Lumbar Nerve Root Blocks: Part C -Technique

Technique: Lumbar Nerve Root Blocks Continued ...

Failure of the contrast medium to spread centrally may indicate epidural pathology-adhesions, scar, disc herniation or stenosis, but inferences as to the nature of the pathology cannot be drawn reliably on the basis of the epidurogram (10). Good-quality imaging studies will better define any pathology. If required, post-injection CT scanning can be particularly illuminating in this regard.

During the injection of contrast medium the opportunity is taken to record any pain response offered by the patient. Notes should be taken of the location and appearance of the leading edge of the spreading contrast medium. Pain reproduction can be interpreted in the light of other imaging studies to determine whether it is consistent or not with the location and nature of the lesion perceived to be responsible for the patient's symptoms. For example, pain reproduction early in the course of the injection when the contrast medium is still in the intervertebral foramen could be consistent with foraminal stenosis or a far, lateral disc herniation. Late reproduction of pain when the contrast medium approaches the disc above could be consistent with the sequestrated fragment from that level (Fig. 5).

Figures 5a, 5b, 5c.

Figures 5d, 5d.

Fig. 5 (d), (e) Lateral and posterior views following the injection of 1.5 ml of contrast medium. In (e) the contrast medium follows the ventral ramus (vr) laterally and inferiorly. Medially it circumvents the pedicle of L5 and extends upwards towards the L4-5 disc. Injection was terminated immediately when the patient reported reproduction of their accustomed radicular pain which coincided with the advancing front of the contrast medium (arrows) reaching the caudal edge of the ruptured L4-5 disc ((d) and (e); compare with (c)).

Failure to outline the nerve root complex can occur if the injection is intravascular. To reduce the risk of intravenous injection, the patient must be encouraged to breathe normally and not hold their breath. This minimizes the pressure in the epidural venus plexuses and reduces their distension. Distended epidural veins not only increase the risk of vascular injection but also impede the flow of contrast medium into the epidural and periradicular spaces. If inadvertent intravenous injection does occur its appearance is obvious; the contrast medium dissipates rapidly into the vessels and is cleared from the field; it does not persist and outline the nerve root complex. Should intravenous injection occur, the needle should be readjusted slightly and a renewed injection of contrast medium undertaken.

Once an appropriate dispersal of contrast medium has been established, the syringe containing the contrast medium is replaced with one containing the next agent. This can be local anaesthetic alone, for purely diagnostic purposes, or a mixture of local anaesthetic and corticosteroid for combined diagnostic and putatively therapeutic purposes. Up to 2 ml of agent should be injected at the same site and at the same rate at which the contrast medium had been injected.

When local anaesthetic is used as a sole agent the intention is to anaesthetize the nerve root and its surroundings. Technical success is evident by the onset of numbness in the appropriate dermatome. For this purpose, a long-acting local anaesthetic such as 0.5% or 0.75% bupivacaine is recommended. This provides a prolonged period of anesthesia during which the patient can evaluate the effect on their symptoms. The same local anaesthetic may be mixed with a corticosteroid preparation in equal parts. The objective is to obtain a more prolonged response. The local anaesthetic component provides an immediate diagnostic effect while the corticosteroid is intended to provide a more sustained, quasi-therapeutic effect.