Tonsillar ectopia, also known as cerebellar tonsillar ectopia, Chiari malformation and Arnold-Chiari malformation, is associated with descent of the bottom portion of the cerebellum into the foramen magnum. In this regard, ectopia means an abnormal location or position of an organ or body part. Hence CTE refers to a downward displacement of the lower portion of the cerebellum, called the tonsils into the foramen magnum. The foramen magnum is the large hole in the base of the skull for the passage of the brainstem and cord.
The arrow in the brain scan above points to the tonsils of the cerebellum. To the right of the tonsils is a thick white, almost horizontal line below the rest of the cerebellum. That white line is the part of the occipital bone which forms the rear wall of the cranial vault, as well as the floor of the posterior fossa. The front tip of the white line is the backside of the foramen magnum. Technically, the backside of the foramen magnum portion of the occipital bone is called the opisthion.
On the other side of the foramen magnum in front of the brainstem is a white triangular shape pointing downward. The white triangular shape is the clivus portion of the base of the skull. The clivus is formed by the union of the sphenoid and occipital bones. The sphenobasilar junction of the clivus is joined together by cartilage which slowly disappears with age. The bottom tip of the clivus is the front side of the foramen magnum. The technical term for the tip of the clivus portion of the base of the skull on the front side of the foramen magnum is the basion.
If you draw a line from the basion at the front of the foramen magnum to the opisthion at the back of the foramen in the MRI above, you will see that the tonsils of the cerebellum fall below the line. (The arrow points to the cerebellar tonsillar ectopia.)
Chiari malformations and cerebellar tonsillar ectopia are technically brain herniations as well. A herniation occurs when tissues, such as the brain or blood vessels in this case, get squeezed through openings in neighboring tissues or bone, compressing them. In the sketch below, an example of a tonsillar ectopia is represented by number six. The number five example represents an upward displacement (ectopia) type herniation, which occurs in the opposite direction of a cerebellar tonsillar ectopia. There is a condition called Dandy-Walker syndrome that is associated with an upward displacement (ectopia) of the cerebellum, that will be discussed further below on this page. In this case, instead of herniating through it, the upward displacement of the cerebellum causes a deformation of its covering called the tentorium cerebelli.
Currently, there are five types of Chiari malformations or cerebellar tonsillar ectopia based on the degree of descent of the cerebellum and braintem, as well as other factors such as congenital defects in the brain, cord and spine, which will be discussed below. They range from a Chiari 0 malformation, with minimal tonsillar etopia (displacement) and herniation into the foramen magnum, to a Chiari 4 with significant tonsillar ectopia and herniation and other problems. The greater the degree of downward displacement the more likely it is to involve other parts of the brainstem along with the cerebellum.
In addition to tonsillar ectopia, some Chiari malformations are associated with syrinxes in the central canal of the cord called syringomyelia or hydromyelia. Still others, such as Arnold-Chiari II and III malformations are associated with herniation of the brainstem or the cord and their coverings called meninges, or both the brain and the cord along with their meninges through defects in the walls of the skull or spine. Example number four in the sketch above is of a herniation of the brain through the skull, which can occur in skull fractures. In contrast to the sketch, Chiari malformations are more typically associated with herniations through the rear wall of the posterior fossa of the cranial vault or a defect in the rear wall (spinal bifida) of the upper cervical vertebra.
Technically speaking, because they are associated with the structural defects mentioned above, such a herniation of the brain and cord through the skull and spine, Arnold-Chiari malformations are still considered by some experts to be different from Chiari malformations. In practice, most neurosurgeons don’t distinguish between the two, as changing the name doesn’t change the tough challenges they face in either case. Neurosurgeons are more concerned with the degree of the herniation of the cerebellum and brainstem, as well as the different types of congenital (birth) defects in the brain and cord and their impact on the health of the patient.
Surgical correction for Chiari malformations with tonsillar ectopia involve removing and shaving down bones in the base of the skull and upper cervical spine to decompress the area. Surgical decompression is followed by stabilzation with plates and screws to safely secure the bones weakened by the surgery. Surgical correction of Arnold-Chiari malformation is the opposite of a Chiari correction for tonsillar ectopia. Arnold-Chiari correction, which is tonsillar ectopia along with protrusion of the brain or cord outside the cranial vault or spinal canal, is more complex. The procedure requires decompression, as well as replacing the protruded contents of the brain and cord back inside the cranial vault and spinal canal and closing up the opening defect in the skull and spine.