Anatomy and function

The first cervical vertebra, atlas, is unique in the sense that it possesses neither a body nor spinous process. It consists of an anterior and posterior arch extending between two lateral masses, forming a closed triangular ‘ring’ that accommodates the brainstem. Each lateral mass contains superior and inferior facets. Since no discs separate the atlas from adjacent bony structures, these facets function to connect C1 to the occiput above, and the axis below. The cranial articular surfaces are large and concave, articulating with the condyles of the occipital bone to form the two atlanto-occipital (AO) joints. The transverse processes of C1 are long, protruding structures containing triangular foramina, through which the vertebral arteries pass on their way to the brain. The posterior arch, devoid of a spinous process, lies deep beneath the skin, making manual palpation difficult.

The articulations of the occipital-atlanto-axial complex are among the most complex in the human body. The primary function of the AO joint is flexion and extension in a sagittal plane (i.e. nodding of the head). While flexion at the AO joint is usually limited to around 10°, the degree of extension is considerably greater, approaching 25°.⁷ The range of motion in the anteroposterior plane is generally restricted to less than 20° rotation. The innervation of the AO joint is from the ventral ramus of C1, with the second dorsal cervical nerve supplying the AO synovial space.^8,9

Mechanisms of injury

Perhaps owing to the relative weight of the cranial contents and the stress induced by frequent movements of the head, atlanto-occipital mediated joint pain has been described in a variety of different inflammatory diseases including rheumatoid arthritis, osteoarthritis, and ankylosing spondylitis.^10,11 Trauma is another common cause of AO joint pain, with hyperextension of the skull, with or without damage to the tectorial membrane, being the proposed mechanism of action. Interestingly, restriction of normal articular motion is also hypothesized to result in pain emanating from the AO joint.¹² Proposed etiologies of this type of injury include intra-articular adhesions, capsular scarring, and localized muscle hypertonicity.¹¹

Clinical presentation

The presentation of patients with AO joint pain is protean, with no known pathognomonic features.¹³ The International Headache Society’s (IHS) diagnostic criteria for cervicogenic headache include:⁶

According to the IHS, cervicogenic headaches tend to occur more frequently in females, are often accompanied by a history of trauma, and show a poor response to nonsteroidal antiinflammatory drugs and medications used to treat migraines, such as ‘triptans’ and ergotamine. Of import, the IHS criteria do not distinguish between the various pain generators involved in cervicogenic headaches.

In a study assessing the response to lateral AO joint injections in five asymptomatic volunteers, Dreyfuss et al. found considerable variability in the provoked pain patterns, with the most inferior area of pain approximating the C5 vertebral level, and the most superior area extending almost to the vertex of the skull.⁸ In one patient, only temporal pain was produced. Most subjects tended to have pain limited to the upper neck and suboccipital regions. The induced pain was typically characterized as being ‘dull,’ ‘aching,’ or ‘pressure-like,’ and unilateral in distribution. In a study by Fukui et al., whose aim was to determine the pain referral patterns for all levels of cervical zygapophyseal joint injections, in the 10 patients with neck and occipital pain who underwent lateral AO joint blocks, pain was referred into the ipsilateral upper posterolateral cervical region in all cases, and into the occipital area in 30% of patients.¹⁴

Outcome studies

While it is widely acknowledged that the AO joint can be a source of head and/or axial neck pain, there is scant evidence to support the therapeutic use of intra-articular joint blocks. In a paper by Dreyfuss et al., they reported three patients with upper neck pain and/or occipital headaches who obtained complete relief after local anesthetic and steroid injections of the AO joints.¹⁵ In one patient, two injections were needed. However, in two of the patients, atlantoaxial (AA) and C2–3 injections were performed in addition to the AO injections. In the third patient, an AA joint injection was concurrently done. In an abstract by Busch and Wilson, the authors presented two patients who obtained significant pain relief following combined repeat AO and AA joint injections.¹⁶