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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.
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