The following is a brief overview of the radiology measurements of the craniocervical junction, and the radiology indicators to assist specialists in the ruling out eagle jugular syndrome and tethered cord syndrome.
Our radiologists complete a comprehensive report of more than these radiology assessments, such as spinal canal dimensions and other key factors, to help provide a detailed and comprehensive radiology report to provide patient’s physicians with as much information as possible.
Overview of Key Radiology Measurements of the Craniocervical Junction’s Structural Integrity
The following radiology measurements are based on current peer-reviewed research. These measurements may be interpreted differently depending on the doctors experience. For the most accurate and up-to-date assessment, consult a radiologist or neurosurgeon familiar with craniocervical instability.
Basion-axial Interval (BAI) (and translational BAI)
Lateral Mass Overhang / Displacement
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Overview of Vascular Eagle Jugular Syndrome Radiology References
Eagle Jugular Syndrome is a subtype of Eagle syndrome where an elongated or medially-angled styloid process (a bone that projects down from the skull base) presses against the internal jugular vein (IJV), often between the styloid process and the transverse process of C1 vertebra. The compression by the transverse process of C1 can also be the sole cause of compression in some cases. This can reduce or block blood flow from the brain to the heart, leading to head pressure symptoms, headaches, cognitive disfunction, tinnitus, dizziness, visual disturbances, involuntary motor episodes or dysautonomia-like symptoms. EJS has gained increasing attention in medical literature over the past decade and is still considered a rare condition. It seems to be common in whiplash patients as well as patients with craniocervical instability, although more research is needed on EJS.
Area of Abnormality: Internal jugular vein at the site where the styloid process and C1 transverse process. Best evaluated with high-resolution (i.e. less than 1 mm slice thickness) dual-phase 3D CT Angiogram of the Neck in neutral, and if possible, head rotated left and right positions.
EJS Radiology Key Basic Reference Values:
- Styloid Process Length
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- Normal: usually less than 25 – 30 mm
- Borderline: 25 mm – 30mm (moderately long)
- Abnormal: > 30 mm (considered an elongated styloid process)
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2. 3D Styloid Process Orientation
The styloid process’ orientation or angulation is still under research, however, researchers, physicians specializing in EJS, and radiologists may also review how the styloid process is angled. If the styloid process points more medially or downward toward the C1 vertebrae, it can press on the internal jugular vein in certain head positions, and potentially reduce blood flow from the brain, even if the bone is only moderately long. In some research studies, they note that the medial angulation and inferior or vertical orientation of the styloid process are more important than length alone.
At the moment, normal orientation is considered angles greater than 75 degrees tend to point laterally and are less likely to compress the vein.
3. Styloid Process – C1 Distance
Sometimes the internal jugular vein can get compressed between the styloid process and C1 transverse process.
Normal distance is considered when there is not close contact to the internal jugular vein. The radiologist measures the shortest distance between the styloid process and the C1 transverse process.
Borderline distance is considered between 5 mm to 7.5 mm, and this warrants further clinical evaluation by an eagle jugular syndrome specialists, typically an ENT or Neurosurgeon.
Abnormal distances is a distance of less than 5 mm between the styloid and C1 transverse process, and has been reported as frequently associated with IJV compression.
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Overview of Radiology Findings in Tethered Cord Syndrome
To detect this difficult to identify condition, a specialized lumbar MRI is done in the supine position and sometimes prone position (laying on back or stomach, respectively). Although the use of prone MRI as a standard for diagnosing tethered cord is not part of mainstream protocols at this time, some specialists consider the prone lumbar MRI as superior to the supine position.
Radiologists that specialize in assessing Tethered Cord Syndrome closely examine the lower spinal cord and its surrounding structures. Key signs of tethered cord on MRI include:
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- a low-lying conus medullaris
- thickening of the filum (greater than 2 mm)
- fat in the filum terminale (a small cord at the base of the spine)
- the terminal filum shows restricted or absent movement in prone position
- restricted conus mobility
- in prone position, the terminal filum is separated from the cauda equina and is shifted caudally to posteriorly in the subarachnoid space. *more peer reviewed research is needed on this topic
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A low-lying conus medullaris, thickened or fatty filum terminale, and limited mobility of the cord / filum, are well-documented criteria.
The benefit of Prone Position Testing (PPT) and the Kazuyoshi Protocol, are the prone (axial) MRI imaging tethered cord protocols, that add important information by showing who the spinal cord and filum behave when the body is in a different position. This can reveal subtle signs of tethering or abnormal stretching that might not appear in the supine scan.
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