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The prevalence of cervicogenic headache ranges from 1% to 4.1% in the general population [1,2] and accounts for 15% to 20% of all headaches [3]. The exact cause(s) of the cervicogenic headache can be difficult to diagnose and differentiate. Inflammation-related (degenerative, traumatic, infectious, rheumatoid) causes, congenital fusion, and adhesions can, individually or in combination, cause pain in the suboccipital region [4,5], which extends from the forehead to the vertex, and, in most cases, to the ear [6] other patterns have also been described, involving supraorbital or even hemicranial pain.

The atlanto-occipital (AO) joint is an encapsulated synovial joint between the first cervical vertebra (C1) and the occipital bone, consisting of a convex occipital condyle and a concave superior articular surface of the C1 lateral mass. In addition to the ligamentous structures of the spinal column, stabilization (and flexibility) is also provided by various additional overlying ligaments. Innervation is provided by the ventral ramus of the first cervical nerve (C1). The vertebral artery runs medially and diagonally and often lies over the medial one-third of the AO joint. The carotid artery and jugular vein run anterolaterally to the joint.

The AO joints, as part of the craniovertebral joints, provide mechanical strength, while enabling complex movements, as they offer a (passive range of motion of) 10 degrees of flexion and 25 degrees of extension (primary plane of motion), along with 5 degrees of (frontal) flexion and 4 degrees of conjugate rotation (minimal movement in these planes). Approximately 50% of the flexion and extension of the cervical spine occurs in the AO joint.

The diagnosis is a clinical one and mainly based on the patients’ history and physical examination. The patient may suffer from (sub) occipital headache, neck pain, and pain with nodding, along with a limited range of motion (ROM) in flexion and extension, crepitus, tenderness at the occipitocervical junction and craniocervical kyphosis. These findings from the physical examination might help to differentiate AO joint pain from other pathologies, for instance occipital neuralgia. In general, there are no pathognomonic symptoms and signs, and history and physical examination cannot reliably predict painful AO joints, but can help guide decision making. A differential diagnosis must be made with various other conditions, such as vascular disorders (aneurysm of the vertebral artery, temporal arthritis, and artery spasm), central nervous system disorders (dura inflammation, nerve root irritation, and Arnold-Chiari malformation), intracranial disorders (tumor, arteriovenous malformation, and hemorrhage) or musculoskeletal disorders (discopathy, muscle ligaments, and C1-2 or C2-3 facet joint arthropathy).

There is scarce literature available on pain originating from the AO joint, as well as its treatment possibilities. Moreover, there is a gap present regarding recent literature on effectiveness of minimal invasive treatment of refractory AO joint pain, as well as on its impact on patients’ lives. Therefore, we present this case series report on PRF lesions in the AO joint in reducing pain and the global perceived effect. We also examine if there is a correlation between pain reduction and the presence of legal claims, as well as the occurrence of complications as a result of the minimally invasive treatment.

Materials and Methods

We conducted a case series report on the effectiveness of PRF in patients with AO joint pain lasting more than three months. Patients included in the study were aged 18 years or older, experienced occipital neck pain and headaches refractory to conservative treatment, had clinical signs and symptoms indicating AO joint involvement, understood the treatment, and provided verbal consent to undergo the treatment.

Patients younger than 18 years old, those who were pregnant, had malignant – or infectious diseases, coagulation disorders, or had previously undergone surgery in the upper cervical spine were excluded for PRF treatment of the AO joint.

All procedures were performed in a specialized pain center by an experienced pain physician. Patients were placed in a prone position, with two pillows under the chest and a pillow supporting the forehead, with slight neck flexion, depending on pre-existing axial spine problems and contour. From an anteroposterior (AP) position of the cervical spine axis, the c-arm is adjusted craniocaudally, with the tip of the jaw in the midline, to expose the AO joint.

All procedures were performed under sedation by an experienced sedation specialist, with monitoring according to American Society of Anesthesiologists (ASA) House of Delegates Standards for Basic Anesthetic Monitoring (ASA) [7], continuous intravenous (IV) propofol Target Controlled Infusion (TCI) 2,0 µg/mL – 4,0 µg/mL and continuous nasal oxygen 3 L/min. Strict aseptic conditions were applied. No skin infiltration with local anesthetic was performed, in fact we did not use any medication at all for the AO joint treatment.

We used a 6 cm 22 Gauge needle with a 5 mm active tip to perform the procedure (Sluijter-Mehta Kit needles, via CoYoMeÒ, Markham, Ontario, Canada). The needle was fluoroscopically guided to the posterior lateral third of the AO joint (Figures 1 and 2). After fluoroscopic placement (no sensory stimulation), we treated the AO joint with PRF (5 Hz, 5 msec, 4 minutes, 45 V, maximum of 42 degrees Celsius) using a lesion generator (Diros OWL URF-3AP lesion generator, via CoYoMeÒ, Markham, Ontario, Canada), after which the patient was awakened.

Figure 1: Anteroposterior view of the AO joints in a patient with placement of the needle in the left posterior lateral one-third of the AO joint (orange arrow depicts the tip of the needle).

Figure 2: Lateral view of the AO joint with placement of the needle in the posterior part of the AO joint (orange arrow depicts the tip of the needle).

The primary outcome parameter was pain reduction at 2 months post-treatment. The numeric rating scale (NRS) from 0-10 that is frequently used in clinical practice because of its ease of use [8-10]. The secondary study outcome parameter was the Global Perceived Effect (GPE) at 2 months post treatment [11-13]. The type of rating of perceived effect is a “transition scale” or Global Perceived Effect (GPE) scale. The GPE scale asks the patient to rate, on a numerical scale, how much their condition has improved or deteriorated since some predefined time point, as well as their satisfaction with the results of the treatment performed.

As explained by Farrar et al [9]. And later by Dworkin et al. [12], a decrease of 2 points (30%) on the NRS represents a meaningful (much improved) change in chronic pain, and a decrease of 4 points (50%) or more represents a substantial (very much improved) change in pain. Both outcome measures are reported in this article. The follow-up period (T1) was two months after treatment. Categorical variables were summarized using frequencies and percentages. Numerical variables were visually assessed for normality. If normally distributed, data were presented as mean (standard deviation); otherwise, median and interquartile range [IQR] were reported.

The effect of treatment on the NRS score was assessed using the Wilcoxon signed-rank test, comparing scores at time point one versus baseline (time zero). To examine the association between changes in NRS scores (ΔNRS) and GPE scores, Mann–Whitney U tests were performed. The same test was used to explore the relationship between ΔNRS and the presence of claims (yes/no).

All statistical tests were two-sided, and a p-value < 0.05 was considered statistically significant. Analyses were performed using IBM SPSS Statistics, version 28.

Results

Descriptive statistics are summarized in (Table 1). The median NRS score decreased from 8.5 to 5 (p-value <0.001). The change in NRS score correlated with GPE scales satisfaction and recovery (p-value <0.001) (Figures 3 and 4). There was a good correlation between the GPE scales for recovery and satisfaction, as shown in (Table 2).

Table 1. Descriptive statistics summarization.

Table 2. Association between GPE scales recovery and satisfaction.

Figure 3: Box plot of 鈭哊RS (T1-T0) and GPE recovery scale.

Figure 4: Box plot of 鈭哊RS (T1-T0) and GPE satisfaction scale.

There was no correlation between ΔNRS and the presence of claims (p-value 0.18). No complications were observed.

Discussion

In 1994, Dreyfuss et al. described an intra-articular injection treatment for patients with AO joint pain [14]. Since then, only a few studies have been published on the treatment of patients with AO joint pain.

In a prospective study to assess the effectiveness of IA steroid injection for AO joint pain, twenty patients underwent two intra-articular (IA) AO injections with local anesthetic and steroid (one week between de injections and after a diagnostic test block with 50% or more pain relief) [15]. These steroid injections were found to be effective for the short-term management of AO pain.

In a prospective randomized controlled pilot study to assess the effectiveness of PRF stimulation on the AO joint in patients with chronic joint pain, Shin et al. compared the effects of AO intra-articular (IA) PRF and AO IA corticosteroid injection [16]. Intra-articular PRF stimulation significantly relieved AO joint pain and the effect persisted for at least six months after treatment.

Tak and Chang investigated the clinical efficacy of IA PRF therapy for the management of refractory chronic AO joint pain [17]. One and three months after PRF therapy, the numerical rating scale (NRS) scores for pain were significantly lower than before treatment. Sixteen of the 20 (80%) patients reported pain relief and were satisfied with treatment results 3 months after PRF.

The results of our case series report (ΔNRS, presence of complications) seem to be in line with existing literature on PRF treatment of the AO joint. We also examined the change in GPE due to PRF treatment of the AO joint, as well as the correlation between ΔNRS and the presence of legal claims, which are not clearly mentioned in the literature.

In line with existing literature on pain treatment, we used the change in NRS (ΔNRS) for pain as primary outcome parameter and the change in GPE as secondary outcome parameter. What actually matters most, pain reduction or restoring function and satisfaction? What we saw in this study is that ΔNRS after the PRF treatment is related to the GPE scales satisfaction and recovery, and there was also a perfect correlation between the GPE scales for recovery and satisfaction as well. Perhaps we can use the GPE scales as primary outcome measures in future studies.

The test-retest reliability of the GPE scale is excellent, but the ratings are influenced by the current status of the patient; do ratings truly reflect change or just current state? The GPE has several qualities that make it an appealing tool for use in clinical practice and research; being a single question, it is easy and quick to administer and the results are seemingly simple to interpret. Such scales have been recommended for use as a core outcome measure for chronic pain trials and been advocated to increase the relevance of information from clinical trials to clinical practice.

Complications with invasive treatment are rare in experienced practitioners and when they do occur, they are mainly due to the injection of medication [4], not the PRF treatment [17]. Therefore, we didn’t inject any fluid in or at the joint, including contrast agent.

If we did inject a contrast agent (despite the increased risk of injecting fluids), we probably would have known whether we were inside or outside the joint. But, is there a difference in outcome parameters when using PRF with the needle in the joint or near the joint i.e., on the capsule? Moreover, what is the predictive validity of a diagnostic test block with local anesthetics in predicting the result of PRF at the AO joint? We don’t seem to know the answers to these questions.

The course of the vertebral artery can be variable and tortuous, increasing the risk of penetration [18]. The literature provides some support for slight rotation (30 degrees) of the head toward the contralateral side, as well as an access point in the posterolateral aspect of the joint capsule.

Recently, Mares and Majdalani described a somewhat different method of injecting the AO joint, where they avoid the vertebral artery and other important structures [19]. No studies have been performed using this technique, so we can’t comment on the number of complications.

In addition to the AO joint, most patients had one or more other less painful structures, such as the atlanto-axial (AA or C1-2) joint, the C2-3 joint, and one or more neck muscles (median decrease in NRS score from 8.5 to 5). From a patients’ perspective, should we focus solely on the AO joint (or another single structure that causes most of the pain), or should we treat two or more painful structures at the same time? Does treatment of the painful structures together at the same time lead to better results than treatment of the AO joint itself? In future studies, we can examine the combined treatment of multiple painful structures.

There was a lack of correlation between pain reduction (ΔNRS) and the presence of legal claims (p-value 0.18). This might be an important point for the objectivity of the results as well as (in part) answering the question what to do with patients who present at the pain center with this type of chronic pain and have started a legal claim.

This retrospective study has several limitations. We didn’t include an experimental control group, and therefore we can’t compare the treatment results with such a control group, for example a placebo procedure. In addition, pain scores were measured at specific times; we don’t know whether the use of pain scores representing specific periods would have led to a different result.

Conclusions

This retrospective study shows that PRF treatment of the AO joint in patients with chronic AO joint pain results in significant pain relief, an improvement in their recovery and satisfaction. However, high quality research is needed to establish the role of PRF treatment in managing AO joint pain and cervicogenic headache.

Future research should focus on several questions that need to receive an answer: 1) what are the long-term effects of PRF treatment; 2) comparing PRF treatment with other treatment modalities; 3) treatment of multiple painful structures (e.g., the AA joint, C2-3 joint, third occipital nerve, and the neck muscles); and 4) the use of GPE scales as primary outcome parameter, as well as other scales that measure the impact the pain (treatment) has on patients’ lives.

Acknowledgements

The authors wish to thank Mrs. M. de Beste, specialist pain nurse, for her participation in gathering the data from the patients.

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