Chapter 5 Epidural Blocks

Epidural nerve block consists of the administration of small volumes of target-specific local anesthetics, corticosteroids, and other agents into the epidural space to interrupt the pain spasm cycle and reduce inflammation of either axial or radicular pain (Boxes 5.1 and 5.2; Table 5.1) [1,2]. These agents are injected through one of three approaches—interlaminar (for cervical, thoracic, and lumbar epidural injections), transforaminal (for cervical, thoracic, and lumbosacral injections), or caudal. The transforaminal block is discussed in Chapter 6.

BOX 5.1 The Concept of Epidural Injections

Approaches: interlaminar, transforaminal, caudal

Target structure: epidural space, especially the ventrolateral area

Agents administered:

Local anesthetics, which interrupt both the pain spasm cycle and reverberating nociceptor transmission

Corticosteroids, which reduce inflammation by inhibiting the synthesis or release of a number of pro-inflammatory substances

Target symptom: axial or radicular pain

Table 5.1 Postulated Mechanisms of Neural Blockade and Corticosteroids

Injections can also be performed under fluoroscopic guidance and with use of a contrast agent in order to deliver cortisone as close to the disc herniation, area of stenosis, or nerve root impingement, as determined by MRI or CT, and with as little morbidity as possible.

Neural blockade alters or interrupts the following (see Table 5.1):

Nociceptive input

Reflex mechanisms of the afferent limb

Self-sustaining activity of the neuron pool and neuraxis

The pattern of central neuronal activities

Neural blockade may be achieved with local anesthetics or corticosteroids (see Table 5.1). Local anesthetics interrupt the pain spasm cycle and transmission by reverberating nociceptors. Corticosteroids reduce inflammation by (1) inhibiting the synthesis or release of a number of pro-inflammatory substances or (2) causing a reversible local anesthetic effect.

The various modes of action of corticosteroids are as follows (see Table 5.1):

Membrane stabilization

Inhibition of neural peptide synthesis or action

Blockade of phospholipase A₂ activity

Prolonged suppression of ongoing neuronal discharge

Suppression of sensitization of dorsal horn neurons

The benefits of neural blockade can outlast the duration of the anesthetics used. This occurs by the following mechanisms:

Influence on the sympathetic nerve system

Temporary abolition of spontaneous ectopic discharges, resulting in suppression of dynamically maintained central hyperexcitability, as well as reinforcement of endogenous G-protein-couple receptor inhibition of N-type voltage-sensitive calcium channels

Glial inactivation

The presumed effect of steroids is to reduce:

Neuropathic pain

Inflammation

Swelling

Scarring

The mechanism of radiculopathy–neuropathic pain and the action of epidural steroid injection

The rupture of the anulus fibrosus causes radiculitis either by mechanical pressure from disc protrusion or by chemical irritation of the nerve root by leaking material from nucleus pulposus (phospholipase A₂) resulting in radiculopathy-neuropathic pain. In a neuropathic pain state, the steroids can decrease the conduction in injured nerves. It also has been observed that steroids can reduce the bulk of a scar by diminishing its hyaline portion, while leaving the fibrous skeleton intact. Tables 5.2 and 5.3 list profiles of, formulations for, and adverse effects of the commonly used epidural steroids.

Table 5.2 Profile of Commonly Used Epidural Steroids

Table 5.3 Formulations of Commonly Used Epidural Steroids and Their Adverse Effects on the Nerves

As already mentioned, the three main approaches for epidural steroid injection (ESI) are transforaminal, the most specific and effective route; interlaminar, via a midline or paramedian approach; and caudal (see Box 5.1).

The potential uses of fluoroscopically guided transforaminal ESIs include:

Treatment of intraspinal inflammation causing axial spine or radicular pain

Management of acute, subacute, or chronic axial spine or radicular pain that is refractory to more conservative care such as rest, analgesics, and physical therapy

Efficacy of the treatment/procedure is signified by the following:

Reduced leg pain, improved leg raising, and improved lumbar flexion are found at 2 weeks after injection.

No pain-related or functional benefits are seen at 4 weeks or 1 year after injection.

Possible problems with epidural blockade

The injection variables that can affect target site concentration are as follows:

ESI performed without fluoroscopic guidance

The presence of epidural plica or scarring

The volume of the injectate

The rate or pressure of the injection

ESI should not be performed without fluoroscopic guidance. The potential risks of performing ESI without fluoroscopic guidance are as follows:

Intraneural injection

Technical difficulty in the presence of spinal fusion or hardware

Intravascular injection

Spinal cord trauma

Failure to deliver medication to the treatment field may be related either to the approach or to patient variables, as follows:

Epidural fibrosis can result in the preferential flow of medication away from a recurrent disc herniation.

Caudal injections may fail to deliver medication above L5-S1, particularly in patients with central canal stenosis.

Interlaminar injection may localize preferentially to the dorsal epidural space.

Transforaminal injections may localize preferentially to the peripheral perineural space particularly in the setting of severe foraminal stenosis.

Procedural morbidity also varies with each approach. With interlaminar injections, there is a risk of intrathecal injection and subsequent arachnoiditis as well as postprocedural headache. Patients receiving transforaminal injections frequently report significant, although in most cases transient, leg pain, and there is a risk of spinal cord infarction.

Recommendations for use

The American Society of Interventional Pain Physicians makes the following recommendations:

Injections should be given at intervals of 2 weeks only as necessary according to medical necessity criteria.

A maximum of 6 injections per body region should be given per year.

All regions should be treated at the same time, if feasible.

The International Spinal Injection Society recommends that no more than 4 injections at intervals of at least 7 to 14 days be given within a 6-month period.

A definite trend toward nonsurgical management of lumbosacral disc herniation with radicular symptoms has been noted. [3] This change is appropriate for the following reasons:

Resorption of lumbar herniated disc fragments can occur.

Some lumbar disc disorders are asymptomatic, including herniations.

There is an inflammatory component to low back and/or radicular pain in the presence of disc herniation; an inflammation reaction may be necessary to precipitate symptoms, even in the presence of mechanical compression.

The contents of the epidural space

The contents of the epidural space are fat, epidural veins and arteries, and lymphatics (Fig. 5-1).

Figure 5–1 Cross-sectional (A) and three-quarters (B) views of the anatomy of the epidural space.

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