Technique

Caudal ESIs were popularly used especially before the routine use of fluoroscopy because of the ease of access via palpable anatomic landmarks and a lower risk of dural puncture. Because the access point for this procedure is more distal to the presumed pathology in most cases, slightly larger volumes of injectate often are instilled. The typical target is pathology at the L5-S1 level, as medication rarely disperses cranial to this level. Some physicians, however, have developed techniques using an epidural catheter to reach more cranially via a caudal entry point.

Patients are placed in the prone position, and the caudal space is identified using palpation of the sacral hiatus and fluoroscopy. The spinal needle is introduced through the sacral hiatus, and accurate needle placement is ensured using radiopaque contrast and fluoroscopy. To lessen the chance of dural puncture, the needle should not be advanced more cranial than S3 given that the dural sac typically ends at the S2 level. Real-time or “live” fluoroscopy is used to rule out placement in an epidural vein, although some techniques also use a test dose of lidocaine with epinephrine. Once placement is verified, the medication is slowly injected. Post-injection wash-out views on fluoroscopy are used to confirm spread of the injectate to the target level.

Evidence

There have been six RCTs on caudal ESIs for the treatment of sciatica, three of which have included a diagnosis of low back pain. Unfortunately, none of these studies were performed with fluoroscopy. One study showed significantly decreased pain after three ESIs, although results were confounded by treatments outside the study protocol.

For the diagnosis of lumbar spinal stenosis (LSS), which often includes symptoms of neurogenic claudication or radicular pain with or without low back pain, caudal injections were recently studied. In 2008, 40 patients were randomized to fluoroscopically guided caudal injection with either 9 mL lidocaine and 2 mL normal saline, or 9 mL lidocaine, 6 mg betamethasone, and 2 mL normal saline. Multiple injections were allowed throughout the 12-month follow-up period. Success was defined as at least a 50% decrease in numeric pain rating scale and a 40% decrease in the Oswestry Disability Index. There were equivalent results, with approximately 60% success in both the anesthetic alone and the anesthetic plus betamethasone groups at 12 months. A significant weakness of this study includes unlimited injections throughout the follow-up period, with an unknown amount of time between last injection and follow-up points and a nonstandard intention-to-treat analysis. Additionally, the level of stenosis being targeted was not reported.

There is not sufficient evidence for the use of caudal ESIs in the treatment of axial low back pain with our without radicular pain. There is also insufficient evidence to support the use of caudal ESIs for the treatment of LSS.

Technique

The IL technique achieves access to the epidural space preferably one spinal segment below the suspected pathology using a paramedian or midline approach. The route of the needle from superficial to deep includes the skin, subcutaneous tissue, paraspinal muscles (paramedian approach) or interspinous ligament (midline approach), ligamentum flavum, then entry into the epidural space. Some practitioners avoid the IL approach if there has been surgery at the site of intended injection, especially laminectomy, given derangement of the anatomy and a higher risk of dural puncture. The most common means to identify entry into the epidural space is the loss of resistance technique, which can be lost with epidural scarring. Fluoroscopy with radiopaque contrast is used to confirm epidural placement and exclude vascular flow.

Evidence

There are two controlled studies and one observational study for the treatment with IL ESI for axial low back pain. Serrao and colleagues compared ESI with intrathecal midazolam and found no significant difference at 2-month follow-up. An earlier RCT included 39 patients with axial low back pain and radicular symptoms treated with ESI or interspinous saline. There was significant improvement in the steroid group at 1- and 3-month follow-up; however, specific results for low back pain are not known, and fluoroscopy was not used. There is also an observational study that reported pain improvement of 25% to 35% of subjects for greater than 1 year after either one TF or IL ESI. Without a control group, however, and a large drop out rate, these results are difficult to interpret. There is insufficient evidence for the use of nonfluoroscopically guided IL ESIs for the treatment of axial low back pain.

For the treatment of LSS, there is a single, small, randomized controlled study on a single fluoroscopically guided IL ESI. In 2009, Koc and colleagues compared IL ESI with a physical therapy control for the treatment of LSS. Patients had both lower extremity and axial low back pain. The ESI was targeted to the most stenotic level via epidural catheter. The therapy group had 2 weeks of inpatient physical therapy, and all groups (including a third control arm) received diclofenac and a home exercise program. All groups had significant improvements in visual analog scale and Roland Morris Disability Inquiry at up to 6-month follow-up. The ESI group was superior only at 2 weeks. There are some who argue that a single epidural injection may not be sufficient, and that better outcomes may be possible with including up to three injections if pain returns. Currently, however, there is inconclusive evidence for IL ESIs for the treatment of LSS.

There are several studies on IL ESIs for the treatment of failed back surgery syndrome (FBSS), also known as postlaminectomy syndrome. These patients usually are symptomatic of axial low back pain and radicular symptoms, and treatment effect is not specified for one or the other in most of the literature. In 1989, Rocco and colleagues studied 22 patients still symptomatic after laminectomy and randomized them to receive epidural lidocaine with triamcinolone or morphine, or both. After three IL injections at the location of the patients’ pain, there was no difference between groups, suggesting no benefit of epidural steroid. Nonetheless, it is unknown if fluoroscopic guidance was used, and there was no clear control group. Another study on recurrent low back pain after laminectomy randomized 206 patients to nonfluoroscopically guided IL injection with anesthetic plus methylprednisolone or indomethicin, or both. All groups had decreased average pain at 2 weeks, and there was no further follow-up. The short-term follow-up is a major limitation of this study and precludes any interpretation of results. Additionally, the injection was done above the level of operation and presumed source of pain. Many would argue that medication flow in the epidural space is predominately cranial, and epidural injection should always be from below the target level to maximize medication delivery to the presumed source of pain. There is no evidence for the use of nonfluoroscopically guided IL ESI for the treatment of low back pain in FBSS. There are no available studies for the treatment of FBSS with fluoroscopically guided IL ESI.

Overall, there are many studies on the use of IL ESIs performed without fluoroscopic guidance that do not show any conclusive benefit for the treatment of axial low back pain in general, or in the setting of LSS or FBSS. One small study suggests that a single, large-volume, fluoroscopically guided IL ESI is not superior to physical therapy in the treatment of LSS.

Technique

The TF approach is considered to be more specific, delivering the injectate directly to the site of pathology at the ventral epidural space next to the disc, the dorsal root ganglion, and the nerve root. The technique involves using fluoroscopy to identify a subpedicular target within the intervertebral foramen. The needle is advanced to achieve a perineural location in the so-called “safe triangle” that lies just below the pedicle and above the dural sleeve of the target nerve root. Precautions are taken to avoid advancing the needle tip medial to the six o’clock position of the pedicle in the AP (anteroposterior) view to avoid dural puncture.

Technique to target the S1 nerve root in its foramen is somewhat analogous to the subpedicular approach at lumbar levels. The target point for an S1 TF ESI is on the caudal border of the S1 pedicle, just dorsal to the internal opening of the S1 anterior sacral foramen. The needle is inserted into the posterior sacral foramen just short of the floor of the sacral canal and lateral to the six o’clock position of the pedicle.

Real-time or (“live”) fluoroscopy or digital subtraction angiography is used to rule out vascular flow. The anesthetic is injected first, and a pause of 60 to 90 seconds is conducted to monitor for central nervous system adverse effects, which are generally thought of as reversible (seizures, transient paresis, and respiratory depression), and an additional indicator of intravascular injection. The corticosteroid or other medication is then injected and post-injection “wash-out” images are taken to confirm flow of the medication to the target.

Selective nerve root block

In practice and in the literature, the terms selective nerve root block (SNRB) and TF ESI are often used interchangeably. The term SNRB describes a highly selective TF injection in which the interventionalist anesthetizes a single specific nerve root/dorsal root ganglion to confirm or refute it as the source of pain. The selective nature of the nerve root block applies if the injectate/anesthetic travels only to the target nerve root and not medially into the epidural space. It has recently been demonstrated, however, that with as little as 0.5 cc of contrast directed via SNRB at L4 and L5, epidural flow to an adjacent level can be seen. With standard doses of 1 to 2 mL of anesthetic used, these injections are likely often nonselective or at least only partially selective.

Evidence

For radicular pain, there are two recent RCTs supporting the efficacy of TF ESI. Decreased surgical rates and decreased pain scores at 12-month follow-up have been noted. There are no RCTs on the use of TF ESIs for the management of axial low back pain in isolation. There are no studies on TF ESI for the treatment of primarily axial low back pain in FBSS.

There have been several studies on the treatment of LSS with TF ESI. They show favorable response, but include patients with axial low back pain and significant radicular pain, making interpretation specifically for the treatment of low back pain difficult. For axial low back pain in isolation, Lee and colleagues compared treatment with TF versus IL ESIs in 102 subjects with LSS and 103 with herniated nucleus pulposus. Success was defined as a decrease in the numeric pain rating scale of at least 2 points and patient satisfaction. All groups showed significant improvement. The LSS patients treated with TF ESI showed statistically significant greater success than those treated with IL ESI. There was not a significant difference in the rate of success between the TF and IL routes in those with herniated nucleus pulposus. The fact that all groups significantly improved suggests that ESIs may be effective for the treatment of axial low back pain caused by herniated disc or LSS. The lack of a control group, however, significantly limits this conclusion. What can be concluded is that TF ESI is likely superior to IL ESI in the treatment of axial low back pain caused by LSS.

Injectate flow

The primary contention for using the TF approach in preference to the IL approach is that it delivers the medication directly to the ventral epidural space, in closer proximity to the presumed target of the disc or dorsal root ganglion. Studies evaluating contrast patterns generally support this concept in the lumbar spine.

Botwin and colleagues evaluated the contrast patterns in lumbar IL injections in 2004 and found that approximately one-third revealed ventral contrast flow, and 16% were bilateral. In contrast, Manchikanti and colleagues evaluated lumbar TF ESI flow patterns and demonstrated ventral epidural filling in 88% of the procedures and nerve root sleeve filling in 97% of procedures. These two studies suggest better epidural flow to the anterior structures with TF approach. Conversely, a prospective evaluation of contrast flow patterns with fluoroscopically guided lumbar ESI compared the lateral parasagittal IL (PIL) approach with the TF approach. The PIL approach was superior to the TF approach for placing contrast into the ventral epidural space, with reduction in fluoroscopy times and improved spread. In general, a more lateral approach, TF or PIL, will be more likely to produce ventral flow than a midline or paramedian IL injection. This may differ from the cervical spine, as a recent analysis of contrast patterns for cervical IL epidural injections using the midline approach found rate of ventral epidural spread from this posterior approach was 56.7% with 1 cc and 90% with 2 cc of injectate.

Evidence

The effectiveness of TF versus IL ESIs has been compared, and the results are mixed. Schaufele evaluated the effectiveness of TF versus IL ESI for the treatment of symptomatic lumbar disc herniations and found TF injections resulted in better short-term pain improvement and fewer long-term surgical interventions than the IL ESI. Ackerman and Ahmad compared TF, IL, and caudal approaches for treating lumbar disc herniations and found that pain relief was significantly more effective with TF injections. As noted in the discussion of TF ESI evidence, treatment with TF versus IL ESIs was compared in patients with axial low back pain, 102 with LSS, and 103 with herniated nucleus pulposus. All groups showed significant improvement, but the LSS patients treated with TF ESI showed statistically significant greater improvement than those treated with IL ESI. There was not, however, a significant difference in success between techniques in those with herniated nucleus pulposus. The evidence suggests that TF ESI may be more effective than IL ESI for treating axial low back pain, especially in LSS. It should be noted, however, that neither the TF nor the IL technique has been shown to have clear benefit over control for the treatment of axial LBP.