CGH is described as a dull, aching, nonthrobbing, often unilateral pain experienced in the distribution of first trigeminal branch, generally thought to arise from pathology in the cervical spine. It may be accompanied by sympathetic symptoms such as conjunctival injection, runny nose and tinnitus [
8]. Severe attacks in some patients have been described with “migrainous” phenomena, such as nausea and vomiting. The headache can be provoked in most patients by certain movements of the neck, coughing, sneezing, bowel movements, and/or Valsalva maneuvers [
8,
9]. Nearly all patients with CGH tend to have myofascial trigger points on the symptomatic side [
10].
CGH is a distinct entity that needs to be differentiated from other causes of headache. Chronic paroxysmal headache (CPH) and C2 neuralgia can closely mimic CGH [
13]. Whereas CGH patients typically have 1 or 2 episodes every day, CPH patients are differentiated by having usually five to 15 episodes per day, that are usually amenable to indomethacin treatment [
8]. Separately, patients suffering from C2 neuralgia typically present with lancinating occipital pain associated with lacrimation and ciliary injection. The underlying cause of C2 neuralgia is the involvement of the C2 nerve root in inflammation and/or fibrosis in conditions such as meningioma, neuroma, or anomalous vertebral arteries. In addition, greater occipital neuralgia is a clinical condition characterized by pain in the occipital region, thought to be mediated through greater occipital nerve entrapment and/or affliction. However, no conclusive evidence has been found in favor of that particular theory, and it is now widely considered to be a referred pain from upper cervical joints. Similarly, the diagnosis of cervical migraine, also known as Barre-Lieou Syndrome, was initially attributed to vertebral ischemia mediated by cervical sympathetic nerve compression/stimulation but was later discredited as it was not supported by experimental evidence [
14-
16]. Overall, given the ICHD-3 diagnostic criteria, the main symptoms, when experienced a by a patient, that should raise suspicion for CGH are unilateral pain, often in the ipsilateral shoulder diffusely as well, reduced range of motion of the neck, and pain relieved by anesthetic blocks [
17].
2. Cervical Spine Innervation- A Review of Anatomy
Paramount to understanding the mechanism of CGH in cervical spondylosis is appreciating the intricacies of the innervation of the cervical spine. The dura mater and extradural structures of the cervical spine are innervated by the anterior and posterior primary rami of that corresponding cervical level’s nerve root. The posterior primary rami innervate the portions of the cervical spine dorsal to the vertebral foramen, i.e., the zygohypophyseal joints and the posterior portion of the cervical vertebrae. The anterior primary rami innervate the portion of the cervical spine anteromedial to the neural foramen, including the intervertebral discs and the anterior dura mater, primarily through the sinuvertebral nerve (SVN).
The SVN was thoroughly detailed in the 1960s by Drs. Edgar and Nundy [
22] through cadaveric studies, ending several controversial ideas associated with its course and innervation. The SVN, also known as the ramus meningismus or recurrent meningeal nerve of Luschka, is described as a group of 5 to 6 perivascular filaments with one usually being more prominent than the other filaments. It originates primarily from the anterior primary ramus of the cervical nerve root with contributing fibers from the sympathetic trunk in the form of the gray ramus communicantes (
Fig. 3A). At certain levels, it is also joined by branches from the vertebral nerve, a sympathetic nerve coursing along the vertebral artery [
23]. After its origination, the SVN travels superomedially in the vertebral foramen, anterior to the cervical nerve root to enter the spinal canal. The SVN divides into 3 main branches inside the spinal canal at each segment (
Fig. 3A,
B). The superior segment interconnects with the descending branches of the SVN from the superior levels. The middle branch runs horizontally and innervates the intervertebral disc, posterior longitudinal ligament, and the ventral dura mater. The inferiorly traversing branch extends inferiorly to join with ascending fibers of the lower segments. At the lateral recess of the spinal canal, in the extradural space posterior to the uncovertebral joint, it is closely associated with the rich venous plexus and arterioles. It then crosses over the vasculature medially, diving into smaller branches to innervate the dura mater and the intervertebral disc. The SVN also innervates the cervical dural sheath, predominantly in the ventral aspect extending up to the dentate ligament laterally.
The small size of the nerve fibers of the SVN and the presence of naked nerve endings are indicative of its nociceptive function. Because the inferior branch of the SVN can reach up to 3 segments below its origin, nociception from the lower cervical segment, as far as C6, can be mediated through the third cervical nerve, which will eventually project to the trigeminocervical nucleus and, thereby, potentially cause CGH. Even SVN branches from C7 can ascend up to the adjacent level and mediate nociception through communication with the aforementioned inferior descending SVN branches of C3. It is also conceivable that SVNs from C1/C2/C3 levels (upper cervical spine) would innervate their corresponding levels more densely than the lower cervical spine (C4/C5/C6/C7) through their descending branches. This may be the explanation for the predilection of CGH associated with lower cervical spine pathology to be less severe than CGH symptoms arising from upper cervical levels [
3].
In regard to neuroforaminal compression seen in cervical spondylosis, the entry zone to the foramen is typically much narrower compared to the exit, thus making it a more likely source of neural compression and, subsequently, radicular symptoms. The ventral border of this foraminal entry zone is formed by the uncovertebral joint, and the dorsal border is formed by the medial facet joint. This narrow corridor (dark shaded area,
Fig. 3A) houses the SVN plexus, the cervical vasculature (anterior interior portion of venous ring and the segmental radicular artery), and the cervical nerve root, forming an unco-vasculoradicular (UVR) junction (
Fig. 3A,
B).
In the pathologically spondylotic cervical spine, the narrow UVR zone is usually further constricted by intervertebral disc protrusion and/or a hypertrophied uncovertebral joint. This can lead to indirect or even direct compression of SVN and sympathetic neural elements resulting in CGH. Additionally, a phenomenon most often noted intraoperatively and less so on radiographic studies, engorgement of the epidural venous plexus, which lie in the UVR zone, could lead to SVN compression and cause CGH. This would be akin to the neurovascular trigger mechanism described in trigeminal neuralgia.
Other possible mechanisms leading to neural compression in the UVR zone may include stretching of the dura mater due to cervical kyphotic deformity or segmental cerebrospinal fluid (CSF) entrapments resulting from central canal stenosis. Since the dural investment and CSF surrounds the nerve root up until the lateral border of the neural foramen, it is possible that mechanisms that increase CSF pressure (such as any type of Valsalva maneuver) can trigger nociception, which is frequently noted clinically in CGH patients.
While these possibilities can explain the actual headache of CGH, the autonomic symptoms commonly seen in CGH patients are not directly accounted for in the above theories. However, autonomic connections are known to exist between the cervical plexus and the hypoglossal and vagal nerves through the C1 and C2 nerve roots. Furthermore, the superior cervical sympathetic ganglion directly connects with the C1, C2, C3, and C4 nerve roots through the vertebral nerves. Thus, the irritation of the ventral dural nerve endings and/or impingement of the cervical nerve roots seen in cervical spondylosis could aggravate the sympathetic afferents, and explain the autonomic symptoms frequently seen in association with CGH.
It has also been shown that treating neuroforaminal compression, whether temporarily or more long-term, leads to decreased CGH symptoms. In regard to the more temporary treatment, Bogduk et al. [
23] showed that in 161 patients undergoing selective nerve root block for cervical radiculopathy, in both upper and lower cervical spine, 59% patients had a reduction or complete relief of headache symptoms. A more long-term solution to neuroforaminal compression is uncoforaminectomy with foraminal decompression. This has been shown to be effective in treating CGH symptoms, and the headache relief is noted to be both immediate and durable in these cases [
8]. Recently, Liu et al. [
5] reported a consecutive series of 34 patients treated with anterior decompression and fusion, leading to headache improvement in all patients. Posterior cervical laminectomies have also been shown to provide positive results in regards to CGH relief and/or reduction (12 of 15 patients, 80% success rate), specifically in patients with a narrow cervical canal [
10]. However, the headache relief with a posterior approach is noted to be less durable than the improvement seen in anterior approach surgeries [
10]. Though the overall severity of headaches in patients treated with a posterior approach was reduced significantly, a small number of patients who were initially relieved of headaches had recurrences at 1-year follow-up [
10]. Yet, none of the patients in that study relapsed to severe headaches after posterior laminectomies [
10]. The authors postulated that upright posture and mobility over time could be the reason for CGH exacerbation [
10]. We theorize that performing posterior laminectomy provides an indirect decompression of the UVR zone and anterior dura mater. This would explain the overall decrease in headache severity at early follow-up and recurrence of a low-grade headache over the long-term.
In contrast, the ACDF and CDA operations provide direct elimination of the anterior underlying pathogenesis and thus provide more durable results. Furthermore, it has been shown that performing an adequate uncoforaminectomy and removal of the posterior longitudinal ligament are critical to headache relief with these anterolateral approaches [
8]. An anterior cervical approach with discectomy provides an additional advantage of eliminating a potentially concomitant discogenic nociception.
In our 7-year follow-up results, the 2-level CDA group showed significantly greater improvement in headaches compared to the 2-level ACDF group. Improved kinematics, CSF flow, and possibly lower stress and less arthropathy at adjacent segments, in contrast to fusion operations, might contribute towards better outcomes observed with CDA over the long-term. It will be of interest and value to compare the results between the 2 groups with even longer follow-up.