Epidural Steroids Injection: Basics and Updates

Christine El-Yahchouchi

Salim M. Hayek

Corning first described the delivery of drugs in the epidural space in 1885, using the local anesthetic cocaine.¹ Lievre and colleagues first described injecting corticosteroids in 1953.² Epidural steroid injection has gained popularity because of its minimally invasive nature, ease of use, and reproducibility. It is usually performed for patients with radicular pain based on the hypothesis that radicular pain is not only caused by mechanical compression from a herniated nucleus pulposus on nerve roots,³ but also by chemical factors and inflammatory mediators (prostaglandin, cytokines, substance P, etc.) released from the herniated nucleus⁴ as demonstrated by histologic and animals studies. In fact, in pig models, the introduction of nucleus pulposus into the epidural space induced nerve dysfunction in the absence of nerve compression.⁵ Pain from ischemia of the nerve roots has also been suggested as a potential cause of radicular pain.⁶

Several mechanisms of action of epidural steroids have been proposed. The most accepted one is based on the antiinflammatory effects of glucocorticoids. Glucocorticoids inhibit the enzyme phospholipase A₂, leading to the inhibition of the formation of arachidonic acid, the prime substrate for cyclooxygenase and lipoxygenase pathways and thus ultimately the formation of prostaglandins and other eicosanoids (leukotrienes, thromboxanes, and prostacyclins). In addition, glucocorticoids block conduction in C fibers,⁷ suppressing the release of neurotransmitters from the dorsal root ganglia.⁸,⁹ Another proposed means of action of epidural steroids is a washout mechanism by which the injectate will wash away and dilute inflammatory mediators. In a systemic review performed by Rabinovitch and colleagues, pain relief correlated with the epidural volume of steroids irrespective of the dose used.¹⁰ This raised the question whether local anesthetics or saline can have the same effect as steroids. In a randomized control trial done by Cohen et al. including 84 patients with lumbosacral radiculopathy, patients who received epidural steroids had greater reduction in leg pain than patients who received saline or etanercept.¹¹

It is important to mention that these mechanisms of action of epidural steroids are subject to debate and an element of placebo effect may account for some of the effects.

APPROACHES

One can access the epidural space through three different approaches:

Interlaminar epidural injections access the dorsal epidural space between the ligamentum flavum and dura mater. Injections can be guided by fluoroscopy (and occasionally other imaging techniques) or performed without image guidance. The most popular technique to confirm entry into the dorsal epidural space using the interlaminar approach is the loss of resistance technique. It has been shown that relying only on loss of resistance, without image guidance, is inadequate to deposit the medications in the epidural space. 25.7% of all unguided loss of resistance injections were not deposited in the epidural space as confirmed by fluoroscopy and contrast medium injection.¹² Another important factor to consider when performing unguided or image-guided interlaminar epidural injections is the unpredictable distribution of medications from the dorsal epidural space to the ventral and lateral spaces where the pain generators are believed to be located. The distribution of medication depends on needle placement, and other factors such as volume of the epidural space and connective tissue septae, which are beyond the operator’s control.
With transforaminal epidural injections, the needle is introduced into the foramen from an oblique approach. This injection was developed to deliver the medications directly to the dorsal root ganglia of the exiting nerve and the ventral epidural space. Three different approaches can be used: supraneural, infraneural, and retroneural. It is critical to review available imaging before performing the procedure in order to determine the most appropriate approach according to the patient’s anatomy and pathology.

The supraneural approach is the most common strategy; in this approach, the needle is located in the upper part of the foramen, superior to the nerve in the traditional “safe triangle.” The supraneural approach typically yields cephalad flow along the neural sleeve to the lateral recess and supra-adjacent disk. However, a recent retrospective review of spinal angiograms of 115 patients performed by Murthy and colleagues showed that in 97% of the patients, the medullary artery, known as artery of Adamkiewicz, was located in the upper half of the foramen, typically between T12 and L3 and more frequently on the left side.¹³ The supraneural approach therefore may place a medullary artery at risk for needle cannulation.

The infraneural approach places the needle inferior and medial to the exiting nerve; it typically provides flow covering the exiting nerve, but with more caudal flow that may also cover the traversing nerve. At the L5 level, an infraneural approach may have dominant caudal epidural flow. However, the needle is more likely to inadvertently cannulate the intervertebral disk with this approach.

The flow pattern typical of the retroneural approach is less documented.
Caudal epidural injections access the epidural space through the sacral hiatus. This injection carries the lowest risk and some have advocated its use after back surgeries where anatomic changes as well as adhesions can make interlaminar or transforaminal injections more challenging. Cohen and colleagues reviewed the randomized trials performed since 1970 comparing transforaminal and interlaminar epidural injections and concluded that approximately 73% of transforaminal injections demonstrated benefit when compared to 62% for the caudal and 50% for interlaminar injections.¹⁴

EFFICACY AND EFFECTIVENESS

One of the most important determinants of efficacy is appropriate patient selection. Smoking, previous back surgery, coexisting psychological disorders, and chronic pain (pain > 6 months) are predictors of poor outcomes after epidural steroid injections.¹⁴ When discussing the injection with the patient, the operator should stress that epidural steroid injections are not a treatment for the underlying pathology, but provide temporary short-term pain relief to allow the natural course of the disease and to prevent central sensitization and chronic pain. More than 70% of patients with radicular pain in the distribution of the sciatic nerve have resolution of their symptoms without interventions within 4 months.¹⁵ In addition, combination of epidural steroid injections with membrane stabilizers and physical therapy may be superior to either epidural steroid injection or medications/physical therapy alone.¹⁶

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