood flow. Herniation also appears to produce dysfunction in important white matter pathways, such as those connecting the MRF to the thalamus, probably via geometrical distortion of the pathways themselves. Extracerebral structures, particularly the third cranial nerve, are also susceptible to distortion when the brain shifts.

The classic herniation syndromes are well established in the clinical neurosciences; recent advances in imaging have changed some of our understanding of their pathological anatomy, but their clinical symptoms remain important.

Central Herniation Syndrome. Central herniation occurs when diffuse brain swelling (e.g., after trauma) or a centrally located mass causes the diencephalon to move caudally through the tentorial notch. Dysfunction of the reticular formation and cerebral hypoperfusion due to ICP elevation are the leading hypotheses explaining alteration of consciousness in this setting. Diencephalic dysfunction initially produces small reactive pupils because of loss of sympathetic output from the hypothalamus. At this stage, decorticate (flexor) posturing may be present spontaneously or is frequently elicited by noxious stimuli. As the midbrain begins to fail, the pupils enlarge to midposition, and posturing becomes decerebrate (extensor). Attempts to elicit horizontal eye movements may reveal failure of adduction with either cervico-ocular reflex (COR) or vestibulo-ocular reflex (VOR) testing; however, this is often only present briefly. Eventually, if the patient is breathing spontaneously, tidal volume decreases and the respiratory rhythm becomes irregular.

The initial cardiovascular response to diminished brain stem perfusion, regardless of etiology, is hypertension. In this setting, bradycardia is a reflex response to the systemic hypertension that is commonly seen in children but is less common in adults. There may be a tendency for bradycardia to occur more commonly in patients with posterior fossa masses, but this observation is confounded by the higher incidence of posterior fossa tumors in children. Respiratory disturbances, the third component of Cushing's triad, reflect the anatomical considerations discussed earlier and are not part of the same reflex system as the cardiovascular responses. In some clinical settings, such as head injury with diffuse brain swelling, spontaneous hyperventilation is commonly noted but its etiology remains obscure.

Lateral Mass Herniation (Uncal or Hippocampal Herniation) Syndrome. Although concepts of herniation due to an expanding lateral cerebral mass have changed in the past decade, there is a consistent sequence of physical findings that develops. The initial signs are usually those related to the mass itself (e.g., contralateral hemiparesis); as the diencephalon begins to shift away from the mass, consciousness begins to diminish, and an ipsilateral third cranial nerve palsy develops in about 85 percent of patients. When the third nerve is compressed against the tentorial notch, the pupillary fibers, located most peripherally in the nerve, are primarily damaged and the prominent sign is a large pupil. The remaining patients develop either simultaneous bilateral third nerve palsies or, on occasion, an isolated contralateral third nerve palsy. These findings presumably follow from the distortion of third nerve anatomy described earlier, although the bilateral disturbance may also relate to midbrain ischemia as the brain stem begins to move away from the basilar artery. Imaging studies (CT or MRI) performed early in this process do not show transtentorial movement of the temporal lobe; this feature occurs later in the process, accounting for its implication as the cause of third nerve dysfunction in autopsy studies. Early in this process, the most common finding on imaging studies is an enlargement of the perimesencephalic cistern ipsilateral to the mass.

As lateral displacement of the midbrain continues, the contralateral corticospinal tract (in the cerebral peduncle) is compressed against the edge of the tentorium. This produces an ipsilateral hemiplegia, called the Kernohan's notch phenomenon. If the herniation process continues, the diencephalon and the ipsilateral temporal lobe may actually herniate downward through the tentorial notch. Because the uncus (containing the amygdala) and the hippocampus are the most medial portions of the temporal lobe, they are the first structures to cross the tentorial edge; this accounts for the older terms of uncal or hippocampal herniation. Movement of the more posterior aspects of the medial temporal lobe may compress the posterior cerebral artery (or arteries), producing ischemia in the medial temporal lobe and sometimes in the occipital lobe.

Subfalcine Herniation Syndrome. Herniation of the medial frontal structures (e.g., the cingulate gyrus) beneath the falx is commonly observed in patients with frontal lobe masses. Most of these patients present with signs related either to the mass itself or to the global increase in ICP that accompanies it. Rarely, this subfalcine herniation causes ischemia in the distribution of the anterior cerebral arteries.

Cerebellar Tonsillar Herniation Syndrome. Posterior fossa masses produce most of their findings by compression of the brain stem and cranial nerves, and by obstructive hydrocephalus. As the pressure gradient across the foramen magnum increases, however, the cerebellar tonsils may be pushed into, and eventually through, the foramen. This compresses the medulla and may produce apnea by inducing dysfunction in the medullary respiratory centers. Before losing consciousness, patients with cerebellar tonsillar herniation may complain of a stiff neck.

Upward Transtentorial Herniation Syndrome. Expanding posterior fossa masses usually herniate caudally because the obstructive hydrocephalus they produce prevents a pressure gradient across the tentorial opening. If these patients undergo ventriculostomy for relief of hydrocephalus, however, the possibility of upward herniation of the contents of posterior fossa into the diencephalic region exists. Many neurosurgeons prepare the patient for an emergent posterior fossa decompression just before performing the ventriculostomy in case this problem occurs.

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Blood Pressure Health

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