A Chiari malformation, or cerebellar tonsillar ectopia (CTE) was once attributed to a malformation of the cerebellum and brainstem in children. It was believed that a slightly oversized brainstem caused crowding in the posterior fossa. In the picture above the posterior fossa lies beneath the cover, called the tentorium cerebelli that separates it from the rest of the brain above.
The posterior fossa contains the cerebellum. Crowding of the posterior fossa forces the inferior (lower) and medial (inner) parts of the cerebellum closest to the foramen magnum (large hole in the base of the skull) down and into it. The tonsils are the lower and inner parts of the cerebellum. Depending on how bad the situation is, it may involve much more of the cerebellum than just the tonsils as in the picture above. Click here for diagnosis and treatment of Chiari.
Later studies demonstrated that Chiari malformations can also be caused by problems in the design of the base of the skull, particularly the posterior fossa, and also an undersized foramen magnum, or tighter type design in the spinal canal of the upper cervical spine. An interesting page on skull design due to race and gender is Cerebellar Tonsillar Ectopia, Race and Gender.
In this regard some babies are born with a shorter length in the base of the skull measured from anterior to posterior, which is front to back. This can crowd the contents of the cranial vault and force them down into the foramen. It is further interesting to note that Down’s syndrome babies are often found to have a short skull base. The picture below is of an infant with a short base that died of SIDS shortly after birth.
In addition to design problems in the brainstem, skull or spine, even more recent studies showed a significant correlation between Chiari malformations and trauma. This means that adults can get it later in life. There is also an association between Chiari, syringomyelia and trauma. Moreover, if trauma can cause it in adults, then surely cervical strains during delivery can cause it in newborns.
Lastly, there is a significant correlation between Chiari malformations and spondylosis (degenerative changes of the spine), stenosis (narrowing of the spinal canal), scoliosis,(lateral bend in the spine), tethered cord syndromes (tension in the spinal cord due a genetically short design relative to the length of the spine, or acquired shortening in its length due to degenerative changes).
Venous Compression versus Stenosis
Just recently, vascular surgeon Dr. Paulo Zamboni of the University of Ferrara in Italy has attributed the cause of multiple sclerosis to chronic cerebrospinal venous insufficiency or CCSVI. Recent studies have shown a correlation between Chiari malformations and multiple sclerosis and they share many similar signs and symptoms as well. This further solidifies the link between Chiari malformations, CCSVI and multiple sclerosis. It also lends further credence to a long recognized suspicion of a connection between multiple sclerosis and trauma.
Contact of the brainstem against the bones of the base of the skull or foramen magnum can cause compression of the venous and cerebrospinal fluid (CSF) pathways. Compression of venous pathways can cause CCSVI. Compression of CSF pathways can lead to NPH. Compression of the brainstem against the bones of the base of the skull can also compress cranial and other nearby nerve structures.
Chiari Malformations are typically graded according to the depth of penetration of the brainstem into the foramen magnum. A new term has been coined however, called Chiari 0. In a Chiari 0 the brainstem does not sink into the foramen magnum. Instead, the space typically filled with fluid between the bottom of the cerebellum and base of the skull gets compressed.
This causes the cerebellum to come in contact with the hard bones of the skull and to compress the venous dural sinus drainage routes that lie beneath it. In particular it can cause compression of the much smaller occipital marginal sinus and accessory emissary venous outlets that drain into the vertebral veins during upright posture.
Some researchers now suggest the current method of determining Chiari malformations based on descent into the foramem magnum may be inadequate. Instead, they recommend comparative analysis of CSF volume in suspected Chiari cases to normal ranges. They further suggest that herniation of the cerebellar tonsils into the foramen magnum is not the real problem. The real problem comes from blockage of CSF flow. Consequently, they maintain that a Chiari malformation occurs anytime the brain sinks in the vault sufficient enough to block CSF flow.
Chiari, CCSVI and Normal Pressure Hydrocephalus
In addition to CCSVI, compression of the brainstem in the foramen magnum also affects the subarachnoid spaces of the brainstem and spinal cord in this area, as well as in the spinal canal. The subarachnoid space contains CSF, thus, Chiari malformations can lead to entrapment of CSF resulting in normal pressure hydrocephalus (NPH) in the brain and subsequent NPH without ventriculomegaly. Ventriculomegaly means an increase in the size of the chambers of the brain called ventricles where CSF is produced. Again, some researchers now suggest that NPH occurs when CSF flow becomes blocked with or without ventriculomegaly.
Compression of venous pathways and subsequent CCSVI can decrease the passive CSF pressure gradient. The job of the passive CSF pressure gradient is to increase CSF production during upright posture to compensate for the increase in outflow. This means that CCSVI could lead to a Chiari malformation and subsequent edema and NPH.
Lastly, in addition to blockage of blood and CSF flow, in severe cases of Chiari conditions in children, the blockage of CSF can cause a condition called hydrocephalus in the brain above the point of constriction, and what are called syrinxes, which are cavities in the central canal of the spinal cord, below the point of constriction.
CSF flow is pulsatile in nature being driven by arterial pressure waves and significantly amplified by respiratory pressure waves. Inhalation sucks CSF into the skull and exhalation pushes it out.
After being produced CSF flows out of the fourth ventricle, which is the last and lowest ventricle, into the cisterns of the subarachnoid space. (Click cisterns for a picture.) Some of it also flows down into the central canal, which runs through the center of the spinal cord. All CSF in the brain and cord flows up through the subarchnoid space of the brain to be absorbed into the superior sagittal sinus (vein) at the top of the skull.
The cavities in the cord called syrinxes are caused by the pulsatile CSF jetting out at the point of constriction on one side like water out of a tight nozzle on a high pressure hose. As it jets out opposite the side of constriction it crashes into the walls of the central canal and over time wears it down like waves beating on coastal cliffs.