Classification Types of<unk> . The tentorium is an extension of the dura mater that separates the cerebellum from the cerebrum. There are two major classes of<unk> : supratentorial and infratentorial. Supratentorial refers to<unk>  of structures normally found above the tentorial notch, and infratentorial refers to structures normally found below it. * Supratentorial<unk>  1) Uncal (transtentorial) 2) Central 3) Cingulate (subfalcine/transfalcine) 4) Transcalvarial 5) Tectal (posterior) * Infratentorial<unk>  6) Upward (upward cerebellar or upward transtentorial) 7) Tonsillar (downward cerebellar) Uncal<unk>  In uncal<unk> , a common subtype of transtentorial<unk> , the innermost part of the temporal lobe, the uncus, can be squeezed so much that it moves towards the tentorium and puts pressure on the<unk> stem, 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 (a.k.a. CN III), which may affect the parasympathetic input to the eye 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). Another important finding is a false localizing sign, the so-called Kernohan's notch, 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<unk>  and contralateral hemiparesis with reference to the cerebral crus. With increasing pressure and progression of the hernia there will be distortion of the<unk> stem leading to Duret hemorrhages (tearing of small vessels in the parenchyma) 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<unk> stem can lead to decorticate posture, respiratory center depression and death. Other possibilities resulting from<unk>  stem distortion include lethargy, slow heart rate, and pupil dilation. Uncal<unk>  may advance to central<unk> . The sliding uncus syndrome represents uncal<unk>  without alteration in the level of consciousness and other sequelae mentioned above. Central<unk>  In central<unk> , the diencephalon and parts of the temporal lobes of both of the cerebral hemispheres are squeezed through a notch in the tentorium cerebelli. Transtentorial<unk>  can occur when the<unk>  moves either up or down across the tentorium, called ascending and descending transtentorial<unk>  respectively; however descending<unk>  is much more common. Downward<unk>  can stretch<unk> ches of the basilar artery (pontine arteries), causing them to tear and bleed, known as a Duret hemorrhage. The result is usually fatal. Other symptoms of this type of<unk>  include small, fixed pupils with 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<unk>  is characterized by obliteration of the suprasellar cistern from temporal lobe<unk>  into the tentorial hiatus with associated compression on the cerebral peduncles. Upwards<unk> , on the other hand, can be radiographically characterized by obliteration of the quadrigeminal cistern. Intracranial hypotension syndrome has been known to mimic downwards transtentorial<unk> . Cingulate<unk>  Subfalcine<unk>  on CT In cingulate or subfalcine<unk> , 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<unk> . Cingulate<unk>  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<unk> stem as the other types of<unk> , 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<unk> . Interference with the blood supply can cause dangerous increases in ICP that can lead to more dangerous forms of<unk> . Symptoms for cingulate<unk>  are not well defined. Usually occurring in addition to uncal<unk> , cingulate<unk>  may present with abnormal posturing and coma. Cingulate<unk>  is frequently believed to be a precursor to other types of<unk> . Transcalvarial<unk>  In transcalvarial<unk> , the<unk>  squeezes through a fracture or a surgical site in the skull. Also called "external<unk> ", this type of<unk>  may occur during craniectomy, surgery in which a flap of skull is removed, the protruding<unk>  region preventing the piece of skull from being replaced during the operation. Upward<unk>  Increased pressure in the posterior fossa can cause the cerebellum to move up through the tentorial opening in upward, or cerebellar<unk> . The midbrain is pushed through the tentorial notch upward. This is also known as ascending transtentorial<unk>  since it occurs across the tentorium cerebelli. Tonsillar<unk>  In tonsillar<unk> , also called downward cerebellar<unk> , transforaminal<unk> , or "coning", the cerebellar tonsils move downward through the foramen magnum possibly causing compression of the lower<unk> stem and upper cervical spinal cord as they pass through the foramen magnum. Increased pressure on the<unk> stem can result in dysfunction of the centers in the<unk>  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<unk>  of the cerebellum is also known as a Chiari malformation (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<unk> . Sometimes these patients are described as having a 'Chiari type 0'. There are many suspected causes of tonsillar<unk>  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<unk>  caudally. Connective tissue disorders, such as Ehlers Danlos syndrome, can be associated. For further evaluation of tonsillar<unk> , 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.
Brain herniation
Epidural hematoma
Subdural empyema
Cerebral contusion
Cerebrospinal fluid leak
Arachnoiditis
Intraparenchymal hemorrhage
Intracerebral hemorrhage