The vertebral artery (VA) is one of a pair of blood vessels that travels through the neck to supply the posterior portion of the brain, known as the posterior circulation. This area includes the brainstem, cerebellum, and parts of the cerebrum, which are responsible for functions such as balance, coordination, and consciousness. To accurately describe the location of disease or injury, clinicians and radiologists use a standardized system that divides the artery into four distinct segments (V1 through V4). This segmentation allows for precise communication regarding diagnostic findings and guides the selection of appropriate management strategies.
Defining the Four Vertebral Artery Segments
The vertebral artery begins as a branch off the subclavian artery and ascends through the neck before entering the skull, where it merges with its counterpart to form the basilar artery. This entire course is systematically divided into four segments, designated V1 through V4, each defined by its relationship to the surrounding bony and soft tissue structures.
The V1 segment, often termed the pre-foraminal segment, extends from the artery’s origin on the subclavian artery up to its entry point into the transverse foramen of the sixth cervical vertebra (C6). This segment typically runs behind the common carotid artery. It is a common site for atherosclerotic disease due to the hemodynamics at the vessel’s origin.
The V2 segment, the foraminal or cervical portion, follows a protected course through the bony tunnels of the cervical vertebrae. It travels superiorly through the transverse foramina of C6 up to the transverse foramen of the axis (C2). This segment is encased by bone and is surrounded by a plexus of veins and branches of the sympathetic nervous system.
The V3 segment, known as the atlantic or extraspinal segment, is characterized by a complex, winding path. After exiting the transverse foramen of C2, the artery turns sharply, looping around the lateral mass of the atlas (C1) and grooving its posterior arch. It then pierces the dura mater to enter the skull.
The V4 segment is the intradural or intracranial portion of the artery. This segment begins where the artery penetrates the dura mater and ascends toward the brainstem. It terminates where it combines with the opposite vertebral artery to form the basilar artery. The V4 segment is the source of important branches, including the posterior inferior cerebellar artery.
Imaging Modalities for Visualization
Visualizing the vertebral artery requires specialized radiological techniques, as the vessel’s deep and tortuous path is obscured by bone and soft tissue. The choice of imaging modality depends on the segment being examined and the type of pathology suspected.
Doppler Ultrasound is the initial, non-invasive imaging tool used to assess the extracranial segments (V1 and V2). This technique evaluates blood flow velocity and direction, providing functional information about the degree of stenosis or occlusion. However, the bony encasement of the V2 segment and the deep course of V3 often limit the completeness of ultrasound visualization.
Computed Tomography Angiography (CTA) utilizes X-rays and an injected contrast agent for detailed anatomical views. CTA is proficient at visualizing the artery’s relationship to surrounding bone structures, making it effective for assessing the V3 segment’s course around the C1 and C2 vertebrae. This modality rapidly provides high-resolution images of the entire vertebral artery.
Magnetic Resonance Angiography (MRA) provides structural and flow information without ionizing radiation, making it valuable for evaluating the intracranial V4 segment. Specialized MRA sequences are preferred for diagnosing subtle changes in the vessel wall, such as intramural hematoma associated with dissection. MRA is also superior for detecting small ischemic strokes resulting from vertebral artery disease.
Digital Subtraction Angiography (DSA) remains the gold standard for its superior resolution, which reveals minute details of the vessel lumen, but it is an invasive procedure. CTA and MRA are typically used for initial diagnosis and follow-up, with DSA reserved for cases requiring intervention or when non-invasive studies are inconclusive.
Pathologies Associated with Specific Segments
The unique anatomical environment of each vertebral artery segment renders it susceptible to distinct types of disease processes. Understanding this segment-specific vulnerability is central to diagnosing and treating vertebrobasilar disorders.
The V1 segment is the most common site for atherosclerotic disease, which is the buildup of plaque within the artery wall. Stenosis, or narrowing of the vessel, typically occurs near the origin from the subclavian artery due to turbulent blood flow dynamics. This narrowing can significantly reduce blood flow to the posterior brain, leading to symptoms of vertebrobasilar insufficiency.
The V2 segment, traveling through the transverse foramina, is protected by bone but is susceptible to external compression from degenerative changes in the cervical spine. Conditions like cervical spondylosis, involving bone spurs (osteophytes), can impinge upon the artery. This mechanical compression is exacerbated by neck movement, leading to transient symptoms of dizziness or vertigo.
The V3 segment is the most frequently affected by traumatic dissection, a tear in the inner lining of the artery wall. Its course involves sharp turns through the highly mobile C1 and C2 joints. Sudden, forceful movements of the neck can stretch and shear the vessel wall, leading to dissection.
The V4 segment, located inside the skull, is the site for pathologies typical of the intracranial circulation, including saccular aneurysms. It is also vulnerable to vasculitis, inflammation of the vessel wall that can narrow the lumen and cause stroke. A dissection extending into this segment carries the risk of causing a subarachnoid hemorrhage.
Treatment Approaches Based on Segment Location
The location of the pathology within the four segments dictates the appropriate treatment strategy, determining whether a procedure will be endovascular, surgical, or managed primarily with medication.
Treatment of V1 segment stenosis, commonly caused by atherosclerosis near the origin, is managed with an endovascular approach. This involves accessing the vessel remotely and placing a stent to open the narrowed segment and restore adequate blood flow to the brain.
For V2 segment compression related to cervical spondylosis, initial management involves physical therapy and anti-inflammatory medications. If symptoms persist and compression is severe, surgical decompression may be necessary to remove the offending bony elements, such as osteophytes, that are pressing on the artery. Surgical access to the V2 segment is challenging due to the artery’s deep, bony encasement.
Pathologies affecting the V3 segment, such as traumatic dissection, are often managed initially with antithrombotic medications (antiplatelet or anticoagulant agents) to prevent blood clots from forming and causing a stroke. Open surgical approaches or bypass procedures may be considered if medical therapy fails to prevent recurrent ischemic events or if the dissection leads to a large, symptomatic aneurysm.
The intracranial V4 segment presents unique challenges, and its pathologies are treated with specialized endovascular techniques. Intracranial aneurysms in V4 are frequently treated by coiling, where soft platinum coils are deployed inside the aneurysm to induce clotting and prevent rupture. If a dissection extends into V4 and is complicated by a subarachnoid hemorrhage, prompt endovascular or surgical intervention is required to secure the ruptured vessel.