These symptoms are known as Cushing's Triad: hypertension (with widening pulse pressure), irregular respirations (commonly Cheyne-Stokes), bradycardia and in severe cases cardiac arrest.

[3][4] Causes of brain herniation include:[5] The tentorium is an extension of the dura mater that separates the cerebellum from the cerebrum.

Another important finding is a false localizing sign, the so-called Kernohan's notch, which results from compression of the contralateral[10] cerebral crus containing descending corticospinal and some corticobulbar tract fibers.

[citation needed] 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.

Other possibilities resulting from brain stem distortion include lethargy, slow heart rate, and pupil dilation.

[7] The sliding uncus syndrome represents uncal herniation without alteration in the level of consciousness and other sequelae mentioned above.

In central herniation, the diencephalon and parts of the temporal lobes of both of the cerebral hemispheres are squeezed through a notch in the tentorium cerebelli.

Also found in these patients, often as a terminal complication is the development of diabetes insipidus due to the compression of the pituitary stalk.

[1] Increased pressure in the posterior fossa can cause the cerebellum to move up through the tentorial opening in upward, or cerebellar herniation.

[8] Increased pressure on the brainstem can result in dysfunction of the centers in the brain responsible for controlling respiratory and cardiac function.

The most common signs are intractable headache, head tilt, and neck stiffness due to tonsillar impaction.

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.

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.

In fact, when herniation is visible on a CT scan, the prognosis for a meaningful recovery of neurological function is poor.

[11] Investigation is underway regarding the use of neuroprotective agents during the prolonged post-traumatic period of brain hypersensitivity associated with the syndrome.