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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 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 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 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. ==Classification== The tentorium is an extension of the dura mater that separates the cerebellum from the cerebrum. 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.〔Orlando Regional Healthcare, Education and Development. 2004. ("Overview of Adult Traumatic Brain Injuries." ) Retrieved on January 16, 2008.〕 * Supratentorial herniation # Uncal (transtentorial) # Central # Cingulate (subfalcine) # Transcalvarial # Tectal (posterior) * Infratentorial herniation
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