This is also known as ascending transtentorial herniation since it occurs across the tentorium cerebelli. Tonsillar herniation
In tonsillar herniation, also called downward cerebellar herniation, transforaminal herniation, or "coning", 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. 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 level of consciousness may decrease and also give rise to flaccid paralysis. Blood pressure instability is also evident in these patients.Congenital tonsillar herniation 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.
Glucarpidase (Voraxaze) is a medication used for the treatment of elevated levels of methotrexate (defined as 1 micromol/L) during treatment of cancer patients who have impaired kidney function (and thus cannot reduce the drug to safe levels sufficiently after the drug has been given). Glucarpidase is an enzyme that inactivates methotrexate rapidly after injection. Because this agent reduces systemic levels of methotrexate and could therefore interfere with efficacy, it is not recommended for use in patients with normal or only slightly impaired kidney function or in whom serum levels are normal. The main antidote for methotrexate overdoses prior to the approval of this drug were high doses of folinic acid. However, this agent was not always sufficient at preventing kidney failure due to methotrexate. Glucarpidase also degrades folinic acid so the two should not be used together (within two hours of one another). Glucarpidase, a recombinant form of the bacterial enzyme carboxypeptidase G2 converts methotrexate into glutamate and 2,4-diamino-N(10)-methylpteroic acid. These are generally much less toxic and are excreted largely by the liver. One case series in children has found that high-dose methotrexate therapy can be resumed after an instance of methotrexate-induced acute kidney injury successfully treated with glucarpidase.Adverse effects include mild and include numbness, tingling, flushing, nausea, vomiting, itching, and headache.