Tonsillar Ectopia and Chiari Malformations
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.
CTE and Skeletal Anomolies
Classic Arnold-Chiari malformations were attributed to malformations in the brain and cord that develop from primitive ectodermal tissues in the embryo. Skin and nerves develop from ectodermal tissues. It was thought that these developmental problems were the cause of the malformations. More recent research, however, is beginning to show that many cases of Chiari malformations are due to design problems in the skull and upper cervical spine. In contrast to ectoderm, bones and muscles develop from primitive mesodermal tissue.
The following information on skeletal anomalies comes from an article in Mediscape called "Imaging in Chiari Malformations," by Nasir H. Saddiqi and others. According to the authors one of the common skeletal anomalies associated with Chiari I malformations is a shorter length and reduced height of the clivus and supraocciput of the base of the skull. Together these structures form the front and rear walls of the posterior fossa respectively. They also determine the ceiling height of the fossa. Moreover, the shorter lengths in the walls and lower ceiling height is associated with a small undersized posterior fossa, which contains the cerebellum and brainstem.
The authors further state that skeletal anomalies of the base of the skull have been seen in about 25-50% of the patients with a Chiari I malformation. Other common anomalies of the base of the skull include platybasia and basilar invagination, which account for about 25-50% of the cases. Platybasia refers to a skull base that is too flat. Basilar invagination occurs when the odontoid process of the second cervical vertebra (C2) called axis, penetrates (herniates) into the foramen magnum above it.
Atlantooccipital assimilation was seen in about 1-5%. It occurs when the first cervical vertebra called atlas or C1, fuses with the base of the skull during development. Another similar conditon called Klippel-Feil syndrome was seen in about 5-10% of the cases. Klippel-Feil syndrome, likewise, occurs when upper or lower cervical vertebra fuse and grow together during development. In contrast, to fusion, incomplete development and unification of the posterior arch (spina bifida) of the atlas or C1 vertebra was seen in another 5% of the cases. Lastly, a retroflexed or backward bend in the odontoid process of the axis (C2) vertebra was seen in 26% of the cases.
As for the rest of the spine, an abnormal sidewards curvature of the spine called a scoliosis was seen in about 42%. Additionally, other less frequent findings were an increase in cervical lordosis and kyphosis which are forward and backward bends in the spine respectively. Elongated processes on the cervical vertebra in the neck, called cervical ribs were also seen, as well as fused thoracic (chest) ribs.
Still other skeletal anomalies included problems with facial development called craniofacial dysmorphism (growth and development problems in the face such as a cleft palatte); occipital bone plagiocephaly (flattening of the bone in back of skull); and os odontoideum (lack of fusion of the odontoid process of C2). Also certain conditions that affect the health of the musculoskeletal system such as juvenile rheumathoid arthritis, and osteogenesis imperfecta have also been associated with Chiari malformations (cerebellar tonsillar ectopia). Among other things rheumatoid arthritis can affect the cervical spine and is associated with degeneration of the ligaments of the upper cervical spine as well as erosion of the odontoid process of C2 and sometimes a fluid filled cyst on C2 called a pannus. Osteogenesis imperfecta is associated with failure of the bones of the body to develop properly and mature. Both conditions are probably predisposed to Chiari malformations because of their impact on the upper cervical spine and base of the skull.
Of the skeletal abnormalities mentioned above, platybasia and basilar invagination were the most frequent finding. These findings were followed in frequency by retroflexion of the odontoid process and scoliosis.
In brief, anomalies of the the upper cervical spine and base of the skull were associated with fifty to seventy-five percent of the cases and scoliosis was associated with about forty percent of the cases of cerebellar tonsillar ectopia. Although it remains unclear, there definitely appears to be a connection between the design of the skull and spine to cerebellar tonsillar ectopia.
CTE and Brain and Cord Anomalies
In addition to anomalies (abnormalities) of the skull and spine, Chiari malformations are also associated with anomalies in the development of the brain and cord. The following findings discussed below on brain and cord anomalies associated with CTE are from a paper published in Pediatric Neurosurgery in 2010 called, "Cerebral anomalies and Chiari I type malformations," by Marcelo Galarza and others.
According to the authors, one of the most common anomalies that occurs in the cord is called syringomyelia mentioned previously. One of the most common anomalies of the brain was hydrocephalus. Hydrocephalus occurs when the volume of CSF in the brain increases. Kinking of the cervicomedullary cord was another common finding. The cervicomedullary cord is the point where the medulla of the lowest part of the brainstem joins to the top most portion of the spinal cord. Other brain anomalies include incomplete development or missing (agenesis) of the corpus callosum (communication system between the hemispheres of the brain) and an undersized (hypoplastic) brainstem.
Interestingly, agenesis of the corpus callosum and a hypoplastic, compressed looking, brainstem are findings similar to those seen in another condition in children called Dandy-Walker syndrome. Dandy-Walker, however, is the complete opposite of cerebellar tonsillar ectopia. As was mentioned above, instead of getting pushed down, in Dandy-Walker the cerebellum and the tentorium cerebelli, which covers the posterior fossa and cerebellum get pushed upwards and into the upper compartment of the brain. It is further interesting to note that Chiari malformations and Dandy-Walker syndrome are seen far more frequently in females than in males. Moreover, females tend to have smaller cranial vaults. In this regard, cerebellar tonisillar ectopia has been associated with a smaller posterior fossa, as indicated by a shorter clivus and supraocciput mentioned previously.
There is a condition seen in adults called multi-system atrophy or olivopontocerebellar atrophy (OPCA), which is a variant of Parkinson’s disease. Brain scans of adults with MSA or OPCA look suspiciously similar in certain ways to children with Dandy-Walker syndrome. I will discuss more about MSA (OPCA) and Dandy-Walker as this site grows.
CTE and Other Ectopias
In addition to the cerebellum, other tissues can be affected by displacement (ectopia) of the brain inside the cranial vault. For example, in a functional Chiari malformation the ventral (belly side) of the brainstem comes in contact with the clivus of the base of the skull during neck flexion. A similar condition occurs in what is called atlantoaxial instability.
Atlantoaxial instability occurs due to weakness, degeneration and tears of the upper cervical ligaments. Atlantoaxial instability is seen in Ehlers-Danlos syndrome and rheumatoid arthritis. It has further been associated with psoriatic arthritis. In Ehlers-Danlos syndrome the problem is due to genetically loose ligaments that allow too much range of motion, such as in the upper cervical spine. In rheumatoid and psoriatic arthritis the problem is caused by chronic inflammation and degeneration of the ligaments, which can affect the cervical spine.
In addition to loose or degenerated ligaments causing CTE, according to recent research, by Dr. Michael Freeman, an expert in public health and forensic medicine, Dr. David Harshfield Jr., a medical radiologist and Dr. Scott Rosa a chiropractor who specializes in the Atlas Orthogonal method used for upper cervical correction of the spine, atlantoaxial instability and cerebellar tonsillar ectopia can be caused by trauma. More recently, research by Dr. Raymond Damadian the inventor of the MRI and president of Fonar Upright MRI, along with David Chu, the head scientist at Fonar, showed that multiple sclerosis is associated with cervical trauma. Furthermore, other studies have linked MS to cerebellar tonsillar ectopia.
Among other things, according to a study by Dr. Thomas Milhorat and others atlantoaxial instability (hypermobility) can cause cranial settling in which the skull slides forward of its normal position during upright posture and sinks down onto the upper cervical spine. Consequently, cranial settling can cause functional Chiari malformation and basilar invagination. Basilar invagination occurs when the odontoid process of C2 is pushed upwards and into the foramen magnum (herniates) as mentioned above. It, likewise, occurs when the skull sinks onto the odontoid process.
Lastly, the tissues or bones, for that matter, such as the odontoid process of C2, don't actually have to herniate through the foramen (openings) in the skull to cause problems. Simple contact with the hard bones of the cranial vault is enough to cause problems in certain cases. Consequently, some researchers have suggested that the definition of a Chiari malformation should be expanded to include any condition that causes contact of the brain with the cranial vault.
Chiari 0 and Chiari 1 malformations are now believed to be acquired and some appear to be related to trauma, especially whiplash. I refer to them in my book as pressure conus type conditions. A pressure conus condition occurs when too much CSF is withdrawn from the cord for testing purposes. The decrease in CSF volume (actually pressure) causes the brain to sink in the cranial vault, which increases pressure on base of the brain with only minimal tonsilar ectopia (displacement) of the brainstem and cerebellum.
There is still one other significant factor to consider when it comes to Chiari malformations and CTE. If neurovascular tissues can herniate through the foramen magnum, then it seems likely that the brain can also be displaced toward and potentially herniate into other foramen in the base of the skull. The problem with herniation is that it causes compression of soft nerves and blood vessels as they get squeezed into the tight rigid tunnels of bone and connective tissues. In addition to compression, displacement of the brain can also cause tension on delicate tissues from stretch. Displacement of the brain also causes sheer forces inside the cranial vault, which can injure delicate nerves and blood vessels where they enter or exit through tunnels.
The optic nerve and ophthalmic artery to the eyes are particularly vulnerable and is discussed on the optic neuritis page. Interestingly, the great sensory nerve of the face, called the trigeminal nerve, is likewise, vulnerable. The acoustic nerve is susceptible to shear stresses. For further information on these nerves go to the cranial nerves page.
In light of all of the above, except in classic cases of Arnold-Chiari malformations, the term malformation has become outdated and inappropriate in describing all of the above variations of Chiari malformations. The term herniation isn't exactly appropriate in all cases either in that many are associated with little or no penetration of neurovascular tissues into the involved tunnels. I further suspect that other cases are associated with sheer forces not compression.
In this regard, cerebellar tonsillar ectopia is a better term since it simply describes diplacement of tissues. The problem with cerebellar tonsillar ectopia (CTE), however, is that it limits the condition to the cerebellum. A better, more encompassing general term, would be encephalic ectopia. The term encephalic refers to the entire brain including the cerebellum. Encephalic ectopia is a displacement of the brain within the cranial vault.
I will be discussing the impact of displacement of the brain in the cranial vault on neurovascular tunnels further as this site develops.
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