The upper two cervical segments formed by atlanto-occiptal & atlanto-axial joints are anatomically dissimilar from the functional units below. In this article, I am going to share some insights that might assist your clinical reasoning for patients with upper cervical neck pain.

The craniocervical junction (CCJ) is a complex transitional region between the base of the skull and the upper cervical spine. It is formed by the occipital bone and the first two cervical vertebrae, C1 or atlas and C2 or axis, both of which contain vital neural and vascular structures (i.e. brainstem, spinal cord, cranial nerves, and the vertebral artery). This articulation is the most mobile of the cervical spine and allows for 40% of all cervical flexion-extension and 60% of all head rotation.

The occiput-C1 junction allows for the majority of flexion-extension (25°). The sagittal displacement of the base and the dens (which should not exceed 1mm) is avoided by the tectorial membrane, alar ligaments, and, in minor contribution, by the C1 facets. This joint also performs 5° of lateral bending and axial rotation. The alar ligament limits both movements, while the superior C1 facets aid in restraining the axial rotation. The atlantoaxial articulation allows for 15° of flexion-extension, limited by the tectorial membrane and dens-C1 arch contact, and 5° of lateral bending, limited by the alar ligament, which is also responsible for the forced rotation of C2. This joint performs most of the rotation of the head (40°), thanks to the rotation of the atlas ring around the dens, the convex articular surfaces of C1, and their loose capsules, which allow sliding on the C2 facet joints.

The first two cervical vertebrae, the atlas (C1) and the axis (C2), have no posterior articulations. Also, they have no intervertebral foramen.

The C1 and C2 nerves are primarily sensory. The sensory distribution of these nerves is the posterior and lateral borders of the scalp. Also, the branches of C3 join the C1 and C2 nerves to form greater and lesser occipital nerves. The sensory pattern extends anteriorly to the supraorbital area, so in this way irritation of the nerves at the base of the skull can lead to referred pain that mimics “sinusitis”.

The nerves at the base of the skull also have a very close relationship to the vertebral artery, as these nerves are very close to the vertebral artery at its point of angulation to enter the skull. The space difference between the body and foramen (3-6mm) and the space between the facet and foramen (2-3 mm) indicate a possibility of vascular impingement and neural irritation. Also, with the close distance between the facet and foramen, we can say that vascular impingement is more likely to occur by the superior articular process and sometimes due to changes in uncovertebral joints.

Finally, I would like to share my thoughts about “the sinuvertebral nerves (SVNs) at the craniovertebral junction”:

Origin of the Upper Three Cervical Sinuvertebral Nerves – somatic and sympathetic: All three SVNs, once formed in the intervertebral foramen, give off very tenuous branches to the dura mater and run in the epidural space anterior to the dorsal root ganglia (DRG). Interestingly, the communication between the ascending branch of the C2 SVN with the hypoglossal nerve confirms the long-held belief that sensory communication does occur between cervical and cranial nerves.

As about 50% of the movement of the neck occurs at the craniovertebral joints, and as maximum movement occurs immediately above the dens, most of the overlapping innervation (C3 and C2 SVNs) appears to arise from a less mobile part of the craniovertebral articulation below the C2 vertebra and extend upward to the basiocciput along a fairly immobile part of the craniovertebral articulation, the anterior part of the spinal canal. Therefore, there is a lower risk of damaging the delicate nerves within the normal range of motion at the craniovertebral junction (CVJ). Such an adaptation of neural structures to the mobility of the CVJ also seems to hold true for the arterial supply of the CVJ, which comes mainly from the C3 intersegmental artery.

In summary, the upper two cervical segments are anatomically dissimilar from the functional units below. The upper cervical SVNs supply the structures within the anterior craniovertebral spinal canal individually and jointly. There is an overlap of innervation of structures by the C2 and C3 SVNs as there is in the lower spinal canal, except for the C1 SVN which has a rather narrow area of supply. Furthermore, the structures responsible for the origin of cervicogenic headache may be related to the upper cervical SVN nerves as they supply the dura of the posterior cranial fossa and ligaments of the CVJ. Also, these nerves have the same root value as the dorsal rami, which relays pain signals from the dorsal head and neck region.

I hope that was interesting for you. Thanks for reading this article. If you find this interesting, please share it with someone who might benefit from it!

References: 

1. Bogduk N. 2001. Cervicogenic headache: Anatomic basis and pathophysiologic mechanisms. Curr Pain Headache Rep 5:382–386.

2. Bogduk N, Tynan W, Wilson AS. 1981. The nerve supply of the human lumbar intervertebral discs. J Anat 132:39–56.

3. Bogduk N, Windsor M, Inglis A. 1988. The innervation of the cervical intervertebral discs. Spine 13:2–8.

4. Rennie C, Haffajee MR, Ebrahim MA. The sinuvertebral nerves at the craniovertebral junction: a microdissection study. Clin Anat. 2013 Apr;26(3):357-66. doi: 10.1002/ca.22105. Epub 2012 Jul 17. PMID: 22806929.

5. Vigo V, Hirpara A, Yassin M, Wang M, Chou D, De Bonis P, Abla A, Rodriguez Rubio R. Immersive Surgical Anatomy of the Craniocervical Junction. Cureus. 2020 Sep 10;12(9):e10364.

Till next time!