Neuroscience Clerkship at UH/VA
 

HERNIATION SYNDROMES

 

Herniation is a neurologic emergency that requires immediate intervention. Close monitoring, preferably by a neurointensivist is required to help avoid neurological catastrophe. A neurosurgical intervention may be necessary depending on the etiology of brain herniation.

In order to understand the clinical aspects of brain herniation syndromes, one must first appreciate the physiology. The Monro-Kellie doctrine, proposed from experiments from more than 200 years ago, discovered that in an adult, the sum of the volumes of brain, blood and CSF are constant. An increase in one of these three substances, requires a decrease in the volume of another.

(Brain parenchyma + CSF + Blood) volume = Constant volume

A main reason for the above phenomenon is that the cranial contents in adults are enclosed (after fontanel closure) within a rigid, poorly compliant cranium composed of bone and dura. Due to the poor compliance of this system, small increases in intracranial volume will result in sharp increases in intracranial pressure. Compliance refers to the ratio of change in volume to change in pressure.

C (compliance) = Δ V (volume) / Δ P (pressure)

In most biological tissues, the relationship between change in volume to change in pressure is not linear. Thus as demonstrated by the graph below, at higher levels of volume, it requires a smaller change in volume to produce a given change in pressure.

Any new masses developing within the cranial cavity, such as hematomas, tumors or edema are initially tolerated by the minimal compliance of the brain. However, after a very small increase of volume to the cranial contents, the brain will displace due to a very large increase in intracranial pressure (ICP). The tissue may displace one brain compartment into another. The physical movement of brain tissue, if not recognized and treated urgently, compromises vital structures and may result in brain death due to irreversible brainstem dysfunction or disruption of respiratory and cardiovascular centers resulting in death from respiratory or cardiac arrest.
 

Above: Herniation syndromes. Red arrow - Transtentorial (uncal) herniation; Blue arrow - Foramen magnum herniation; Yellow arrow - Subfalcine herniation
Transtentorial Herniation
 

 The tentorium is a dural structure that separates the cerebrum from the brainstem and cerebellum in the posterior cranial fossa below. The opening in the tentorium through which the brainstem, specifically the midbrain, connects to the cerebrum is the tentorial incisura. The presence of a large supratentorial mass in one hemisphere often results in subfalcine herniation, where the cingulate gyrus of the ipsilateral hemisphere is compressed and herniates under the falx to compress the contralateral hemisphere. Subsequently, progression of the herniation then becomes transtentorial with the supratentorial contents moving through the tentorial incisura.
 

Above: Uncal herniation. Arrows point to the medial temporal lobe that has herniated through the tentorial incisura to compress the midbrain
 
The structure that herniates first is usually the uncus on the medial temporal lobe. As the uncus herniates, it first presses against the midbrain, resulting in an ipsilateral third nerve palsy. As the parasympathetic fibers are on the outside of the third nerve, the first sign of uncal herniation is usually pupillary dilation. Further compression results in paralysis of extraocular muscles.
 

Because transtentorial herniation occurs most commonly from a supratentorial mass, the patient usually will already have a contralateral hemiplegia. If the brainstem is torqued, the contralateral cerebral peduncle can be compressed against the tentorial notch (i.e., Kernohan’s notch), resulting in quadriplegia (contralateral hemiplegia from the initial lesion, ipsilateral hemiplegia from Kernohan’s notch phenomena). As herniation proceeds, dysfunction of both hemispheres occurs followed by dysfunction of the brainstem. As this occurs, abnormal posturing is seen.
 

Decorticate posturing (figure top) is recognized as bilateral flexion at the elbows and wrists, shoulder adduction and extension of the lower extremities occurring with lesions above the midbrain’s red nucleus. As brainstem dysfunction proceeds inferiorly, decerebrate posturing  (figure bottom) occurs, recognized as rigid extension of the arms with internal rotation, and extension of the legs with internal rotation and downward pointing of the toes. Retraction (backward arching) of the head may occur. This is followed by respiratory insufficiency and death.

 

Above: Duret hemorrhages. These are the terminal event in transtentorial herniation.

Pathologically, the downward herniation stretches the penetrating branches of the basilar artery, which then rupture, causing secondary linear hemorrhages in the midbrain and pons, known as Duret hemorrhages. These are generally fatal.


Also passing through the tentorial incisura are both posterior cerebral arteries (PCA). Although deficits secondary to PCA ischemia are difficult if not impossible to determine clinically at the time; patients who survive an episode of transtenorial herniation may be left with bilateral PCA infarcts (i.e., cortical blindness or other bilateral visual field abnormalities; and marked short term memory dysfunction from involvement of the medial temporal lobes).


Foramen Magnum Herniation


Foramen magnum herniation occurs from either a posterior fossa space-occupying lesion or from further progression of a supratentorial mass lesion. Foramen magnum herniation causes compression of structures that lie above and pass through the foramen magnum, (i.e., the cerebellar tonsils and medulla). In the medulla are located the vital centers regulating respiration and cardiac function, and the reticular activating system for maintaining consciousness. In foramen magnum herniation, the following develop: changes in the level of consciousness, extensor posturing, apnea, and then circulatory collapse followed by death.

Above: Pathologic specimen of foramen magnum herniation. Below the arrows mark where the cerebellum had herniated downwards into the foramen magnum, resulting in compression of the medulla.

 

Above: Herniation of the cerebellar tonsils (red arrow) and compression of the medulla. The dotted white line shows the level of the foramen magnum.