The transverse foramen is a distinctive anatomical feature found exclusively within the cervical vertebrae (bones of the neck). This structure is a small, ring-like opening located on the transverse process, the bony projection extending out to the side of a vertebra. Its function is to create a protected, vertical passageway for structures traveling between the chest and the base of the skull. The presence of these openings distinguishes the seven cervical vertebrae from the rest of the spine, forming a bony canal on both sides of the neck.
Unique Location and Structure
Every one of the seven cervical vertebrae (C1 through C7) contains a transverse foramen on each side. When stacked, these individual openings align to form a continuous, protective channel ascending the neck toward the head. The vertebral artery, a major blood vessel, typically begins its ascent by entering the foramen of the sixth cervical vertebra (C6). It then travels upward through the successive foramina of C5 through C1, the uppermost vertebra.
The pattern of passage is not uniform across all levels, as the structure of the foramen in C7 is often different. The C7 transverse foramen is frequently smaller and often does not transmit the vertebral artery. Instead, the artery usually bypasses C7 and enters the bony canal at C6, leaving the C7 foramen to accommodate only accessory vessels or veins. The foramen of C1 (the atlas) is also unique because the artery exits it to loop around the bone before entering the skull.
The Contents of the Foramen
The primary purpose of the transverse foramen is to serve as a secure conduit for a trio of neurovascular structures. The most significant content is the vertebral artery, a major vessel branching off the subclavian artery in the chest. This artery supplies the posterior part of the brain, including the brainstem and the cerebellum. The bony protection shields this supply route from external compression or injury during head and neck movements.
Accompanying the artery is the vertebral vein, which generally follows the same path, draining blood from the head and neck. This vein is often surrounded by a plexus (network) of smaller veins as it descends through the foramina. The final component traveling through this bony canal is a collection of sympathetic nerve fibers. These nerves regulate involuntary functions, such as the constriction and dilation of blood vessels.
The vertebral artery merges with the artery from the opposite side at the base of the brain to form the basilar artery. This combined system, known as the vertebrobasilar system, supplies brain areas responsible for balance, coordination, vision, and consciousness. Disruption to the blood flow in this pathway can have immediate neurological consequences.
Clinical Importance and Related Conditions
The close confinement of the vertebral artery and nerves within the transverse foramina makes them susceptible to damage from degenerative changes and acute trauma. A common concern is cervical spondylosis, a condition involving age-related wear and tear of the spinal disks and joints. Degenerative changes can lead to bony outgrowths called osteophytes, which can narrow the transverse foramen. This narrowing (stenosis) can compress the vertebral artery, potentially restricting blood flow to the brain.
Compression of the artery can result in vertebrobasilar insufficiency, leading to symptoms like vertigo, dizziness, or visual disturbances, especially when the head is turned or extended. Compression of the nerve fibers within the foramen can also cause localized neck pain or radiating symptoms in the arm. The artery’s path through the foramina makes it vulnerable to traumatic dissection, a tear in the inner lining of the blood vessel wall.
Dissection can occur after sudden, forceful neck movements, such as whiplash or aggressive neck manipulation. The middle segment of the artery (V2), which is fixed inside the bony canal, is susceptible to stretching and tearing during these rapid movements. Arterial dissection is a severe medical event because it can lead to a clot that may travel to the brain and cause a stroke. Recognizing the anatomical vulnerability created by the transverse foramen is paramount in diagnosing and treating these neurovascular conditions.