Nonselective Epidural Steroid Injection - General Principles

Nonselective Epidural Steroid Injection: General Principles

Anatomy

Meninges

The spinal cord and nerve roots are surrounded by three meningeal layers: the dura mater, arachnoid mater and pia mater (Figure 1). The dura mater consists of dense fibrous tissue and forms a long tube-like enclosure or sac for the spinal cord.  Regular bilateral outpouchings of dura mater are present at the level of each neural foramen, where the dura evaginates to form a dural sleeve.  These sleeves envelope the dorsal and ventral nerve roots and spinal ganglia, which themselves are covered by the more delicate arachnoid mater.  The pia mater represents the deepest meningeal layer and is adherent to the spinal cord and spinal nerve roots.

Epidural Space

The spine may divided anatomically into three compartments: anterior, posterior and neuroaxial. The anterior compartment includes the vertebral bodies and intervening discs, while the posterior compartment contains the structures posterior to the transverse process including the facet joints.  The neuroaxial compartment lies in between and includes the epidural space and neural structures. The epidural (extradural) space of the spine surrounds the dural sac and is located between the dura mater and the posterior longitudinal ligament anteriorly, the pedicles and intervertebral foramina laterally, and the ligamentum flavum and lamina posteriorly (Figures 2, 3). The space is composed predominantly of fat and loose connective tissue but also contains arteries, lymphatics and the posterior and anterior venous plexuses. It extends from the level of the foramen magnum to the sacral hiatus and communicates with the paravertebral space through the neural foramen. In adults the posterior epidural space of the lumbar spine exhibits a characteristic undulating appearance with its greatest dimensions at the level of the intervertebral disc. This appearance is best appreciated on sagittal sections (Figure 3). In the cervical spine, the posterior epidural space is typically widest at the C7-T1 and C2-C3 levels but remains considerably smaller than the lumbar epidural space and lacks its characteristic segmented appearance.

The epidural space terminates inferiorly at the level of the sacral hiatus (Figure 3).  The hiatus is formed by the developmental absence of lamina and spinous processes at S5 and occasionally S4.  At its inferior margin, the hiatus is flanked by two osseous protuberances known as the sacral cornua, which reflect the inferior articular processes of S5.  These are typically palpable on physical examination, and can serve as a guide to needle placement during caudal epidural injections. The sacral hiatus leads into the sacral canal (the most inferior portion of the intervertebral canal), which contains epidural fat and veins, the termination of the dural sac, the cauda equina and filum terminale (Figure 4).  The dural sac generally terminates between S1 and S2.

Pertinent Ligaments

The ligamentum flavum (yellow ligament) is a firm but elastic ligament that binds the lamina of adjacent vertebra and helps to define the posterior boundary of the epidural space (Figure 5).  The ligaments join the anteroinferior margin of the lamina above to the posterosuperior margin of the lamina below and extend laterally to cover portions of the facet (zygapophysial) joint capsules.  In a similar fashion neighboring spinous processes are bound by the interspinous and supraspinous ligaments (Figure 5, 6).  The interspinous ligaments are physically weaker and lie between each spinous process.  The tougher and more superficial supraspinous ligament runs along the posterior tip of each spinous process.

Rationale and Clinical Indications

Diagnostic Information

ESIs have little diagnostic utility and are not typically used to elucidate a cause for a patient's back pain.  With other spine injections, such as facet and selective nerve blocks, administration of a local anesthetic like bupivicaine may help to localize the source of pain by providing symptomatic relief at a specific injection site.  With nonselective epidural injections, however, relief of pain is not sufficient to establish the exact cause of symptoms, particularly in those patients who have multiple co-existing reasons for back pain.  Therefore the use of a local anesthetic in conjunction with steroid adds little value to ESIs from a diagnostic point of view.  

The immediate pain reduction achieved by using a local anesthetic, however, helps to create a more positive patient attitude toward the procedure and may result in a better clinical outcome. This expectation that the injection will work takes advantage of the placebo effect.  A further advantage of using a local anesthetic is that the patient's immediate response after injection can help distinguish between correct needle placement in the epidural space versus inadvertent dural puncture.  For example, proper epidural injection can result in localized pain reduction and mild lower extremity numbness, while intrathecal injection often produces "saddle anesthesia" and significant bilateral lower extremity weakness. 

Therapeutic Effect

The therapeutic efficacy of non-selective epidural injections has been examined in numerous studies over the past several decades.  However, only six prospective randomized control trials have been undertaken since the early 1970s.  The majority of these studies demonstrate statistically significant improvement in symptoms of radicular pain compared to placebo in the short term.  Two of these studies which failed to demonstrate any significant differences in outcome have been criticized for their short follow-up periods (e.g. 24 to 48 hours), as symptomatic relief related to ESIs can be more apparent several days after injection.  Any long-term benefit of ESIs remains unproven in these trials, and it is likely that the chances of a positive long-term outcome depend more on the nature of the pain generating lesion and its potential to resolve spontaneously than on any effect of epidural steroid treatment.

Thus the primary goal of a nonselective epidural injection (ESI) is to achieve at least short-term symptomatic relief from pain originating from the anterior and/or neuroaxial compartments.  Patients referred for an ESI may present with radicular or localized back pain from various causes such as spinal stenosis, disc bulges/herniations or spondylolisthesis. These conditions may produce pain from nerve root compression and associated intraneural edema and inflammation.  Direct nerve impingement, however, need not be present to produce radicular symptoms.  With some disc herniations, for example, chemical mediators such as phospholipase A2 escape from the nucleus pulposus and likely play an equally, if not more important role in pain pathogenesis.  In either case ESIs are believed to utilize the potent anti-inflammatory properties of long-acting steroids to reduce inflammation and pain associated with nerve root irritation.  Other theories have described the lysis of adhesions or alterations in the spatial relationship between the disc and nerve root as mechanisms accounting for the therapeutic benefit of ESIs.  Still others have looked at the role of local anesthetics like bupivicaine in breaking the "pain cycle" (Wall and Melzac) and the potential effects of local anesthetics as anti-inflammatory agents.

Whatever the mode of action, the pain relief afforded by steroid/anesthetic injections may allow patients to delay surgery and initiate a structured regimen of physical therapy.  Indeed, the combination of physical rehabilitation, pain medications and epidural injections may be sufficiently therapeutic over time to allow patients to avoid surgery altogether.  Repeat injections may also be performed as needed, particularly if previous ESIs have been shown to be effective.  These are typically limited to three or four per year.

In selecting patients who may benefit from ESIs it is important to note that such injections have been shown to be more effective for treatment of radicular rather than axial pain and when symptoms have been present for less than 3 months duration.  In addition ESIs appear to be more effective at relieving "positive" rather than "negative" symptoms.  Positive symptoms include pain and paresthesias (tingling, pins and needles), which reflect nerve root irritation and excessive conduction activity.  They may result from mechanical effects causing nerve root pressure and stretching or local inflammation.  Negative symptoms, on the other hand, include numbness and weakness and usually reflect nerve root damage and conduction failure with decreased sensory and/or motor function.  They result from severe mechanical effects of nerve root compression.

Clinical Scenario

Patient Symptoms

Patients undergoing epidural steroid injections may have variable clinical symptoms including lower back pain (lumbago), radiculopathy or neurogenic claudication.

Imaging Findings

MRI findings are highly variable and may include central canal, lateral recess or neural foraminal stenosis, disc bulge, protrusion or extrusion, degenerative changes of the disc with or without associated annular tear, and spondylolisthesis.  In some patients referred for ESI, MR imaging may provide no clear explanation for clinical symptoms.

Contraindications

Absolute Contraindications

• Poorly controlled bleeding diathesis or anticoagulation
• Local cellulitis or pilonidal cyst near anticipated needle entry site
• Systemic infection (bacteremia)
• Uncontrolled diabetes mellitus
• Progressive neurologic disorder
 

Relative Contraindications

• Prior laminectomy at proposed level of injection
  - causes effacement or obliteration of posterior epidural fat
  - post-operative MR images must be reviewed or an alternative level/approach selected
• Allergy to contrast media
  - pretreat with corticosteroid and antihistamines.
• Allergy to local anesthetic
  - consider alternate class of anesthetic to which the patient is not allergic

Consent Process

Potential Complications and Adverse Reactions

• Bleeding (epidural hematoma)
• Infection (e.g. cellulitis, epidural abscess)
• Contrast reaction
• Temporary leg weakness
• Temporary increase in pain
• Intrathecal needle placement
• Intrathecal steroid/anesthetic injection
  - self-limited saddle anesthesia and lower extremity weakness
  - spinal headache
• "Steroid flush"
  - facial flushing
  - related to systemic absorption of intraarticular steroid
• Hyperglycemia in diabetics
• HPA axis suppression
  - typically self-limited

Documents

• Patient information sheet
• Consent form