See Article, page 1139

In this issue of Anesthesia & Analgesia, the pros and cons of placement of the needle in the safe triangle: supraneural, upper portion of the foramen to avoid the nerve, or in Kambin’s triangle: infraneural, lower portion to avoid the radiculomedullary artery, have been eloquently discussed by Doshi et al.1 The interventional pain physician is consulted to perform a cervical and lumbar transforaminal (TF) epidural steroid injection (ESI) for therapeutic and/or diagnostic purposes. Therapeutic injectate consists of a steroid with or without a local anesthetic. There have been reports of paraplegia and cerebral embolism resulting from embolization of the particulate steroid through a radicular artery accompanying the nerve root; the adjacent vertebral artery; a communication through segmental medullary arteries of the ascending cervical and deep cervical arteries with the anterior spinal artery; the artery of Adamkiewicz in the thoracolumbar region; and trauma or spasm of these arteries.2–4 These catastrophic events occur more frequently with cervical TF injections due to the prevalence of radiculomedullary arteries within the foramen, the proximity of the vertebral artery to the foramen, and the location of the ascending cervical and deep cervical arteries in the vicinity of the superior articular process.3 To diminish central nervous system (CNS) injury, therapeutic cervical TF ESIs are now performed only with nonparticulate dexamethasone. Diagnostic/prognostic injections, performed when the surgeon wants to confirm the involved nerve root before a selective surgery, are usually done with local anesthetic alone. For the lumbar spine, the exact risk of CNS injury due to injury of the artery of Adamkiewicz, or the radicular artery accompanying the nerve root, during TF ESI is unknown. The risk is likely lower than in the cervical spine due to anatomical differences, but evidence shows it is clearly a pertinent risk.2–7

To reduce catastrophic events from TF ESIs, the Multidisciplinary Pain Workgroup (MPW), in conjunction with the Food and Drug Administration and the World Institute of Pain Benelux (Belgium, the Netherlands, and Luxembourg) Work Group, has separately established recommendations.5–7 Their proposals include the use of contrast medium under real-time fluoroscopy and/or digital subtraction angiography, use of extension tubing, and the avoidance of moderate-to-heavy patient sedation. The Benelux work group also recommended the injection of a local anesthetic test dose. The specific type of steroid injectate is controversial and beyond the scope of the current Pro Con discussion by Doshi et al; hence, only a few comments are made. Both the MPW and Benelux groups recommended against the use of particulate steroid in cervical TF injections. For lumbar TF injections, the MPW recommended the initial use of a nonparticulate steroid and supported the use of a particulate steroid in certain situations (eg, short duration of relief). For L3 and lower levels, the Benelux work group concluded that a particulate steroid may be used.7 This recommendation was partly due to the limited availability of the nonparticulate dexamethasone in Belgium, the Netherlands, and Luxemburg. The Benelux work group endorsed placement of the needle tip in the posterior portion of the neuroforamen but did not specifically state the safe triangle or Kambin’s triangle. The MPW also did not comment on the preferred placement of the needle tip. Practice guidelines from professional organizations have not firmly endorsed one approach for needle placement over another.

As noted by Doshi et al in their Pro Con paper, contrast spread and clinical efficacy appear to be similar between different foraminal needle placements in TF ESI. However, intradiscal injection is higher and nerve contact less with infraneural injection.

For safety and efficacy, either of the TF ESI approaches can be more or less favorable depending on the spinal anatomy, the patient’s specific anatomy, and the target of the injectate. The clinician must understand the anatomical (nerve, disc, and vascular) risks based on the typical anatomy of the spinal level. The clinician should also consider the patient’s specific anatomy and pathology. For example, an infraneural injection may not be feasible due to disc protrusion into the inferior foramen. In another patient with lateral recess impingement, the same target may more adequately deliver injectate posterior to the disc. Intraprocedural evaluation of injectate flow is critical for a safe and effective TF ESI. Furthermore, the location of the blood vessels can be identified, and placement of the needle can be accordingly planned (Figure 1).

Figure 1.:

A 44-year-old man with left low back pain radiating to the hip and lateral thigh. A, A sagittal T1-weighted MR image of the left-sided lumbar neural foramina shows an L4-5 foraminal disc herniation largely obliterating the inferior neural foramen in the region of Kambin’s triangle. The area of the target zone is markedly decreased, and advancement of a spinal needle inferiorly into this area may result in intradiscal injection. B, A slightly more lateral image from the same sequence shows the relationship of the exiting left L4 nerve root (arrowhead) with one of the radicular vessels (arrow). In this case, the nerve is slightly superior to the vessel, and the epidural fat is preserved posteriorly and superiorly. C, An even more lateral image shows the relation of the nerve root (arrowhead) to 2 radicular vessels (arrows), which are located deep and inferior to the nerve root. In this case, the epidural fat is most preserved superiorly and posteriorly, and a “safe triangle” approach just under the L4 pedicle may be helpful to avoid vascular injury. MR indicates magnetic resonance.

Preprocedural and intraprocedural imaging is, therefore, invaluable to the interventional pain physician performing a TF ESI. High spatial resolution magnetic resonance imaging (MRI) obtained before an injection is most helpful because it identifies the course of the nerve root. This imaging can demonstrate the relationship of the nerve root to the radicular vessels within the neural foramen, especially in the lumbar spine, and can suggest anatomic variability and show disc herniations or disc-osteophyte complexes. These findings help the interventional pain physician safely plan the needle trajectory to reduce the risk of radicular arterial injury and intradiscal injection. Finally, the physician can evaluate the contrast spread during injection—with a secondary injection planned if the injectate does not cover the intended target.

When performing a TF ESI, the interventional pain physician needs to maintain a flexible approach and to be familiar with both the supraneural and infraneural approaches to safely and effectively deliver the intended injectate. As noted above, foraminal disc herniations and disc-osteophyte complexes often obliterate the inferior neural foraminal recess in the region of Kambin’s triangle, and a supraneural approach may offer the only option for a successful injection. On the other hand, even though an ESI performed at one spinal level often covers adjacent levels, for cases in which multiple levels could contribute to the patient’s symptoms and a strictly selective nerve root injection is not a priority, the interventionist can perform a paramedian interlaminar injection.

The reason for the TF injection should be borne in mind in deciding which approach to take. If done as a diagnostic and prognostic aid before surgery, it is probably more appropriate to place the tip of the needle in the posterior portion of the neural foramen to better outline the target nerve root and to limit the spread of the injectate into the epidural spread, thereby preventing the blockade of unintended nerve roots.

For therapeutic injections at low cervical and midcervical levels, especially when >1 level is involved, interlaminar injections at low cervical levels can be performed. A catheter can be threaded up to the level of the pathology. Injections of 2 to 4 mL volumes have been shown to cover 3.6 ± 1 levels (up to C2-3) bilaterally.8 Relief and functional improvement from a C7-T1 interlaminar steroid injection was noted to be comparable to targeted steroid injection via an epidural catheter.9 These improvements were sustained at 1-year follow-up.10

For therapeutic TF injections during which CNS vascular injury is a concern, the infraneural approach has lower risk. If the contrast spread into the epidural space is minimal and the relief mild, any subsequent injections can target the supraneural foramen. For safety, if the supraneural approach is chosen, the needle tip is preferably in the posterior-superior aspect of the neural foramen. Blockade of the involved nerve root is ideal in terms of etiology of the radicular pain. There is a trend toward greater efficacy when medication reaches the preganglionic nerve.11

Fluoroscopy is the standard imaging modality to guide routine lumbar spine ESI and has been successfully used in the cervical spine. Computed tomography (CT) is being used by radiologists for a cervical TF ESI to more precisely delineate complex anatomic relations in the cervical region (Figure 2). However, CT-guided interventions require dedicated imaging resources with much greater financial cost and technical expertise; increase radiation exposure, particularly to radiosensitive organs such as the thyroid gland in the head and neck; and often prolong procedural time. Cervical TF ESI with CT guidance has also been associated with adverse outcomes, and fastidious technique is still warranted.

Figure 2.:

A 29-year-old man with 1-mo history of right neck and upper extremity pain and weakness, with progressive numbness, tingling, and weakness in the triceps and wrist flexors. A, Sagittal T2-weighted nonfat-suppressed MR image of the right paramedian cervical spine through the lateral recess shows a C5-6 lateral recess disc herniation (arrow). B, Axial CT intraprocedural image of the cervical spine at the C5-6 level during an ESI shows percutaneous needle access of the posterior margin of the right neural foramen (arrow) with iodinated contrast in the epidural space outlining the exiting nerve root (arrowhead). The DLP , which can be a measurement of the effective radiation dose, was 164 mGy*cm, and the examination took 34 min, while a comparable fluoroscopically guided procedure had a DLP of 17.8 mGy and took 12 min. CT indicates computed tomography; DLP, dose-length product; ESI, epidural steroid injection; MRI, magnetic resonance.

In summary, the interventional physician should base placement of the needle on the purpose of the TF injection: whether diagnostic, prognostic, or therapeutic. The Kambin’s/infraneural approach is the safer approach in therapeutic injections. In view of the highly variable size and shape of the infraneural foramen, and variations in anatomy, one should not persist with a chosen approach since complications occur in the absence of predisposing risk factors when one does not accept failure and makes multiple attempts. The response of the patient can help determine placement of the needle and nature of the injectate in subsequent injections.

Further studies may elucidate the best TF ESI approach for both safety and efficacy. Until then, matching the planned TF ESI target to the objective of the intervention (diagnostic versus therapeutic), patient-specific anatomy on advanced imaging, and then determining outcomes are warranted. Correlating the contrast coverage of the target with the patient’s response may help determine the subsequent approach. Additionally, utilizing more detailed and standardized anatomic descriptions of the TF ESI targets12 may facilitate future outcome studies.

DISCLOSURES

Name: Alison Stout, DO.

Contribution: The author was involved in the writing of the manuscript.

Name: Imran M. Omar, MD.

Contribution: The author provided the figures and was involved in the writing of the manuscript.

Name: Honorio T. Benzon, MD.

Contribution: The author was involved in the writing of the manuscript.

This manuscript was handled by: Thomas R. Vetter, MD, MPH, MFA.

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