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 refers to herniation of structures normally found above the
tentorial notch, and infratentorial refers to structures normally found below it. •
Supratentorial herniation 1)
Uncal (transtentorial) 2) Central 3)
Cingulate (
subfalcine or 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
tentorium and puts pressure on the
brainstem, 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 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 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 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. 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. 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 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. Cingulate herniation can be caused when one hemisphere swells and pushes the
cingulate gyrus by the falx cerebri. Symptoms for cingulate herniation are not well defined. 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 [type] 0'. 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, 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==