Spondylotic radiculopathy is a common complication following spondylosis. Foraminal stenosis or disc herniation provoked cervical root resulting in radicular pain. Conservative treatment, like resting, medication, and exercise, has traditionally been a treatment option at first. When conservative treatment failed, C-arm guided cervical transforaminal epidural steroid injection (C-TFESI) has emerged as an effective treatment option. C-TFESI allows precise delivery of injection materials to the targeted pathological site, so that making it a logical approach for spondylotic radiculopathy. However, concerns regarding the safety of C-TFESI have arisen due to reports of severe adverse reactions, including cervical cord infarction.¹

Concurrently, the development of ultrasonography evaluation for musculoskeletal diseases has led to efforts to utilize this technique in treating spondylotic radiculopathy.² Ultrasonography offers distinct advantages, including excellent soft tissue discrimination and visualization of vascular structures.³ In addition, the target area of the cervical root is typically located relatively superficially, allowing for accurate identification and assessment of soft tissue in the neck and cervical root region. These inherent characteristics of ultrasonography have highlighted its potential to reduce complications associated with vascular events, such as thromboembolism-induced spinal cord and brain infarction following cervical root block. Consequently, several studies have reported on the feasibility and effectiveness of ultrasound-guided cervical selective root block, positioning it as a safe and efficient therapeutic option.^2,4 However, the validation of these known advantages remains limited.

Therefore, this review article discusses the clinical significance and safety considerations of ultrasound-guided selective cervical root block.

1. Anatomical consideration of ultrasonographic images

1) Leveling of cervical spine

For an accurate cervical selective root block, precise identification of the target level is essential. C-arm imaging offers the advantage of a broader field of view, facilitating the confirmation of cervical spine levels. Conversely, ultrasound imaging provides cross-sectional images, in which examiners reconstruct three-dimensional anatomical structures based on multiple cross-sectional images. The cervical spine has a complex three-dimensional structure of soft tissues and bones. Therefore, the distinctive shape of the cervical bones and the anatomical relationship between the vertebral artery and the cervical spine is utilized to determine the level accurately.

The transverse process of the cervical spine exhibits a characteristic shape, making the differentiation of cervical levels relatively straightforward. The transverse process of C7 is characterized by the absence or small size of the anterior tubercle. A rudimentary anterior tubercle ca be visualized at 1%.⁵ Furthermore, the C7 root and vertebral artery are positioned laterally and medially on the transverse apophysis. So, it is essential to note that misinterpretation can occur, mistakenly identifying the vertebral artery as the C7 cervical root, depending on the positioning of the ultrasound probe, due to their proximity within the C7 transverse process (Fig. 1). Therefore, it is crucial to utilize a color doppler to confirm blood vessels’ presence and precise location. Since the vertebral artery enters the transverse foramen within transverse process from C6 to C2, it is not visible in ultrasound images obtained at the level of the transverse process.⁶ Above the C7 level, the transverse process has anterior and posterior tubercles, with each cervical root situated between these two tubercles. The transverse process of C6 exhibits a V-shape, while the transverse process of C5 appears U-shaped and is narrower than C6, resulting in a narrower space between the tubercle and cervical root compared to C6.

Figure 1. (A) In the ultrasound image at the C7 level, the C7 root (white arrow) and vertebral artery (white arrowhead) are seen lying on the transverse process. (B) The vertebral artery can be visualized using color Doppler. (C) However, depending on the angle between the probe and the transverse process, the C7 root may not be clearly visible. At the same time, the vertebral artery may appear like the C7 root, requiring confirmation using color Doppler. (D) In the root image at the C6 level, the vertebral artery is positioned within the vertebral foramen of the transverse process and is not visible. The C6 cervical root (empty white arrowhead) is positioned between the anterior and posterior tubercles of the transverse process of C6.

2) Vascular structure around cervical spine

The cervical spine contains a variety of vessels that necessitate caution during ultrasound-guided selective cervical root block. Procedures involve navigating the needle through the trajectory where several vessels are located, including the deep cervical artery, ascending cervical artery, radicular artery, and vertebral artery.⁷ The cervical radicular arteries arise from the vertebral arteries, deep cervical arteries, and ascending cervical artery.⁸ After originating, the cervical radicular arteries traverse the intervertebral foramina and their respective spinal nerve roots. Subsequently, the radicular arteries join the anterior spinal artery to supply blood to the spinal cord. The anatomical relationship between a radicular artery and cervical root around the transverse process varies depending on the artery’s point of origin. For example, the branches from the ascending cervical artery enter the anteroinferior and anterosuperior aspects of the spinal nerve root. On the other hand, branches originating from the deep cervical artery pass through the inferior and posterior aspects of the spinal nerve root, which is the target area of ultrasound-guided selective root block.

About 3% of blood vessels are at the target nerve and needle pathways of the C5 and C6 root. However, at the C7 root, there are about 23% and 10% vessels around the target nerve and needle pathway, respectively.⁹ Therefore, it is crucial to use Doppler to identify vessels around the target root during ultrasound-guided procedures to prevent unexpected adverse effects.

2. Ultrasonography and cervical root

1) Echogenicity of cervical root

This histological difference results in variations in echogenicity.¹⁰ As a result, the connective tissue within the epineurium appears hyperechoic background, while the fascicles exhibit low echogenicity. On ultrasound imaging, peripheral nerves display a honeycomb pattern due to multiple branched fascicles and connective tissue.¹¹ Conversely, cervical roots appear as round structures with low echogenicity in ultrasound imaging due to the absence of branched fascicles and less connective tissue than distal nerves. In ultrasound imaging, blood vessels like cervical roots appear as round hypoechoic structures. Therefore, it is crucial to differentiate between vessels and nerves by observing pulsation or utilizing Doppler to identify the vessels.

2) Root edema

The measurement of the cross-sectional area of nerves in ultrasound examination is an essential diagnostic tool for assessing focal neuropathy.¹¹ Changes in nerve structure caused by focal compression can be observed in ultrasound examinations, including enlargement of the nerve proximal to the site of compression, flattening, and increased vascularity.¹² In the case of cervical roots, nerve swelling caused by inflammation is manifested as an increase in cross-sectional area in ultrasound imaging.¹³ Disc herniation can trigger inflammation around the herniated disc.¹⁴ Additionally, uncompressed disc herniation can lead to inflammation in the epidural space resulting in edema of the adjacent cervical root.¹⁵ Considering the narrow dimensions of the cervical epidural space, cervical disc herniation can cause inflammation around the herniated disc and within the epidural space, extending to the nerve root. However, there is no definite cut-off value for cervical root edema, so comparing the affected and unaffected sides is necessary to gain a clinical perspective. In this case, differences can be observed due to measurement location and probe angle variations. So, the clinical application of root edema measurement for diagnosing radiculopathy has not been fully validated.

3. Ultrasound guided selective cervical root block

1) Spread patterns of injection materials

To maximize the efficacy of the root block, it is essential to accurately identify the specific level of the cervical spine associated with the pain. In addition, to enhance the effectiveness of the root block, it is crucial to consider how accurately the injectate spreads within the foramen and epidural space. Because, spondylotic radiculopathy and cervical disc herniation, which can cause radicular pain, can lead to inflammation in both the cervical root and the epidural space.¹⁶

Although the accuracy of ultrasound-guided procedures can vary depending on the operator’s proficiency, ultrasound-guided selective root block showed higher accuracy.¹⁷ In ultrasound-guided cervical root block procedures, contrast spreads to the medial foramen in approximately 50% of cases.¹⁸ Additionally, in only 5% of cases, the contrast exhibits an intramuscular pattern without involving the perineural area, indicating injection failure. A small injection volume decreased the proximal spreading of the injectant to the cervical spine.¹⁹ After the procedure, contrast was observed to spread into the epidural space in approximately 10% of cases.²⁰ Most cervical injections with a volume of 1 mL exhibited an extraforaminal contrast pattern, while 25% of injection trials with a volume of 4 mL showed an intraforaminal contrast pattern. The volume of the injectant can influence the spreading pattern during the procedure. In summary, when performed with an appropriate volume, the ultrasound-guided cervical injection can effectively target the perineural space and the area around the foraminal region.

2) Effectiveness of ultrasound guided selective root block

The contrast pattern observed during ultrasound-guided injections is closely related to the effectiveness of the injection. A good contrast pattern is characterized by the perineural spread of contrast, indicating that the injectant has reached the targeted area surrounding the nerve root. The perineural pattern suggests that the medication or anesthetic is more likely to have the intended therapeutic effect. Conversely, if the contrast spreads in an intramuscular pattern or fails to reach the desired area, it may indicate a suboptimal injection and potentially lower effectiveness of the treatment. Intramuscular patterns show 100% unfavorable outcome.¹⁸ However, intraforaminal and extraforaminal spreading patterns did not show statistical difference in pain relief after 2 weeks.¹⁹ Moreover, In comparative studies evaluating the therapeutic effects of ultrasound-guided and C-arm-guided cervical root procedures for cervical radicular pain, both guidance methods have demonstrated statistically comparable effectiveness.^21,22

As previously mentioned, ultrasound-guided injections have a high success rate in achieving perineural injection, which in turn leads to higher effectiveness of the injection. Therefore, despite the limitation of epidural injection, the ultrasound-guided injection can expect good clinical outcomes.

3) Comparative advantage of ultrasound guided root block to C-TFESI

C-arm guided cervical transforaminal steroid injection has been reported rare serious adverse effect like spinal cord and brain stem infarction.^23,24 In C-arm guided cervical transforaminal steroid injections, the rate of vascular injection is from 19.4% to 32.8%.^25,26 Among these cases, 18.9% exhibited inadvertent contrast spreading simultaneously into the blood vessels and the epidural space. The high incidence of vascular injection and the rare but significant risk of adverse events have raised concerns regarding the safety of cervical procedures.²⁷

There was no report of vascular injection following ultrasound-guided injection.^2,21,22,28 Most studies on vascular injection report no evidence of entering the blood vessels based solely on the absence of aspiration or flask back. However, the absence of blood aspiration after needle insertion does not guarantee the absence of medication entering the vasculature.²⁹ Additionally, it has been recognized that particulate steroids are a significant contributor to catastrophic events following transforaminal epidural steroid injection.³⁰ Therefore, subsequent guidelines recommend using non-particulate steroids have led to a decrease in catastrophic events. Based on this notion, most ultrasound-guided cervical root blocks utilize non-particulate steroids. In addition to the advantage of visualizing blood vessels, the utility of non-particulate steroids is considered one of the reasons why catastrophic events, as reported in C-arm-guided cervical procedures, are not observed.

Ultrasound-guided cervical root block has emerged as a potentially safe and effective treatment modality for spondylotic radiculopathy. However, further validation for vascular event is required.