Brain herniation
| Brain herniation | |
|---|---|
| Specialty | Neurology, neurosurgery  |
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.
Classification
There are a number of different types of herniation, classified by which structures the brain shifts across: transtentorial, upward, tonsilar, central, cingulate, and transcalvarial herniation.[1]
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.[1][2] 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.[2]
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.[1] 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 (McCaffrey, 2001). 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.[2] 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.[3] 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 (McCaffrey, 2001). 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.[1] The midbrain is pushed through the tentorial notch. This also pushes the midbrain down.[3]
Tonsillar herniation
In tonsillar 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..[1] Increased pressure on the brainstem can result in dysfunction of the respiratory and cardiac centers. [3]
Tonsillar herniation of the cerebellum is also known as an Arnold Chiari Malformation (ACM). This condition can range in severity of symptoms from none/minimal to life-threatening. This condition once thought to be very rare is now being discovered more frequently with the use of MRI. In some radiologic reports, this may also be referred to as tonsillar ectopia or low lying tonsils. There is also discussion occurring as to the definition of a Chiari Malformation, the currently accepted standard is 5mm. below the level of the foramen magnum. This is being challenged, in fact no longer accepted, by many specialists in the field, noting that some people experience symptoms with no herniation or 'Chiari 0' meaning no herniation of the tonsils.
The most widely recognized symptom is known as a 'Chiari Headache'; this is a headache that is worsened with the valsava maneuver. Unfortunately, most health care providers still believe a symptomatic Chiari Malformation will exhibit this type of headache. This is not necessarily true. It should also be noted that there are many comorbid conditions that can cause, occur with or result from a Chiari Malformation. Some of the suspected causes of tonsillar herniation include(but not exclusively)- Spinal cord tethering or occult tight filum terminale (pulling down on the spinal cord/brainstem and surrounding structures); a 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). Among the more well known comorbid conditions are- Syringomyelia, a condition many have heard of because of golfing great, Bobby Jones, -and Spina Bifida (associated most often with ACM Type II). A Chiari Malfomation is often 'coincidentally' found when tests are being run for scoliosis in children. It is now believed that this is not an incidental finding.
For further evaluation of tonsillar herniation, CINE flow studies are suggested. For people 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. It is also recommended that persons with a complicated diagnosis or with comorbid conditions, especially those with connective tissue disorders, be treated by a specialist whose primary focus is ACM.
Further information regarding Arnold Chiari Malformation (ACM) can be found though the following organizations: WACMA (World Arnold Chiari Malformation Association) [1] Chiari Connection International [2] and Conquer Chiari.org [3]. These sites also have groups for discussion and support of this 'invisible illness' that is not well understood.
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.[4][1] 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.[1][3] Usually occurring in addition to uncal herniation, cingulate herniation presents with abnormal posturing and coma.[3]
Transcalvarial herniation
In transcalvarial herniation, the brain squeezes through a fracture or a surgical site in the skull.[3]
Results of herniation
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.[2] Damage to the midbrain, which contains the reticular activating network that regulates consciousness will result in coma.[2] Damage to the cardio-respiratory centers in the medulla will cause respiratory and cardiac arrest.[2]
References
- ^ a b c d e f g Shepherd S. 2004. "Head trauma." Emedicine.com. Retrieved on January 28, 2007.
- ^ a b c d e f Cornell. 1998. Introduction to neuropathology. Reaction to injury: Brain histology." Cornell University Medical College.
- ^ a b c d e f Orlando Regional Healthcare, Education and Development. 2004. Overview of adult traumatic brain injuries. Retrieved on January 28, 2007.
- ^ Dawodu ST. 2007. Traumatic brain injury: Definition, epidemiology, pathophysiology. Emedicine.com. Retrieved on January 28, 2007.