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Keywords and synonyms: Cerebral herniation, cerebellar herniation, uncal herniation, central herniation, supratentorial herniation, infratentorial herniation, transtentorial herniation, tonsillar herniation, cingulate herniation, transcalvarial herniation

# Overview
Herniation, a deadly side effect of very high intracranial pressure, occurs when the brain shifts across structures within the skull.  The brain can shift by such structures as the falx cerebri, the tentorium cerebelli, and even through the hole called the foramen magnum 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 increase intracranial pressure such as traumatic brain injury.  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 pressure.

# Historical Perspective
In 1783, a medical educator and anatomist from Scotland, Alexander Monro, first proposed the concept of Intracranial pressure in his paper 'Observations on the Structure and Functions of the Nervous System'. In this paper, he stated his observations about brain, cranial cavity, and cranial blood flow. He mentioned that the cranial cavity is rigid and it encloses the incompressible brain. He hypothesized that the blood volume is constant in the cranial cavity at all times. There is a continuous balance between outflowing venous and incoming arterial blood.
This hypothesis was supported by George Kellie's (Monore's former student) experiments, and 'MonroKellie hypothesis' came into existence. According to this, the sum of brain volume, cerebrospinal fluid, and intrcranial blood flow is constant.

# Classification
There are two major classes of herniation: supratentorial and infratentorial.. Depending on which structures of the brain have been displaced, herniation is further sub-classified.
Tentorium Cerebelli is one of the four duramater folds (Extention of outermost meningeal layer, duramater). It serves as a landmark and divides the cranial cavity structures into supra and infra tentorial. This dural extention has a fixed and free margin. The free margin of tentorium cerebelli is called as 'Tentorial notch".

## Central herniation
In central herniation, (also called "transtentorial herniation") the diencephalon and parts of the temporal lobes of both of the cerebral hemispheres are squeezed through a notch in the tentorium cerebelli.  Downward herniation can stretch branches of the basilar artery (paramedian artery), causing them to tear and bleed, known as a Duret hemorrhage.  The result is usually fatal.

## 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 goes by the tentorium and puts pressure on the brainstem. The tentorium is a structure within the skull formed by the meningeal layer the dura mater.  Tissue may be stripped from the cerebral cortex in a process called decortication.  The uncus can squeeze the third cranial nerve, which controls parasympathetic input to the eye on the side of the affected nerve.  This interrupts the parasympathetic neural transmission, causing the pupil of the affected eye to dilate 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). Cranial arteries may be compressed during the herniation.  Compression of the posterior cerebral artery may result in loss of the contralateral visual field.  A later important finding, the false localizing sign, results from compression of the contralateral cerebral crus, which contains descending corticospinal fibers. This leads to ipsilateral (to herniating uncus) hemiparesis of the body. This type of herniation can also damage the brain stem, causing lethargy, slow heart rate, respiratory abnormalities, and pupil dilation.  Uncal herniation may advance to central herniation.

## Cerebellar 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 is pushed through the tentorial notch. This also pushes the midbrain down.

## Tonsillar herniation
In tonsillar herniation, also called downward cerebellar herniation, 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.
Tonsillar herniation of the cerebellum is also known commonly as a Chiari Malformation (CM)  which previously was called 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 are cerebellar tonsils that lay 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 0'.
There are many suspected causes of tonsillar herniation including (but not exclusively)- 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 as well.(These may affect the ability of the brain, as well as the supporting joints, to maintain proper position/strength; in pediatric cases, ACM is often an 'incidental' finding while doing a work up for scoliosis, which is usually associated with certain connective tissue disorders).
For further evaluation of tonsillar herniation, CINE flow studies are suggested. This type of MRI will look at the flow of CSF at the cranio-cervical joint.  For persons experiencing symptoms with seemingly minimal herniation, especially if the symptoms are better in the suppine position and worse upon standing/upright, an upright MRI may be useful.

## 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. 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.  Cingulate herniation is frequently believed to be a precursor to other types of herniation.
- Subfalcine-herniation

## Transcalvarial Herniation
In transcalvarial herniation, the brain squeezes through a fracture or a surgical site in the skull.

# Natural History, Complications and Prognosis
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 consciousness will result in coma. Damage to the cardio-respiratory centers in the medulla will cause respiratory and cardiac arrest.

# Pathophysiology
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# Causes
Conditions that lead to elevation of inracranial pressure increase the risk of brain herniation. These include-
Localized or generalized swelling of the brain
Space-occupying lesions
Increase intracranial venous blood pressure (venous sinus thrombosis, Heart failure, jugular venous obstruction)
Obstruction of Cerebro-spinal Fluid flow
Traumatic (bRAIN HEMORRHAGES, CONTUSION)
Space-occupying lesions (eg, brain tumor, edema, or abscess; contusions; hematomas)
Generalized swelling or edema of the brain (eg, due to acute liver failure or hypertensive encephalopathy)
Increased venous pressure (eg, due to heart failure, obstruction of superior mediastinal or jugular veins, or venous sinus thrombosis)
Obstruction of the CSF flow (eg, due to hydrocephalus or extensive meningeal disease)
There are multispectral factors that can predispose to raised intracranial pressure and brain herniation syndrome such as:
Hematoma (traumatic epidural and subdural hematoma, contusions, intracerebral hemorrhage)
Malignant infarction
Tumors
Infections (abscess, empyema, hydatid cyst)
Hydrocephalus
Diffuse subarachnoid hemorrhage
Pneumocephalus (traumatic or postoperative)
CSF over drainage 
Metabolic-hepatic encephalopathy
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# Diagnosis

## Diagnostic Study of Choice
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## History and Symptoms
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## Physical Examination
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## Laboratory Findings
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 is usually normal among patients with .
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## Electrocardiogram
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## X-ray
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## Echocardiography or Ultrasound
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## CT scan
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 CT scan may be helpful in the diagnosis of . Findings on CT scan suggestive of/diagnostic of  include , , and .
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There are no CT scan findings associated with . However, a CT scan may be helpful in the diagnosis of complications of , which include , , and .

## MRI
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 MRI may be helpful in the diagnosis of . Findings on MRI suggestive of/diagnostic of  include , , and .
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There are no MRI findings associated with . However, a MRI may be helpful in the diagnosis of complications of , which include , , and .

## Other Imaging Findings
There are no other imaging findings associated with .
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 may be helpful in the diagnosis of . Findings on an  suggestive of/diagnostic of  include , , and .

## Other Diagnostic Studies
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 may be helpful in the diagnosis of . Findings suggestive of/diagnostic of  include , , and .
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Treatment
Medical Therapy
Brain Herniation is a medical emergency and requires prompt management. Cautious adjustment of intracranial pressure should be done in all patients who develop displacement of the brain tissue. Based on Monroe-Kellie hypothesis, balance in brain volume, CSF volume and intracranial blood
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 is recommended among all patients who develop .
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Surgery
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Primary Prevention
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 vaccine is recommended for  to prevent . Other primary prevention strategies include , , and .
Secondary Prevention
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