The Upper Cervical Angle
The upper cervical angle and intervals are important to the health and function of the brainstem and cord. The brain scan below is from a case study by Drs. Milhorat, Bolognese and Nishikawa published in the Journal of Neurology in December 2007, using a Fonar Upright MRI in evaluating excess motion in the upper cervical spine.
This particular case was affected by an inherited disorder of connective tissue called Ehler's-Danlos. Among other things, Ehler's-Danlos is associated with loose ligaments predisposing joints to strains, subluxations and dislocations. The excess motion in the upper cervical spine can cause neurological signs and symptoms.
The upper cervical angle, technically known as the clivoaxial angle, is determined by drawing a line along the base of the skull (clivus) and a second line down the backside of the body of the second cervical vertebra (axis) and measuring the degrees of separation between them. The angle is noted as CXA on the brain scan below and should be approximately 140 degrees.
The intervals are critical spaces that separate the base of the skull from the upper cervical spine, as well as spaces between the atlas (C1) and axis (C2) vertebrae of the upper cervical spine. The two spaces in the brain scan are denoted as BDI and BAI and will be explained further below.
The atlas vertebra (C1) of the upper cervical spine connects to the condyles of the posterior fossa of the base of the skull. The front border of the posterior fossa is called the clivus portion of the base of the skull. It is represented by line C in the brain scan above. The brainstem lies parallel to the clivus. (The following link is to a page on this site that discusses the design and angles of the posterior fossa).
The pitch of the clivus varies in different races and in different individuals. In certain cases its pitch is too flat, platybasia, which predisposes the brain to a Chiari malformation when the head is tilted forward. A Chiari malformation occurs when the bottom of the brain comes in contact with the base of the cranial vault.
In addition to the angle of the base of the skull and the pitch of the clivus, the angle of the clivus relative to the upper cervical spine, clivoaxial angle (CXA) mentioned above and shown in the brain scan above, is also important. The pitch of the upper cervical spine is represented by line D drawndown the backside of the odontoid process of the axis or second cervical vertebra (C2). The odontoid, also known as the dens, is the upward pointing tooth-like projection as seen in the sketch below of the axis (C2).
In contrast to the clivoaxial angle, the upper cervical intervals are several specific spaces between certain points of the base of the skull and upper cervical spine, and between the upper cervical vertebrae atlas (C1) and axis (C2).
Conditions Resulting from Changes in Angles and Intervals
It is important to maintain a proper angle and spacing between the base of the skull and the upper cervical spine to prevent compression problems of the brainstem and cord. Decreases in the upper cervical angle and intervals in relationship to the base of the skull have been associated with respiratory problems in rheumatoid arthtitis, autism in children and brainstem signs and symptoms in Ehler's-Danlos and Chiari malformations. In addition to the above, decreases in the upper cervical angle and base of the skull may play a role in Alzheimer’s, Parkinson’s and multiple sclerosis, as well as other neurodegenerative diseases.
Some of the design problems that affect the intervals and upper cervical angles to the base of the skull are genetic (inherited). Consequently, beginning signs and symptoms associated with the condition tend to show up earlier in life. In other cases, the normal upper cervical angle and intervals are decreased by misalignments in the upper cervical spine due to aging, injuries and trauma.
In either case, a decrease in the normal upper cervical angle and intervals can kink the brainstem and cord, compress the contents of the foramen magnum and spinal canal, and cause Chiari malformations. In addition to the spinal cord, the contents of the foramen magnum also include: 1)the vertebral veins, which drain the brain during upright posture; 2)the vertebral arteries, which supply blood to the brainstem, cerebellum and inner aspects of the temporal and occipital lobes of the brain; and 3) the subarachnoid space of the cord, which contains cerebrospinal fluid (CSF) and is continuous with the cisterns of the brain.
As mentioned, the normal upper cervical angle or clivoaxial angle, should be about 140 degrees. As seen on the right side in the brain scan above, when the clivoaxial angle falls below the normal range it causes a kink or kyphosis in the spinal cord. This is similar to the kinking of the brainstem caused by platybasia (flatter than normal base of the skull). The area of the spinal cord affected by a less than normal angle is called the cervicomedullary connection. It is the connection between the upper cervical spinal cord and the lower end of the brainstem. In addition to kinking the cord, a decrease in the clivoaxial angle also predisposes the brainstem to compression by a functional Chiari malformation likewise similar to platybasia.
As mentioned above, the intervals or distances between the upper cervical spine and the clivus and other points on the base of the skull and between C1 and C2 are, likewise, important. One of the important intervals is the distance between the bottom of the clivus called the basion, and the tip of the dens (odontoid process) of C2. In the brain scan above the basion-dens interval is noted as BDI. There are other important intervals radiologists check as well.
Basilar invagination, also called basilar impression, occurs when the odontoid process of C2 actually penetrates the foramen magnum. In cases with basilar invagination there is no BDI. Basilar invagination is the opposite of a Chiari malformation in which the brainstem and cerebellum descend into the foramen magnum. Instead, in basilar invagination the odontoid either pushes up into the foramen magnum or the skull sinks down onto it, which is called cranial settling. In either case the odontoid process penetrates the foramen magnum. The consequence is compression of the cervicomedullary junction between the medulla at the lower end of the brainstem and the upper end of the cervical portion of the spinal cord.
Several genetic conditions and acquired diseases can cause basilar invagination or impression. For example, different types of design flaws in the upper cervical spine can cause it such as Kleppel-Feil in which the neck is too short. Endocrine diseases such as Morquios (dwarfism) and cleidocraniodysostosis (failure of cartilage conversion to bone during development) can also cause it. In adults Marfans (giantism) and Paget’s disease (enlarged head size), also cause changes in bone with similar outcomes.
Inherited disorders of connective tissues such as rheumatoid arthritis and Ehler’s-Danlos syndrome can cause ligament laxity and subsequent upper cervical instability called atlantoaxial instability. Atlantoaxial instability can result in cranial settling and basilar invagination. In the brain scan above the scan on the left was done with the patient lying down in the supine, face-up, position. The scan of the right is with the patient sitting up.
In this particular case, both the upper cervical angle and the intervals relative to the base of the skull decreased when the patient sat up. Rather than the cerebellum descending into the foramen magnum in a classic type Chiari malformation, in this case the upper cord gets kinked and the front side of the lower brainstem comes in contact with the clivus of the base of the skull and the cerebellum comes in contact with the floor of the posterior fossa.
As upright MRI becomes more commonly used we will, more than likely, see more of these types of situations. Chiari type malformations in which parts of the brain come in contact with the floor of the cranial vault or penetrate its foramen are probably more common than we think and the source of many neurological and neurodegenerative conditions we now see.
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