Lumbar Nerve Root Blocks: Technique Part A

Technique

Early techniques of lumbar nerve root blocks required that the needle be directed at the target nerve, usually just outside the intervertebral foramen; the objective was to reproduce the patient's pain by striking the nerve (1, 3, 6, 7, 9, 10). Concordance between the evoked pain and the patient's accustomed pain was taken as the cardinal indication that the 'impaled' nerve was responsible for the patient's pain. In contemporary practice this aspect is regarded as inappropriate and unnecessary.

Modern techniques avoid deliberately striking the nerve. This renders the procedure more tolerable for the patient and reduces the potential risk of nerve damage. Instead, the diagnostic decision is based on whether or not blocking the target nerve reduces the patient's symptoms. Observations are made opportunistically as to if and when the patient's pain is reproduced, but this is not an essential component of the diagnostic process.

Lumbar Nerve Root Blocks

The most practicable approach for nerve root blocks at lumbar levels involves the target point at the base of the pedicle immediately above the target nerve. Radiographically, the target point lies infero-lateral to the pedicle, i.e. at the 5:30 position on the right and at the 6:30 on the left, using an analogy with a clock-face (Fig. 2). This target point lies at the medial apex of what can be portrayed as a 'safe triangle' (Fig. 1). The triangle has a base tangential to the pedicle, a side in line with the outer margin of the intervertebral foramen and a hypotenuse coincident with the upper margin of the spinal nerve and dorsal root ganglion. A needle tip directed into this triangle will therefore lie above and lateral to the nerve and will not incur any other structure of significance to risk of morbidity.

Figure 1.

Fig. 1 Right lumbar spinal nerve viewed from the rear with the dural sleeve opened, showing the relationship of the spinal, its roots and its ventral ramus to the pedicle. The 'safe triangle' is the region where a needle may be introduced without striking the neural elements or entering the dural sleeve.

Figure 2.

Fig. 2 A postero-anterior view of lumbar pedicles showing the target points (arrowed) for selective nerve root blocks.