Selective Epidural Injection: Section 3

If a selective epidural injection is being done for radicular pain the fact that these structures are also anesthetized is irrelevant, as they are sources of axial pain, not radicular pain. However, if an injection is being done for axial pain, it is important to realize that pain relief after an injection can occur with pathology in any of he structures innervated by the nerve.

A selective epidural injection can potentially be helpful in diagnosing axial pain.

Derby, et al. Precision percutaneous blocking procedures for localizing spinal pain. Part 2: The lumbar neuraxial compartment. Pain Digest 1993;3:175-88.

Any lesion that affects a nerve root also necessarily affects it’s dural sleeve, and therefore is a potential cause of axial pain. In the case of HNP, disc material in the epidural space has been demonstrated to elicit an inflammatory response not only in the nerve root but also the dura. Relief of both axial and radicular pain with a selective epidural injection suggests that the same lesion is responsible for both. However, one cannot be certain that the axial component of the pain is arising from structures above and/or below the injected level. In patients with predominantly axial pain, fully characterizing the source of pain may require synovial joint and or disc injections, depending on the clinical situation.

A selective epidural injection has two components. The first is the provocative pain response resulting from contrast injection, and the second is the analgesic response resulting from injection of local anesthetic and/or corticosteriod.

In evaluating provoked pain it is important to compare the location and character of the provoked response to the patients typical symptoms. Furthermore, the onset of provoked pain should be related to where the leading edge of the contrast solution is when pain begins.

Derby, et al. Precision percutaneous blocking procedures for localizing spinal pain. Part 2: The lumbar neuraxial compartment. Pain Digest 1993;3:175-88.

Normal epidural tissue is not painful when gently stimulated by contrast solution. In the absence of scar tissue, pain provocation indicates that the tissue stimulated is irritated. For example, early pain provocation of pain, when contrast is still in the foramen, suggests foraminal stenosis or a foraminal disc herniation. Late pain provocation when the contrast approaches the disc above is consistent with a paramedian disc herniation.

Immediately after the injection the effect of the local anesthetic injected on the patients symptoms should be assessed, both at rest and in response to mechanical stimulation. Studies on selective nerve root injections have used the criterion for a positive analgesic response to be from 80% (1) to 100% (2) relief.

(1) Derby, et al. Precision percutaneous blocking procedures for localizing spinal pain. Part 2: The lumbar neuraxial compartment. Pain Digest 1993;3:175-88.

(2) Dooley, et al. Nerve root infiltration in the diagnosis of radicular pain. Spine 1988;13(1): 79-83.

The significance of lesser degrees of pain relief in response to an injection is uncertain. If corticosteroid is included in the injection the patient should be re-evaluated one week afterwards, as the degree of pain relief at that interval can provide important information.