Clivo-axial angle

The clivo-axial angle (CXA) is a measure of the angle between the clivus, a bony part of the base of the skull, and the spine. It is used to aid in the diagnosis of craniocervical instability. It is sensitive to horizontal instability and characterizes the relationship between the skull, the brainstem, and the odontoid process, including abnormalities due for example, to a compromised transverse ligament.

Definitions
Also called the clivus canal angle, the clivus vertebral angle, the clivus spinal angle or the clivus cervical angle, terms and measurement methodology can vary from study to study and from neurosurgeon to neurosurgeon. In an effort to standardize terminology and measurement, a 2014 consensus statement defined the CXA as "the angle between the clivus line and the posterior axial line." "The clivus line is drawn along the lower third of the clivus, from the spheno-occipital synchondrosis to the basion; in the case of basilar invagination, it is drawn from the spheno-occipital synchondrosis to the top of the odontoid process. The posterior axial line is differentiated to reflect either the bone contour of the axis on CT, the so-called bone CXA, or the ligamentous margin of the odontoid—the soft tissue CXA. The soft tissue CXA, necessarily including thickening of the posterior ligament due to pannus, may be more pertinent in identifying possible ventral brainstem compression, and is therefore more representative of the pathology."

Epidemiology
The average CXA in healthy or nonsymptomatic population is estimated to be approximately 150 degrees. Several studies have shown that a CXA lower than 135 degrees, a frequently used cutoff, is uncommon in the healthy population  (see table below) and is associated with pathological conditions such as brainstem compression in patients with rheumatoid arthritis. A consensus statement formed at the second International CSF Dynamics Symposium of the Chiari and Syringomyelia Foundation in 2013, proposed that a CXA lower than 135 degrees could be seen as “potentially pathological.”

Gender
Women have increased range of motion (change in CXA from flexion to extension) than men.

ME/CFS
A Swedish study of 234 ME/CFS patients meeting the Canadian Consensus Criteria found an average CXA of f 148 ±10 degrees, measured as:"A straight line was drawn along the caudal edge of the clivus bone, and a second was drawn by extending a straight line along the posterior axial line from the base of the axis to the dens"

Ehlers-Danlos Syndrome
A study of the CXA in patients with Ehlers-Danlos syndrome and a control group of patients with cervical spondolysis found that EDS patients had an average CXA of 139.7±10.4 degrees as compared to the control group 148.9±8.4. They also had a greater change in CXA between flexion and extension: 74.6±24.4 in the EDS group vs 39.4±11.3 in controls.

Migraine
The clivo-axial angle of 65 patients with migraine and 65 controls were compared. The mean CXA in the migraine group was 142.65 ± 8.73°, 153.66 ± 6.35° in controls.

Basilar invagination
Bothelo & Ferreira (2013) documented 25 patients with basilar invagination and found a mean CXA of 120° (range 79°–145°).