Brain herniation is a potentially deadly side effect of very high pressure within the skull that occurs when a part of the brain is squeezed across structures within the skull. The brain can shift across such structures as the falx cerebri, the tentorium cerebelli, and even through the

foramen magnum The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblon ...

(the hole in the base of the skull through which the spinal cord connects with the brain). Herniation can be caused by a number of factors that cause a mass effect and increase intracranial pressure (ICP): these include traumatic brain injury, intracranial hemorrhage, or brain tumor. Herniation can also occur in the absence of high ICP when

mass lesion In medicine, a mass effect is the effect of a growing mass that results in secondary pathological effects by pushing on or displacing surrounding tissue. In oncology, the mass typically refers to a tumor. For example, cancer of the thyroid glan ...

s such as

hematoma A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...

s 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, a characteristic positioning of the limbs indicative of severe brain damage. These patients have a lowered level of consciousness, 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 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 an ...

in the

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

. Severe headaches and seizures as a result of increased intracranial pressure are not uncommon. Cardiovascular and pulmonary symptoms may also be present as the brain loses function, but might also be associated with bleeding. They can include:

hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...

respiratory depression Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...

arrhythmia Arrhythmias, also known as cardiac arrhythmias, heart arrhythmias, or dysrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. A resting heart rate that is too fast – above 100 beats per minute in adults ...

and in severe cases cardiac arrest.

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 A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...

Stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

* Tumour * Infection

Classification

The

tentorium The tentorium (plural tentoria) is a term used to refer to the framework of internal supports within an arthropod head. The tentorium is formed by ingrowths of the exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "sk ...

is an extension of the dura mater that separates the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

from the cerebrum. There are two major classes of herniation: supratentorial and infratentorial. Supratentorial refers to herniation of structures normally found above the

tentorial notch The tentorial notch (also known as the tentorial incisure or incisura tentorii) refers to the anterior opening between the free edge of the cerebellar tentorium and the Clivus (anatomy), clivus for the passage of the brainstem. The midbrain cont ...

, 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 (subfalcine/transfalcine) 4) Transcalvarial 5) Tectal (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 uncus, can be squeezed so much that it moves towards the

and puts pressure on the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

, most notably the midbrain. The tentorium is a structure within the skull formed by the dura mater of the meninges. Tissue may be stripped from the cerebral cortex in a process called decortication. The uncus can squeeze the

oculomotor nerve The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...

(a.k.a. CN III), which may affect the

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...

input to the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

on the side of the affected nerve, causing the pupil of the affected eye to dilate 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 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 (innervated by abducens nerve (a.k.a. CN VI) and the superior oblique (innervated by trochlear nerve 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 will result in ischemia of the ipsilateral primary visual cortex and contralateral visual field deficits in both eyes (contralateral

homonymous hemianopsia Hemianopsia, or hemianopia, is a visual field loss on the left or right side of the vertical midline. It can affect one eye but usually affects both eyes. Homonymous hemianopsia (or homonymous hemianopia) is hemianopic visual field loss on the sa ...

). Another important finding is a ''false localizing sign'', the so-called

Kernohan's notch Kernohan's notch is a cerebral peduncle indentation associated with some forms of transtentorial herniation (uncal herniation). It is a secondary condition caused by a primary injury on the ''opposite'' hemisphere of the brain.#refWolf, Wolf et al, ...

, which results from compression of the contralateral cerebral crus containing descending corticospinal and some corticobulbar tract fibers. This leads to Ipsilateral hemiparesis 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 hemorrhage Duret haemorrhages are small linear areas of bleeding in the midbrain and upper pons of the brainstem. They are caused by a traumatic downward displacement of the brainstem. They are named after Henri Duret. Causes Duret haemorrhages are named af ...

s (tearing of small vessels in the

parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...

) in the median and paramedian zones of the mesencephalon and pons. The rupture of these vessels leads to linear or flamed shaped hemorrhages. The disrupted brainstem can lead to decorticate posture, respiratory center depression and death. Other possibilities resulting from brain stem distortion include lethargy, slow heart rate, and

pupil dilation Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve. A constriction response (miosis), is the narrowing of the pupil, which may be caused by scleral buckles or drugs such a ...

. 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 The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...

and parts of the temporal lobes of both of the cerebral hemispheres are squeezed through a notch in the tentorium cerebelli. 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 (pontine arteries), causing them to tear and bleed, known as a

. 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 due to the compression of the pituitary stalk. Radiographically, downward herniation is characterized by obliteration of the

suprasellar cistern The sella turcica (Latin for 'Turkish saddle') is a saddle-shaped depression in the body of the sphenoid bone of the human skull and of the skulls of other hominids including chimpanzees, gorillas and orangutans. It serves as a cephalometric la ...

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 is scraped under part of the falx cerebri, 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 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 vessels 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 and

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...

. 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, the protruding brain region preventing the piece of skull from being replaced during the operation.

Upward herniation

Increased pressure in the posterior

fossa Fossa may refer to: Animals * Fossa (animal), the common name of a carnivoran mammal of genus ''Cryptoprocta'' endemic to Madagascar * ''Fossa'', the Latin genus name of the Malagasy civet, a related but smaller mammal endemic to Madagascar Pla ...

can cause the

to move up through the tentorial opening in ''upward'', or ''cerebellar herniation''. The midbrain 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

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

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, difficulty ...

(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 ype0'. 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 Ehlers may refer to: * Ehlers–Danlos syndrome * Ehlers (surname) Ehlers is a German surname. Notable people with the surname include: * Arthur Ehlers, executive in minor and Major League Baseball * Beth Ehlers (born 1968), American actress * ...

, 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. Brain herniation can cause severe disability or death. In fact, when herniation is visible on a

CT scan A computed tomography scan (CT scan; formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers ...

, 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 The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formation ...

which regulates consciousness, will result in

. Damage to the cardio-respiratory centers in the

will cause

respiratory arrest Respiratory arrest is a sickness caused by apnea (cessation of breathing) or respiratory dysfunction severe enough it will not sustain the body (such as agonal breathing). Prolonged apnea refers to a patient who has stopped breathing for a long p ...

and (secondarily) cardiac arrest. Investigation is underway regarding the use of neuroprotective agents during the prolonged post-traumatic period of brain hypersensitivity associated with the syndrome.

References

External links

{{DEFAULTSORT:Brain Herniation Hernias Neurotrauma