For the structure in the cerebellum, see cerebellar tonsil.

Such cerebellar tonsil herniation may occur in up to 70% of children with M-CM.

The currently accepted radiographic definition for a Chiari malformation is that cerebellar tonsils lie at least 5mm below the level of the foramen magnum.

The portion of the cerebellum that protrudes into the spinal canal can become elongated and is called the "cerebellar tonsils" because it resembles the tonsils.

The flocculonodular lobe of the cerebellum which can also be confused for the cerebellar tonsils, is one of three lobes that make up the overall composition of the cerebellum.

Increased intracranial pressure may result in herniation (i.e. displacement) of certain parts of the brain, such as the cerebellar tonsils or the temporal uncus, resulting in lethal brainstem compression.

Cerebellar ectopia 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.

His name is lent to a disorder known as Arnold-Chiari malformation, which occurs when the cerebellar tonsils and the medulla oblongata protrude through the foramen magnum into the spinal canal.

It consists of a downward displacement of the cerebellar tonsils and the medulla through the foramen magnum, sometimes causing hydrocephalus as a result of obstruction of cerebrospinal fluid outflow.

Elongation of the cerebellar tonsils can, due to pressure, lead to this portion of the cerebellum to slip or be pushed through the foramen magnum of the skull resulting is tonsillar herniation.

The displacement of the cerebellar tonsils into the spinal canal may be mistaken for a Chiari I malformation, and some patients with spontaneous intracranial hypotension have undergone decompressive posterior fossa surgery.

A consequence of this disproportionate brain growth appears to be a significantly increased risk of cerebellar tonsillar herniation (descent of the cerebellar tonsils through the foramen magnum of the skull, resembling a Chiari I malformation neuroradiologically) and ventriculomegaly/hydrocephalus.

In tonsillar herniation, also called downward cerebellar 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.