What is the v3 segment of the vertebral artery?

V3 Segment of the Vertebral Artery

The V3 segment is the third extracranial portion of the vertebral artery that extends laterally from where the artery exits the C2 transverse foramen, courses cephalad and posterior to the superior articular process of C2, then travels cephalad and medially across the posterior arch of C1 (the atlas groove), and continues into the foramen magnum before piercing the dura. 1

Anatomical Course and Boundaries

The V3 segment follows a complex three-dimensional path that can be divided into distinct portions:

• Initial vertical portion: Travels between the C2 and C1 transverse foramina after exiting the C2 transverse foramen 2
• Horizontal portion: Runs over the atlas groove (also called the "J-groove") on the posterior arch of C1, which cradles the vertebral artery 3
• Final oblique portion: Courses upward and medially before entering the foramen magnum and piercing the dura mater 2

The segment terminates at the point where the artery enters the dura, after which it becomes the V4 (intracranial) segment 1

Key Anatomical Relationships

The V3 segment is uniquely connected to highly mobile cervical vertebrae (C1 and C2), making it mechanically vulnerable to injury from sudden or excessive neck movement. 1

Important spatial relationships include:

• Distance from midline: Averages 27-28 mm at the upper border of atlas, and approximately 11 mm at dural penetration 4
• Arterial diameter: Approximately 4.4-4.6 mm in the V3 segment 5, 4
• Length: Approximately 14.7-16.4 mm depending on the specific subsegment measured 5

Vascular Branches and Anastomoses

The V3 segment gives off important collateral branches:

• Branches typically anastomose with branches of the occipital artery at the levels of the first and second cervical vertebrae 1
• Cerebrospinal branches are present in approximately 12% of individuals 5
• The segment covers the paravertebral venous plexus 3

Clinical Significance

This segment is particularly vulnerable to dissection and injury due to its mechanical relationship with mobile cervical vertebrae. The V3 segment's connection to C1 and C2 underlies the risk of vertebral artery dissection from cervical manipulation, with an incidence of approximately 1.3 per 100,000 in patients under 45 years within one week of manipulative therapy 1

Symptoms of V3 Segment Pathology

When the V3 segment is compromised, patients may present with:

• Headache and neck pain (often unilateral) 1
• Vertigo, nausea, and visual disturbances 1, 6
• Syncope and ataxia 6
• Horner syndrome (ptosis, miosis, anhidrosis) in cases of dissection 1

Surgical Considerations

The space between the transverse processes of C1 and C2 is the most favorable site for surgical access to the V3 segment, as the diameter of the vertebral canal allows arterial manipulation up to 5 mm, and the length permits mobilization without transverse process resection 5

The V3 segment can be exposed through an anterolateral approach by:

• Passing medially to the sternomastoid muscle 2
• Identifying the C1 transverse process below and in front of the mastoid tip 2
• Unroofing the C1 transverse foramen while preserving the subperiosteal plane 2

A critical pitfall is blind dissection below the foramen magnum level, which can lead to vertebral artery injury. 3 Safe identification requires systematic detection of four anatomical landmarks: the inferior retrosigmoid point, the occipital midline dural point, the posterior tubercle of C1, and the J-groove on the posterior arch of C1 3

Imaging Evaluation

For patients with suspected V3 segment pathology, CTA or MRA is superior to ultrasound imaging, particularly for dissections beginning cephalad to the angle of the mandible 1

MDCT angiography provides detailed demonstration of the V3 segment and its relationships with atlas and axis, allowing preoperative evaluation to prevent intraoperative vascular injuries 4