Epidural Steroid Injections for Chronic Back Pain: Piecing the Puzzle Together

Solutions containing steroids have been injected into the epidural space for the treatment of chronic back pain for over 50 years. Currently, epidural steroid injections (ESIs) are the most commonly performed intervention in pain clinics worldwide [1]. However, despite the widespread use of ESIs, the efficacy of this procedure remains controversial. Whereas a wealth of controlled and anecdotal evidence supports their use, dozens of other studies have attempted and failed to conclusively prove that ESIs are an effective means of analgesia in patients with chronic back pain [2]. This poses a dilemma for the modern pain practitioner and begs the question as to why the efficacy of such a ubiquitous intervention remains unproven in the literature.

In all likelihood, this apparent paradox arises partly from an oversimplification of the question itself. As most practitioners are aware, the term ‘chronic back pain‘ is one that encompasses a vast array of etiologies that are often difficult to distinguish from one another due to significant overlap in symptoms [3]. Even in patients with classical radiculopathy, these symptoms rarely occur in isolation (i.e., without concomitant mechanical low back pain).

From a pathophysiological standpoint, the theoretical benefit of ESIs is in patients with neuropathic spinal pain (NSP). The most widely supported mechanism of analgesia in these patients is ascribed to the anti-inflammatory properties of corticosteroids counteracting the high concentrations of inflammatory enzymes that are often found at the site of pathology [4]. Although other mechanisms have been postulated, such as a direct antinociceptive action [5] and osmotic dilution of inflammatory cytokines [6], epidural steroids should not be expected to be as efficacious in patients without an inflammatory component to their pain.

Even differentiating between NSP and mechanical back pain can be difficult, as factors that predispose to NSP also result in changes in regional biomechanics leading to nociceptive pain [7]. For example, the presence of facet joint arthritis predisposes patients to radiculopathy secondary to foraminal stenosis [8], and internal disc disruption decreases the amount of force needed for herniation of the nucleus pulposus [9]. Several investigators have attempted to design instruments that can reliably distinguish between neuropathic and nociceptive pain, but establishing validity is fraught with limitations; hence, these tools tend to be better at eliminating individuals with unambiguous nociceptive pain than they are at identifying those with predominantly neuropathic pain [10]. The challenges for the diagnostician are quite literally multiplied when researchers attempt to enroll large numbers of these patients into a study, which is necessary to detect what is likely a modest treatment effect. The obstacles investigators face when designing a study aimed at ascertaining the degree of efficacy – if any – of ESIs in patients with NSP, can seem nearly insurmountable due to the complexity of the condition, combined with the scarcity of patients with well-defined, isolated NSP.

Researchers attempting to design a study to answer the question of whether ESIs are suitable for long-term pain relief in this population are forced to walk a fine line. Excluding patients with mixed neuropathic and nociceptive presentations is likely to result in a study that falls short of recruitment goals, whereas including these patients tends to result in equivocal data. In addition to patient symptoms, the chronicity of the patient‘s pain poses a problem for investigators. In patients with longstanding back pain, the failure rate of ESIs is high, and studies targeting this population group need to enroll large numbers due to the high number needed to treat. Conversely, patients with back pain of a shorter duration often have resolution of their symptoms as part of the natural course of their condition, or with conservative therapy alone, making it difficult to detect a statistical difference compared with the control group. In view of these challenges, some investigators have turned to studying indirect markers of ESI effectiveness.

Indirect markers of ESI efficacy include the reduction in need for surgical decompression. In a randomized controlled study evaluating the effect of transforaminal ESI on the need for operative treatment of lumbar radicular pain, Riew et al. found that corticosteroids were more effective than epidural bupivacaine alone for up to 13–28 months [11]. A minimum 5-year follow-up of these patients was subsequently performed, and the authors found that most patients with lumbar radicular pain who did not require an operation for at least 1 year after receiving a nerve root injection continued to not need operative intervention for the minimum follow-up of 5 years [12]. Herein lies the conceptual appeal of ESIs, in that they can decrease pain acutely, allowing the body to ‘heal itself‘. Although other studies were unable to replicate these findings, they were conducted by nonsurgeons who utilized nonstandardized operative criteria [13,14]. In light of the inherent, often prohibitive risk associated with surgical intervention and general anesthesia, the variable response to these ‘definitive‘ procedures, and the poor prognosis associated with failed back surgery syndrome [15], the possibility that ESI could prevent even a small percentage of surgeries is a powerful argument in support of early ESI.

In addition to a reduced need for surgical intervention, when comparing return-to-work or ‘sick days‘, most [16,17], but not all [18], studies evaluating this end point showed a modest short-term improvement associated with ESIs. The ability of these patients to return to work sooner after ESIs than with medical management may be an indirect marker of short-term pain relief that was not found when measuring pain scores directly. When functionality, as opposed to simple pain scores, is used as the ultimate marker for overall treatment success, ESI appears to improve or preserve patients‘ ability to perform activities of daily living more than after placebo or medical management.

If clinical observation and clinical trials are seemingly at odds with one another, how can these differences be explained, and how do we answer the question regarding whether or not ESIs are effective in the management of chronic back pain? The answer likely lies in the fact that each element seems to only tell part of the story. Due to the difficulties associated with designing studies in this population, indirect markers of pain control can be analyzed for evidence of pain relief and preservation of function. Combining what we learn from the available data and what we see in our clinical practice, it seems that ESIs can definitely have short-term and possibly long-term efficacy in carefully selected patients with a clear neuropathic component to their pain.

Disclaimer

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.