Lumbar Nerve Root Blocks: Part B - Technique

Technique: Lumbar Nerve Root Blocks Continued ...

To access this target point a 22-G or a 25-G spinal needle is inserted through the skin and back muscles along an oblique approach. The puncture point is determined by obtaining an oblique view of the target intervertebral foramen such that the apex of the superior articular process of the ipsisegmental zygapophyseal joint points directly upwards towards the target pedicle (Fig. 3a). The needle is passed through the skin just above and lateral to this apex. Under repeated fluoroscopic screening, the needle is advanced slowly towards the base of the pedicle until its further advance is arrested by bony contact (Fig. 3b). At this stage, its tip should be in correct position, which should be confirmed by postero-anterior and lateral views (Fig. 4). If not, the tip should be readjusted until it assumes correct position.

Figures 3a and 3b.

Fig. 3 (a) Oblique view of a right L4-5 intervertebral foramen to illustrate how the superior-articular process points towards me target point (arrowed) for a selective nerve root block. (b) A needle in correct position for a selective lumbar nerve root block.

Once the needle is in correct position, two staged injections are made. The second will be an injection of local anaesthetic or local anaesthetic mixed with corticosteroid. The first will be an injection of contrast medium, the purpose of which is to verify correct placement of the needle but also to determine the volume of injectate that can and should be injected to achieve a block without compromising its selectivity.

One milliliter of contrast medium should be injected slowly under direct visualization to indicate the direction and extent of spread of any solutions that might subsequently be injected. An appropriate pattern of spread is one in which the contrast medium flows along the surface of the nerve root complex outlining the bulge of the dorsal root ganglion and the course of the nerve root sleeve (Fig. 4). Centrally, the contrast medium spreads ventral to the nerve root sleeve curving upwards and medially around the pedicle and extending medially into the epidural space. Peripherally, the contrast medium outlines the course of the ventral ramus to greater or lesser extents.

Figure 4a.

Fig 4a. Stages in the execution of a left L5 selective nerve root block. Posterior, oblique, and lateral views of a needle in correct position prior to the injection of contrast medium.

Figure 4b.

Fig 4b. Posterior, oblique and lateral views following the injection of 1.5 ml of contrast medium.

A sufficient volume should be injected to outline the target nerve but not more. The contrast medium should not be allowed to reach the next spinal nerve lest the selectivity of the block be jeopardized. Usually about 1.0 ml is sufficient to outline the target nerve; by 2.0 nil the contrast medium starts to reach the nerve next above. Not more than 2.0 ml of contrast medium should be injected unless the flow is predominantly in a peripheral direction. In that event it is better to readjust the position of the needle slightly to achieve a predominantly central dispersal.

As a precaution against the injection of excessive volumes of contrast medium or subsequent solutions, only 2-ml or 3-ml syringes should be used. All injections should be performed slowly at the rate of about 1.0 ml per 20 seconds. The patient should be warned to expect pain during the injection of contrast medium and should be asked to report whether or not the evoked pain is concordant in quality and distribution with the pain they usually suffer.