The posterior fossa is the lower rear compartment of the skull. Due to upright posture, the design of the posterior fossa predisposes humans to Chiari malformations and neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis (MS), as well as neurodegenerative conditions of the cord. It’s layout is discussed below.
The Base of the Skull
The base of the skull in most mammals is flat. In humans it is bent to accommodate upright posture. The angle of the base is formed by a line from drawn from a point inside the skull at the level of the bridge of the nose, called the the nasion, to the middle of the sella turcica as seen in the brain scan on the left. The sella turcica (named due to its resemblance to a turkish saddle) is a small separate subcompartment in the cranial vault that houses the pituitary gland. The other line is along the length of the clivus, which goes from the top of the backside of the sella turcica down to the front side of the inside of the foramen magnum. The foramen magnum is the large hole in the base of the skull for passage of the brainstem and cord. The two lines can be seen in blue. The normal angle between the two blue lines should be between 125 – 145 degrees. (This picture is used with permission from a contribution by Dr. Frank Gaillard to Radiopaedia.org).
For simplicity sake, there are three major races to consider when it comes to the design of the skull. They are European, Asian and African. The most obvious difference between the races is the face. Not only are facial designs different as discussed elsewhere on this site. For example, the height of the upper face above the anterior fossa, which contains the forehead and forebrain, and the width of the face show considerable racial differences. The face affects the design and layout of the cranial vaults of the different races as well.
In any case, the angle of the base of the skull is about the same in all three races. The orientation of the two lines, however, that form the base of the skull relative to upright posture and the horizontal plane line differ slightly. In Europeans and Asians it tends to incline upwards. In African designs is it less inclined and more horizontal. Lastly, the length of the clivus differs in the different races. It’s longer in Asians and Africans and shorter in Europeans. For additional information see the page Venous Inversion Flows and Skull Shape.
The Posterior Fossa
The brain scan on the right is from a paper called “Dimensions of the posterior fossa in patients symptomatic for Chiari 1 malformamtion but without cerebellar tonsilar descent,” by Raymond F. Sekula Jr. et al., and published in Cerebrospinal Fluid Research in 12/18/05. The posterior fossa is the area within the white lines. The bottom part is shaped like a soup bowl. The front borderline is the clivus portion of the base of the skull. The bottom borderline is the large opening in the base of the skull called the foramen magnum.
On the backside of the fossa the lower rear borderline is the portion of the occipital bone located beneath the bump on the back of the outside of the head known as the knowledge bone. The top rear borderline is the covering over the cerebellum and posterior fossa called the tentorium cerebelli.
There is no line across the front side of the top of the fossa because part of it is an open. It’s called the tentorial notch or incisura tentorii. The brainstem extends from the posterior fossa through the notch and into the middle fossa.
The design of the base of the skull including its angle, inclination, height and width, affects the design and inclination of the posterior fossa. It consequently determines the position of the brainstem and cerebellum within the posterior fossa and over the foramen magnum.
In the topmost picture the brainstem it slanted slightly forward but its shape is nearly straight. In the second picture the brainstem is more upright but it is slightly bent where it connects to the cord through the foramen magnum. Everybody is a little different but there are some significant racial differences that can have an impact on neurodegenerative conditions.
The angle of the base of the skull and the design of the posterior fossa also affects the postion of the brainstem over the foramen magnum. A Chiari malformation occurs when the brain sinks in the cranial vault and comes in contact with the bones of the base of the skull or penetrates into the foramen magnum below it.
Two factors predispose certain people to Chiari malformations. The first is a short clivus portion of the base of the skull. The second is a steep tentorium cerebelli, which is the membranous cover over the top of the posterior fossa mentioned above. Both findings are also often associated with hypoplastic posterior fossa. Hypoplastic means that it is too small for the size of the cerebellum and brainstem. Consequently, the cerebellum and brainstem get crowded and pushed downward toward the bones and foramen of the base of the cranial vault. In this regard, Europeans tend to have a shorter clivus by design compared to Asian and African races. This makes Europeans somewhat more susceptible to Chiari malformations. The slope of the tentorium in the different races has yet to be studied. Typically, however, the slope of the tentorium is fairly close to the slope of the clivus. Platybasia
Platybasia is a condition in which the base of the skull is too flat as in the brain scan below. When the base is too flat it tends to bend the cord along with it. If you look at the first two brain scans above you can see that the brainstem in these two images is in a more upright position. In the first scan the brainstem is almost vertical and very straight. In the second it is slightly bent. In the skull with platybasia it is bent over almost forty-five degrees. Also note that the covering over the posterior fossa, the tentorium cerebelli, is flat and close to the same pitch as the clivus. Platybasia can also cause a functional Chiari malformation. A functional Chiari occurs when the patient bends the head forward toward the chest. Bending the head and neck forward pulls the cord further down toward the clivus of the base of the skull, which can compress the brainstem and cause signs and symptoms in certain head positions.
The signs and symptoms a patient gets from a Chiari malformation depends on which structures in particular get compressed. Again, it depends on the design of the skull and position of the brainstem and cerebellum in the posterior fossa. While they don’t get MS, Asians and Africans do get a variant of MS called optic spinal multiple sclerosis and Devic’s disease. In contrast to MS, when it comes to Chiari malformations, no one appears to be spared. All races are equally vulnerable due to the precarious position of the brainstem above the foramen magnum. It is possible that Chiari type malformations may play a role in optic spinal multiple sclerosis and Devic’s disease seen in Asians and Africans, as well as the classic form of MS seen in Europeans. The signs and symptoms are determined by the nerves that get compressed against the base of the skull or within its openings called foramen.
The design of the base of the skull and posterior fossa, due to upright posture, predisposes humans to neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis. In addition to Chiari malformations and platybasia there are many other types of inherited (genetic) disorders in the design of the base of the skull and upper cervical spine that can cause problems with blood and CSF flow. Likewise, there are many types of acquired conditions, such as aging, injuries and diseases that can affect the upper cervical spine and base of the skull and affect blood and CSF flow in the brain and cord.
Upright cine MRI, brain scans, arteriograms (MRAs) and venograms (MRVs) are starting to show the effects of upright posture and the design of the base of the skull on the position of the brain inside the cranial vault, as well as blood and CSF flow. The design and health of the base of the skull and posterior fossa and their close relationship to the upper cervical spine are important to the health of the brain and cord.