Brain herniation, also known as
cistern obliteration, is a deadly side effect of very high
intracranial pressureIntracranial pressure is the pressure in the cranium and thus in the brain tissue and cerebrospinal fluid ; this pressure is exerted on the brain's intracranial blood circulation vessels. ICP is maintained in a tight normal range dynamically, through the production and absorption of CSF and...
that occurs when the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
shifts across structures within the
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
. The brain can shift by such structures as the
falx cerebriThe falx cerebri, also known as the cerebral falx, so named from its sickle-like form, is a strong, arched fold of dura mater which descends vertically in the longitudinal fissure between the cerebral hemispheres....
, the
tentorium cerebelliThe tentorium cerebelli or cerebellar tentorium is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes.-Anatomy:...
, and even through the hole called the
foramen magnumIn anatomy, in the occipital bone, the foramen magnum is one of the several oval or circular apertures in the base of the skull , through which the medulla oblongata enters and exits the skull vault.Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum...
in the base of the skull (through which the
spinal cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. The brain and spinal cord together make up the central nervous system. It is around 45 cm long in men and around 43 cm long in women. The length of the spinal cord is much shorter than...
connects with the brain). Herniation can be caused by a number of factors that cause a
mass effectIn medicine, a mass effect is the effect of a growing mass , for example the consequences of a growing cancer....
and increase
intracranial pressureIntracranial pressure is the pressure in the cranium and thus in the brain tissue and cerebrospinal fluid ; this pressure is exerted on the brain's intracranial blood circulation vessels. ICP is maintained in a tight normal range dynamically, through the production and absorption of CSF and...
(ICP): these include
traumatic brain injuryTraumatic brain injury occurs when an outside force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...
,
strokeA stroke is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by thrombosis or embolism or due to a hemorrhage...
, or
brain tumorA brain tumor is an abnormal growth of cells within the brain or inside the skull, which can be cancerous or non-cancerous .It is defined as any intracranial tumor created by abnormal and uncontrolled cell division, normally either in the brain itself , in the cranial nerves...
. Because herniation puts extreme pressure on parts of the brain, it is often fatal. Therefore, extreme measures are taken in hospital settings to prevent the condition by reducing
intracranial pressureIntracranial pressure is the pressure in the cranium and thus in the brain tissue and cerebrospinal fluid ; this pressure is exerted on the brain's intracranial blood circulation vessels. ICP is maintained in a tight normal range dynamically, through the production and absorption of CSF and...
. Herniation can also occur in the absence of high ICP when mass lesions such as hematomas occur at the borders of brain compartments.
Classification
There are two major classes of herniation: supratentorial and infratentorial. Supratentorial herniation is of structures normally above the tentorial notch and infratentorial is of structures normally below it.
- Supratentorial herniation
- Uncal
- Central (transtentorial)
- Cingulate (subfalcine)
- Transcalvarial
- Infratentorial herniation
- Upward (upward cerebellar or upward transtentorial)
- Tonsillar (downward cerebellar)
Uncal herniation
In uncal herniation, a common subtype of transtentorial herniation, the innermost part of the
temporal lobeThe temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both the left and right hemispheres of the brain....
, the
uncusThe anterior extremity of the Parahippocampal gyrus is recurved in the form of a hook, the uncus, which is separated from the apex of the temporal lobe by a slight fissure, the incisura temporalis....
, can be squeezed so much that it goes by the
tentoriumThe tentorium cerebelli or cerebellar tentorium is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes.-Anatomy:...
and puts pressure on the brainstem, most notably the midbrain. The tentorium is a structure within the
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
formed by the
meningealThe meninges is the system of membranes which envelops the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater...
layer the dura mater. Tissue may be stripped from the
cerebral cortexThe cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It constitutes the outermost layer of the cerebrum. In preserved brains, it has a grey color, hence the name "grey matter"...
in a process called
decorticationDecortication is a medical procedure involving the surgical removal of the surface layer, membrane, or fibrous cover of an organ. The procedure is usually performed when the lung is covered by a thick, inelastic pleural peel restricting lung expansion. In a non-medical aspect, decortication is...
. The uncus can squeeze the third cranial nerve, which controls parasympathetic input to the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
on the side of the affected nerve. This interrupts the parasympathetic neural transmission, causing the pupil of the affected eye to
dilateDilate is the seventh studio album by singer-songwriter Ani DiFranco, released in 1996. Dilate is her highest-selling record, with U.S...
and fail to constrict in response to light as it should, so a dilated unresponsive pupil is an important sign of increased intracranial pressure. Pupillary dilation often precedes a later finding of cranial nerve III compression, which is deviation of the eye to a "down and out" position due to loss of innervation to all ocular motility muscles except for the lateral rectus (innervated by cranial nerve VI) and the superior oblique (innervated by cranial nerve IV). The symptoms occur in this order because the eccentric parasympathetic fibers surround the motor fibers of CNIII and, hence, are first to be compressed. Cranial arteries may be compressed during the herniation. Compression of the posterior cerebral artery may result in loss of the contralateral visual field (contralateral homonymous hemianopia). A later important finding, the
false localizing sign, results from compression of the contralateral
cerebral crusThe cerebral crus is the anterior portion of the cerebral peduncle which contains the motor tracts, the plural of which is cerebral crura.In some older texts, it is used as a synonym for the entire cerebral peduncle, not just the anterior portion of it....
, which contains descending
corticospinalThe corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....
and
corticobulbarThe corticobulbar tract is a white matter pathway connecting the cerebral cortex to the brainstem. The term "bulbar" referrers to the brainstem, as "bulb" was a historical term meaning the area currently called the brainstem....
fibers. This leads to ipsilateral (to herniating uncus)
hemiparesisHemiparesis is weakness on one side of the body. Contrast with Hemiplegia, which is total paralysis of the arm, leg, and trunk on the same side of the body...
. Since the corticospinal tract predominately innervates flexor muscles, extension of the leg may also be seen. With increasing pressure, decorticate posture may develop. This type of herniation can also damage the brain stem, causing lethargy, slow heart rate, respiratory abnormalities, and
pupil dilationMydriasis is an excessive dilation of the pupil due to disease, trauma or the use of drugs. Normally, the pupil dilates in the dark and constricts in the light to improve vividity at night and to protect the retina from sunlight damage during the day, respectively...
. Uncal herniation may advance to central herniation.
Central herniation
In
central herniation, (also called "transtentorial herniation") the
diencephalonThe diencephalon is the region of the brain that includes the thalamus, hypothalamus, epithalamus, prethalamus or subthalamus and pretectum. The diencephalon is located at the midline of the brain, above the mesencephalon of the brain stem...
and parts of the temporal lobes of both of the
cerebral hemisphereA cerebral hemisphere is defined as one of the two regions of the brain that are delineated by the body's median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres...
s are squeezed through a notch in the
tentorium cerebelliThe tentorium cerebelli or cerebellar tentorium is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes.-Anatomy:...
. Transtentorial herniation can occur when the brain moves either up or down across the tentorium, called ascending and descending transtentorial herniation respectively; however descending herniation is much more common. Downward herniation can stretch branches of the
basilar arteryIn human anatomy, the basilar artery is one of the arteries that supplies the brain with oxygen-rich blood.The two vertebral arteries and the basilar artery are sometimes together called the vertebrobasilar system, which supplies blood to the posterior part of circle of Willis and anastomoses with...
(paramedian artery), causing them to tear and bleed, known as a Duret hemorrhage. The result is usually fatal. Radiographically, downward herniation is characterized by obliteration of the suprasellar cistern from temporal lobe herniation into the tentorial hiatus with associated compression on the cerebral peduncles. Upwards herniation, on the other hand, can be radiographically characterized by obliteration of the quadrigeminal cistern. Intracranial hypotension syndrome has been known to mimic downwards transtentorial herniation.
Cingulate herniation
In
cingulate or
subfalcine herniation, the most common type, the innermost part of the
frontal lobeThe frontal lobe is an area in the brain of mammals. It is located at the front of each cerebral hemisphere and positioned anterior to the parietal lobes and above and anterior to the temporal lobes...
is scraped under part of the
falx cerebriThe falx cerebri, also known as the cerebral falx, so named from its sickle-like form, is a strong, arched fold of dura mater which descends vertically in the longitudinal fissure between the cerebral hemispheres....
, the dura mater at the top of the head between the two hemispheres of the brain. Cingulate herniation can be caused when one hemisphere swells and pushes the
cingulate gyrusCingulate gyrus is a gyrus in the medial part of the brain. It partially wraps around the corpus callosum and is limited above by the cingulate sulcus.The cortical part of the cingulate gyrus is referred to as cingulate cortex....
by the falx cerebri. This does not put as much pressure on the brainstem as the other types of herniation, but it may interfere with
blood vesselThe blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart, the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s in the frontal lobes that are close to the site of injury (anterior cerebral artery), or it may progress to central herniation. Interference with the blood supply can cause dangerous increases in ICP that can lead to more dangerous forms of herniation. Symptoms for cingulate herniation are not well defined. Usually occurring in addition to uncal herniation, cingulate herniation may present with
abnormal posturingAbnormal posturing is an involuntary flexion or extension of the arms and legs, indicating severe brain injury. It occurs when one set of muscles becomes incapacitated while the opposing set is not, and an external stimulus such as pain causes the working set of muscles to contract. The posturing...
and
comaIn medicine, a coma is a profound state of unconsciousness. A comatose person cannot be awakened, fails to respond normally to pain or light, does not have sleep-wake cycles, and does not take voluntary actions....
. Cingulate herniation is frequently believed to be a precursor to other types of herniation.
Transcalvarial herniation
In
transcalvarial herniation, the brain squeezes through a fracture or a surgical site in the skull. Also called "external herniation", this type of herniation may occur during craniectomy, surgery in which a flap of skull is removed, preventing the piece of skull from being replaced.
Upward herniation
Increased pressure in the posterior
fossaThe posterior cranial fossa is part of the intracranial cavity, located between the foramen magnum and tentorium cerebelli. It contains the brainstem and cerebellum.This is the most inferior of the fossae. It houses the cerebellum, medulla and pons....
can cause the
cerebellumThe cerebellum is a region of the brain that plays an important role in the integration of sensory perception, coordination and motor control...
to move up through the tentorial opening in
upward, or
cerebellar herniation. The midbrain is pushed through the tentorial notch. This also pushes the midbrain down.
Tonsillar herniation
In
tonsillar herniation, also called
downward cerebellar herniation, or "coning", the cerebellar tonsils move downward through the
foramen magnumIn anatomy, in the occipital bone, the foramen magnum is one of the several oval or circular apertures in the base of the skull , through which the medulla oblongata enters and exits the skull vault.Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum...
possibly causing compression of the lower brainstem and upper cervical spinal cord as they pass through the foramen magnum. Increased pressure on the brainstem can result in dysfunction of the centers in the brain responsible for controlling respiratory and cardiac function.
Tonsillar herniation of the
cerebellumThe cerebellum is a region of the brain that plays an important role in the integration of sensory perception, coordination and motor control...
is also known as a Chiari Malformation (CM), or previously an Arnold Chiari Malformation (ACM). There are at least three types of Chiari malformation that are widely recognized, and they represent very different disease processes with different symptoms and prognosis. These conditions can be found in asymptomatic patients as an incidental finding, or can be so severe as to be life-threatening. This condition is now being diagnosed more frequently by radiologists, as more and more patients undergo MRI scans of their heads. Cerebellar ectopia is a term used by radiologists to describe cerebellar tonsils that are "low lying" but that do not meet the radiographic criteria for definition as a Chiari malformation. The currently accepted radiographic definition for a Chiari malformation is that cerebellar tonsils lie at least 5mm below the level of the foramen magnum. Some clinicians have reported that some patients appear to experience symptoms consistent with a Chiari malformation without radiographic evidence of tonsillar herniation. Sometimes these patients are described as having a 'Chiari [type] 0'.
There are many suspected causes of tonsillar herniation including: spinal cord tethering or occult tight filum terminale (pulling down on the brainstem and surrounding structures); decreased or malformed posterior fossa (the lower, back part of the skull) not providing enough room for the cerebellum; hydrocephalus or abnormal CSF volume pushing the tonsils out. Connective tissue disorders, such as Ehlers Danlos Syndrome, can be associated.
For further evaluation of tonsillar herniation, CINE flow studies are used. This type of MRI examines flow of CSF at the cranio-cervical joint. For persons experiencing symptoms with seemingly minimal herniation, especially if the symptoms are better in the supine position and worse upon standing/upright, an upright MRI may be useful.
Signs and symptoms
Brain herniation frequently presents with
abnormal posturingAbnormal posturing is an involuntary flexion or extension of the arms and legs, indicating severe brain injury. It occurs when one set of muscles becomes incapacitated while the opposing set is not, and an external stimulus such as pain causes the working set of muscles to contract. The posturing...
a characteristic positioning of the limbs indicative of severe brain damage. These patients have a lowered
level of consciousnessLevel of consciousness is a measurement of a person's arousability and responsiveness to stimuli from the environment. A mildly depressed level of consciousness may be classed as lethargy; someone in this state can be aroused with little difficulty. People who are obtunded have a more depressed...
, with Glasgow Coma Scores of three to five. One or both pupils may be dilated and fail to constrict in response to light. Vomiting can also occur due to compression of the vomiting center in the
medulla oblongataThe medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
.
Treatment and prognosis
Treatment involves removal of the etiologic mass and decompressive craniectomy. Brain herniation can cause severe disability or death. In fact, when herniation is visible on a CT scan, the prognosis for a meaningful recovery of neurological function is poor. The patient may become paralyzed on the same side as the lesion causing the pressure, or damage to parts of the brain caused by herniation may cause paralysis on the side opposite the lesion. Damage to the midbrain, which contains the reticular activating network that regulates
consciousnessConsciousness is subjective experience or awareness or wakefulness or the executive control system of the mind. It is an umbrella term that may refer to a variety of mental phenomena...
will result in
comaIn medicine, a coma is a profound state of unconsciousness. A comatose person cannot be awakened, fails to respond normally to pain or light, does not have sleep-wake cycles, and does not take voluntary actions....
. Damage to the cardio-respiratory centers in the
medulla oblongataThe medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
will cause
respiratoryRespiratory arrest is the cessation of breathing. It is a medical emergency and it usually is related to or coincides with a cardiac arrest. Causes include opiate overdose, head injury, anaesthesia or drowning...
and
cardiac arrestA cardiac arrest, also known as cardiopulmonary arrest or circulatory arrest, is the abrupt cessation of normal circulation of the blood due to failure of the heart to contract effectively during systole....
. Current investigation is underway regarding the use of neuroprotective agents during the prolonged post-traumatic period of brain hypersensitivity associated with the syndrome.
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