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Brain herniation is a potentially deadly side effect of very high pressure within the skull that occurs when a part of the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...
is squeezed across structures within the
skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
. The brain can shift across such structures as the
falx cerebri The falx cerebri (also known as the cerebral falx) is a large, crescent-shaped fold of dura mater that descends vertically into the longitudinal fissure between the cerebral hemispheres of the human brain,Saladin K. "Anatomy & Physiology: The Uni ...
, the
tentorium cerebelli The cerebellar tentorium or tentorium cerebelli (Latin for "tent of the cerebellum") is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes. Structure The cerebellar tentorium is an arch ...
, and even through the foramen magnum (the hole in the base of the skull through which the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
connects with the brain). Herniation can be caused by a number of factors that cause a
mass effect ''Mass Effect'' is a military science fiction media franchise created by Casey Hudson, Drew Karpyshyn and Preston Watamaniuk. The franchise depicts a distant future where humanity and several alien civilizations have colonized the known unive ...
and increase
intracranial pressure Intracranial pressure (ICP) is the pressure exerted by fluids such as cerebrospinal fluid (CSF) inside the skull and on the brain tissue. ICP is measured in millimeters of mercury ( mmHg) and at rest, is normally 7–15 mmHg for a supine adult ...
(ICP): these include
traumatic brain injury A traumatic brain injury (TBI), also known as an intracranial injury, is an injury to the brain caused by an external force. TBI can be classified based on severity (ranging from mild traumatic brain injury TBI/concussionto severe traumatic br ...
,
intracranial hemorrhage Intracranial hemorrhage (ICH), also known as intracranial bleed, is hemorrhage, bleeding internal bleeding, within the Human skull, skull. Subtypes are intracerebral bleeds (intraventricular bleeds and intraparenchymal bleeds), subarachnoid bleed ...
, or
brain tumor A brain tumor occurs when abnormal cells form within the brain. There are two main types of tumors: malignant tumors and benign (non-cancerous) tumors. These can be further classified as primary tumors, which start within the brain, and seconda ...
. Herniation can also occur in the absence of high ICP when mass lesions such as hematomas occur at the borders of brain compartments. In such cases local pressure is increased at the place where the herniation occurs, but this pressure is not transmitted to the rest of the brain, and therefore does not register as an increase in ICP. Because herniation puts extreme pressure on parts of the brain and thereby cuts off the blood supply to various parts of the brain, it is often fatal. Therefore, extreme measures are taken in hospital settings to prevent the condition by reducing intracranial pressure, or decompressing (draining) a hematoma which is putting local pressure on a part of the brain.


Signs and symptoms

Brain herniation frequently presents with
abnormal posturing Abnormal 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 cause ...
, a characteristic positioning of the limbs indicative of severe brain damage. These patients have a lowered
level of consciousness An altered level of consciousness is any measure of arousal other than normal. Level of consciousness (LOC) is a measurement of a person's arousability and responsiveness to stimuli from the environment. A mildly depressed level of consciousn ...
, with
Glasgow Coma Score The Glasgow Coma Scale (GCS) is a clinical scale used to reliably measure a person's level of consciousness after a brain injury. The GCS assesses a person based on their ability to perform eye movements, speak, and move their body. These thre ...
s 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 The area postrema, a paired structure in the medulla oblongata of the brainstem, is a circumventricular organ having permeable capillaries and sensory neurons that enable its dual role to detect circulating chemical messengers in the blood and ...
in the medulla oblongata. Severe
headaches Headache is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of depression in those with severe headaches. Headaches can occur as a result ...
and
seizures An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with l ...
as a result of increased intracranial pressure are not uncommon.
Cardiovascular The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
and pulmonary symptoms may also be present as the brain loses function, but might also be associated with
bleeding Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, vag ...
. They can include: hypertension, respiratory depression, arrhythmia and in severe cases
cardiac arrest Cardiac arrest is when the heart suddenly and unexpectedly stops beating. It is a medical emergency that, without immediate medical intervention, will result in sudden cardiac death within minutes. Cardiopulmonary resuscitation (CPR) and poss ...
.


Causes

Causes of brain herniation include: *
Brain edema Cerebral edema is excess accumulation of fluid (edema) in the intracellular or extracellular spaces of the brain. This typically causes impaired nerve function, increased pressure within the skull, and can eventually lead to direct compressio ...
* Hematoma * Stroke *
Tumour A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
*
Infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...


Classification

The tentorium is an extension of the dura mater that separates the cerebellum from the
cerebrum The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb ...
. There are two major classes of herniation: supratentorial and infratentorial. Supratentorial refers to herniation of structures normally found above the tentorial notch, and infratentorial refers to structures normally found below it.Orlando Regional Healthcare, Education and Development. 2004
"Overview of Adult Traumatic Brain Injuries."
Retrieved on January 16, 2008.
* Supratentorial herniation 1) Uncal (transtentorial) 2) Central 3)
Cingulate Cingulata, part of the superorder Xenarthra, is an order of armored New World placental mammals. Dasypodids and chlamyphorids, the armadillos, are the only surviving families in the order. Two groups of cingulates much larger than extant arm ...
(subfalcine/transfalcine) 4) Transcalvarial 5)
Tectal The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', "m ...
(posterior) * Infratentorial herniation 6) Upward (upward cerebellar or upward transtentorial) 7) Tonsillar (downward cerebellar)


Uncal herniation

In uncal herniation, a common subtype of transtentorial herniation, the innermost part of the
temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pro ...
, the
uncus The uncus is an anterior extremity of the parahippocampal gyrus. It is separated from the apex of the temporal lobe by a slight fissure called the incisura temporalis (also called rhinal sulcus). Although superficially continuous with the hippo ...
, can be squeezed so much that it moves towards the tentorium and puts pressure on the brainstem, most notably the midbrain. The tentorium is a structure within the
skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
formed by the dura mater of the meninges. Tissue may be stripped from the
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
in a process called
decortication Decortication 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 ex ...
. The uncus can squeeze the oculomotor nerve (a.k.a. CN III), which may affect the parasympathetic input to the eye on the side of the affected nerve, causing the
pupil The pupil is a black hole located in the center of the Iris (anatomy), iris of the Human eye, eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing ...
of the affected eye to
dilate Dilation (or dilatation) may refer to: Physiology or medicine * Cervical dilation, the widening of the cervix in childbirth, miscarriage etc. * Coronary dilation, or coronary reflex * Dilation and curettage, the opening of the cervix and surgic ...
and fail to constrict in response to light as it should. Pupillary dilation often precedes the somatic motor effects of CN III compression called
oculomotor nerve palsy Oculomotor nerve palsy or oculomotor neuropathy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements (fou ...
or third nerve palsy. This palsy presents as 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 Lateral is a geometric term of location which may refer to: Healthcare * Lateral (anatomy), an anatomical direction *Lateral cricoarytenoid muscle *Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...
(innervated by
abducens nerve The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a cranial nerve in humans and various other animals that controls the movement of the lateral rectus muscle, one of the extraocul ...
(a.k.a. CN VI) and the
superior oblique The superior oblique muscle, or obliquus oculi superior, is a fusiform muscle originating in the upper, medial side of the orbit (i.e. from beside the nose) which abducts, depresses and internally rotates the eye. It is the only extraocular musc ...
(innervated by
trochlear nerve The trochlear nerve (), ( lit. ''pulley-like'' nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pu ...
a.k.a. CN IV). The symptoms occur in this order because the parasympathetic fibers surround the motor fibers of CN III and are hence compressed first. Compression of the ipsilateral
posterior cerebral artery The posterior cerebral artery (PCA) is one of a pair of cerebral arteries that supply oxygenated blood to the occipital lobe, part of the back of the human brain. The two arteries originate from the distal end of the basilar artery, where it bifur ...
will result in ischemia of the ipsilateral primary visual cortex and contralateral visual field deficits in both eyes (contralateral homonymous hemianopsia). Another important finding is a ''false localizing sign'', the so-called Kernohan's notch, which results from compression of the contralateral
cerebral crus The cerebral crus (crus cerebri) is the anterior portion of the cerebral peduncle which contains the motor tracts, travelling from the cerebral cortex to the pons and spine. The plural of which is cerebral crura. In some older texts this is call ...
containing descending
corticospinal The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (''co ...
and some
corticobulbar In neuroanatomy, the corticobulbar (or corticonuclear) tract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the brainstem's medulla oblongata (also called ...
tract fibers. This leads to Ipsilateral
hemiparesis Hemiparesis, or unilateral paresis, is weakness of one entire side of the body ('' hemi-'' means "half"). Hemiplegia is, in its most severe form, complete paralysis of half of the body. Hemiparesis and hemiplegia can be caused by different medi ...
in reference to the herniation and contralateral hemiparesis with reference to the cerebral crus. With increasing pressure and progression of the hernia there will be distortion of the brainstem leading to Duret hemorrhages (tearing of small vessels in the parenchyma) in the median and paramedian zones of the
mesencephalon The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', "m ...
and
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
. The rupture of these vessels leads to linear or flamed shaped hemorrhages. The disrupted brainstem can lead to
decorticate posture Abnormal 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 cau ...
, respiratory center depression and death. Other possibilities resulting from brain stem distortion include
lethargy Lethargy is a state of tiredness, sleepiness, weariness, fatigue, sluggishness or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overwo ...
, slow heart rate, and pupil dilation. Uncal herniation may advance to central herniation. The sliding uncus syndrome represents uncal herniation without alteration in the level of consciousness and other sequelae mentioned above.


Central herniation

In ''central herniation'', the diencephalon and parts of the
temporal lobes The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pro ...
of both of the
cerebral hemisphere The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s are squeezed through a notch in the
tentorium cerebelli The cerebellar tentorium or tentorium cerebelli (Latin for "tent of the cerebellum") is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes. Structure The cerebellar tentorium is an arch ...
. 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 artery 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 known as the vertebral basilar system, which supplies blood to the posterior part of the circle of W ...
(pontine arteries), causing them to tear and bleed, known as a Duret hemorrhage. The result is usually fatal. Other symptoms of this type of herniation include small, fixed pupils withToronto Notes paralysis of upward eye movement giving the characteristic appearance of "sunset eyes". Also found in these patients, often as a terminal complication is the development of
diabetes insipidus Diabetes insipidus (DI), recently renamed to Arginine Vasopressin Deficiency (AVP-D) and Arginine Vasopressin Resistance (AVP-R), is a condition characterized by large amounts of dilute urine and increased thirst. The amount of urine produced ...
due to the compression of the pituitary stalk. 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 lobe The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove be ...
is scraped under part of the
falx cerebri The falx cerebri (also known as the cerebral falx) is a large, crescent-shaped fold of dura mater that descends vertically into the longitudinal fissure between the cerebral hemispheres of the human brain,Saladin K. "Anatomy & Physiology: The Uni ...
, the dura mater at the top of the head between the two hemispheres of the brain.Dawodu ST. 2007
Traumatic brain injury: Definition, epidemiology, pathophysiology.
Emedicine.com. Retrieved on January 28, 2007.
Cingulate herniation can be caused when one hemisphere swells and pushes the
cingulate gyrus The cingulate cortex is a part of the brain situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the ...
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 vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away ...
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 posturing Abnormal 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 cause ...
and coma. 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 Decompressive craniectomy ('' crani-'' + '' -ectomy'') is a neurosurgical procedure in which part of the skull is removed to allow a swelling brain room to expand without being squeezed. It is performed on victims of traumatic brain injury, st ...
, surgery in which a flap of skull is removed, the protruding brain region preventing the piece of skull from being replaced during the operation.


Upward herniation

Increased pressure in the posterior fossa can cause the cerebellum to move up through the tentorial opening in ''upward'', or ''cerebellar herniation''. The
midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
is pushed through the tentorial notch upward. This is also known as ascending transtentorial herniation since it occurs across the tentorium cerebelli.


Tonsillar herniation

In ''tonsillar herniation'', also called ''downward cerebellar herniation'', transforaminal herniation, or "coning", the cerebellar tonsils move downward through 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. The most common signs are intractable headache, head tilt, and neck stiffness due to tonsillar impaction. The level of consciousness may decrease and also give rise to flaccid paralysis. Blood pressure instability is also evident in these patients. Congenital tonsillar herniation of the cerebellum is also known as a
Chiari malformation Chiari malformation (CM) is a structural defect in the cerebellum, characterized by a downward displacement of one or both cerebellar tonsils through the foramen magnum (the opening at the base of the skull). CMs can cause headaches, difficu ...
(CM), or previously an Arnold-Chiari malformation (ACM). There are four types of Chiari malformation, 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 patients undergo MRI scans of their heads, especially upright MRI, which is more than twice as sensitive for detecting this condition. Cerebellar tonsillar ectopia (CTE) 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
ype Peace River Airport is a municipally owned airport located west of the Town of Peace River, Alberta, Canada. The airport has one runway, which is , and a terminal building, which is . Northern Air is based at the airport and provides scheduled ...
0'. There are many suspected causes of tonsillar herniation including: 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; or dural tension pulling the brain caudally. 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 but without clear MRI evidence, especially if the symptoms are better in the supine position and worse upon standing/upright, an upright MRI may be useful.


Treatment

Treatment involves removal of the etiologic mass and
decompressive craniectomy Decompressive craniectomy ('' crani-'' + '' -ectomy'') is a neurosurgical procedure in which part of the skull is removed to allow a swelling brain room to expand without being squeezed. It is performed on victims of traumatic brain injury, str ...
. 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 The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
, which contains the reticular activating network which regulates
consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
, will result in coma. Damage to the cardio-respiratory centers in the medulla oblongata will cause respiratory arrest and (secondarily)
cardiac arrest Cardiac arrest is when the heart suddenly and unexpectedly stops beating. It is a medical emergency that, without immediate medical intervention, will result in sudden cardiac death within minutes. Cardiopulmonary resuscitation (CPR) and poss ...
. Investigation is underway regarding the use of neuroprotective agents during the prolonged post-traumatic period of brain hypersensitivity associated with the syndrome.


See also

*
Second-impact syndrome Second-impact syndrome (SIS) occurs when the brain swells rapidly, and catastrophically, after a person has a second concussion before symptoms from an earlier one have subsided. This second blow may occur minutes, days, or weeks after an initial ...


References


External links

{{DEFAULTSORT:Brain Herniation Hernias Neurotrauma