Uncal Herniation

Any condition that considerably increases intracranial pressure can cause uncal herniation. Usually, a driving force applied by nearby anatomical structures within an environment of increased intracranial pressure, as seen during swelling or bleeding of the brain, can cause the uncus to slide over the tentorial notch (i.e., the opening between the brain and the brainstem). This phenomenon is known as a transtentorial herniation and can result in an uncal herniation. 

The presence of expanding mass lesions within the skull, such as brain tumors or abscesses, can pose an increased risk for uncal herniation. Accordingly, severe head trauma leading to rapidly intensifying subdural or epidural hemorrhage (bleeding below or above the dura mater, respectively), as well as intracerebral bleeding, can also cause uncal herniation. Lastly, there is an  increased risk for uncal herniation in individuals with a history of large ischemic strokes and obstructive hydrocephalus (i.e., excessive accumulation of cerebrospinal fluid within the skull due to a blockage in its flow).

Any individual with impending uncal herniation will first experience symptoms similar to those of increased intracranial pressure, including headache, nausea, vomiting, and changes in mental status. Upon physical examination, individuals may present with the Cushing triad, which includes hypertension, bradycardia, and irregular respiration or apnea. 

A more detailed ophthalmologic exam can also reveal papilledema (i.e., swelling of the optic disc). This along with other ophthalmologic symptoms are caused by the compression of the oculomotor nerve (CN III). CN III, or the third cranial nerve, innervates the pupils and lens as well as the muscles for visual tracking and gaze. Therefore, a classic finding of CN III compression involves downward and outward eye deviation as well as decreased eye movement over time. Another distinctive feature of uncal herniation is acute loss of consciousness with affiliated ipsilateral dilation of the pupil. Sometimes the pupils can also be unresponsive to light, and individuals may experience hemiparesis (i.e., weakness or inability to move on one side of the body) on the contralateral, or opposite, side of pupillary dilation. Additionally,  individuals presenting with unilateral anisocoria (i.e., unequal pupil size) should be suspected for an impending uncal herniation.  

Without considerable impairment in the level of consciousness or movement, individuals experiencing uncal herniation may initially present with a unilateral dilated pupil. Further compression of the midbrain may progressively lead to lethargy, coma, or even death. If left untreated, uncal herniation can progress to a central herniation (i.e., downwards transtentorial herniation of the diencephalon and midbrain).

Initial treatment for uncal herniation should be aimed at reducing intracranial pressure. Initial management to alleviate pressure can involve elevation of the head at a thirty degree angle; hyperventilation; or hyperosmolar therapy, such as mannitol or hypertonic fluids (e.g., 3% Saline).

In more severe cases when symptoms do not resolve with conservative treatment, surgical interventions may be considered. If present, surgical removal of a brain tumor, abscess, or hematoma may be required. Additional treatment may involve the placement of a ventricular drain through a hole in the skull in order to get rid of excessive cerebrospinal fluid. If all of the other treatment options have been exhausted, a decompressive craniectomy, involving the removal of a portion of the skull, can also relieve pressure.