Brain Herniation - Classification - Tonsillar Herniation

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

Tonsillar herniation of the cerebellum is also known as a Chiari Malformation (CM), or previously an Arnold Chiari Malformation (ACM). There are at least three types of Chiari malformation that are widely recognized, 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 and more patients undergo MRI scans of their heads. 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. 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 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. 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 with seemingly Max herniation, especially if the symptoms are better in the supine position and worse upon standing/upright, an upright MRI may be useful.