[1] 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 in tonsillar herniation.

This is a life-threatening condition as it causes increased pressure on the medulla oblongata which contains respiratory and cardiac control centres.

A Type I Chiari malformation is a congenital anomaly of the brain in which the cerebellar tonsils are elongated and pushed down through the opening of the base of the skull (see foramen magnum), blocking the flow of cerebrospinal fluid (CSF) as it exits through the medial and lateral apertures of the fourth ventricle.

Although often congenital, Chiari malformation symptoms can also be induced due to physical head trauma, commonly from raised intracranial pressure secondary to a hematoma, or increased dural strain pulling the brain caudally into the foramen magnum.

[2] This article incorporates text in the public domain from page 791 of the 20th edition of Gray's Anatomy (1918)