1.
What is the cerebellar tonsil herniation?
Cerebellar tonsil herniation(CTH) is called as the tonsillar herniation,
the downward cerebellar herniation and the transforaminal herniation.
Depending on the degree of retraction,
the degrees of symptoms also vary from none to serious.
With wide foramen magnum or slowly progressive situation,
it is only accompanied by slight symptoms or sometimes no symptoms.
CTH may be accompanied by increased intracranial pressure and myelopathy associated with syringomyelia,
may also require the decompression by the surgery.
Due to retraction of the medulla oblongata,
impairment of respiratory center induces ataxic breathing and respiratory arrest.
CTH may cause disturbance of consciousness after apnea.
CTH may be lethal condition.
The tonsils more than 5mm below the line connecting with the lower pole of the clivus and the occipital bone are considerd abnormal(Fig.1).
Some authors reports that CTH is diagnosed by tonsils more than 3mm below the foramen magnum because of higher sensitivity,
but it may be observed 3-5mm descent in healthy group.
The physiological range of descent of the tonsils is 6mm or less in 10-year-old or younger and 5mm in from 11 to 33-year-old.
Seventy percents of the patients with 5-10mm below the foramen magnum and nearly all of patients with 12mm below it,
have symptoms.
2.
What causes the cerebellar tonsil herniation?
Causes of the CTH are mostly disproportion between volume of the intracranial contents and skull structure,
the other causes are mechanical or functional factors accompanied with the pressure gradient above and below the foramen magnum.(table.1)
Increased volume of intracranial contents include megalencephaly, hydrocephalus and space-occupying lesions such as tumors or hemorrhage.
The structural abnormalities of the skull induced CTH is a decrease of the intracranial volume,
in particular a decrease of the volume of the posterior fossa associated with malformations or developmental disabilities.
These are mostly congenital pathology.
Malformations include Chiari malformation,
and developmental disabilities include skeletal dysplasia leading to craniosynostosis such as Crouzon disease.
Functional factors such as intracranial hypotension by spontaneous or secondary cerebrospinal fluid leakage and increased intracranial pressure,
may cause CTH.
3.
Development of the posterior fossa
In order to understand dysplasia of the posterior fossa ,
it is necessary to understand the development of it.
Most of bones of the trunk is derived from mesoderm.
Skull including the frontal bone and facial bones are derived from the neural crest.
In contrast parietal bone and occipital bone ,
petrous portion of the temporal bone are derived from the paraxial mesoderm.(fig.2)
The cranium can be developmentally divided into neurocranium to protect the brain and viscerocranium to form the skeleton of the face.
I will describe with respect to the neurocranium involved in the development of the posterior fossa.
In the classification of ossification,
neurocranium is further divided into cartilaginous neurocranium and membranous neurocranium.
- cartilaginous neurocranium
It consists of cartilaginous primordia of the skull base,
bones of the skull base is formed by endochondral ossification.
The occipital bone,
the sphenoid bone and the ethmoid is in order ossified.
Parachordal cartilage or basal plate is formed around the cephalic end of the notochord ,
these fuse with cartilage from sclerotome of the occipital somites.
The cartilage mass involves in the formation of a basilar portion of the occipital bone, extends while surrounding the cephalic end of the spinal cord and forms a peripheral portion of the foramen magnum.
Intramembranous ossification of the neural crest mesenchyme at the top and lateral portion of the brain leads to form calvaria.
This means that occipital bone is derived from the paraxial mesoderm unlike the frontal bone and is formed by endochondral ossification unlike the frontal bone and the parietal bone.
So these abnormalities causes defective development of the posterior fossa.
It can be said that the occipital bone is distinctive among bones of the skull.
There is fibrous sutures in the calvaria of newborn,
so the brain volume can increase through in infancy and childhood.
The volume of the calvaria increases the most until 2-year-old,
the brain volume increases rapidly from 2 to 5-year-old.
Calvaria can extend until 16-year-old.
By thickening of the skull for 3-4 years after 16-year-old ,
calvaria can extend slightly further.
3.5.
Does the abnormality of the enchondral ossification cause the cerebellar tonsil herniation?
Abnormality of the endochondral ossification inhibits the development of the posterior fossa including skull base.
Achondroplasia representing abnormality of the endochondral ossification is accompanied not only with hypoplasia of the posterior fossa,
but also with narrowing of the foramen magnum at high rates.(fig.3)
Achondroplasia has a direct compression of the medulla oblongata rather than descent of the cerebellar tonsil in many cases.
However,
some case accompanied with CTH have been reported.
It often merge hydrocephalus,
but not severe.
4.
Assessment of the posterior fossa
Several indicators for the evaluation of the posterior fossa have been considered.
Compared with healthy group and Chiari malformation type I,
Remy et al.
discussed with respect to the neural and the bone structure of the posterior fossa.
It have been showed a trend of shorter clivus,
shorter supraocciput,
smaller midsagittal area of the posterior fossa and increased tentorial angle(normal degree from 27 to 52°).
There have been a significant difference in the length of the clivus.(fig.4)
There are some reports that the volume of the posterior fossa is not correlated with CTH.