Nerve blocks in the diagnosis of low back pain.

The use of diagnostic injection techniques in the evaluation of low back pain disorders is a clinical judgment skill, based on applied anatomy, pharmacology of the agents employed, and the neurophysiology of pain. In earlier years, the focus was primarily on the anatomic basis of pain, with sometimes oversimplistic interpretations of the results obtained. Although an anatomic correlation of a subjective complaint remains the same diagnostic objective today, a clearer understanding of pain concepts has also given these injection techniques a greater sensitivity but narrower spectrum in their application. Local anesthetic injections can readily identify sources of pain in soft tissues, scar tissue, nerve injury, and ligaments. They have particular application in this regard in postsurgical and postinjury back pain states in which the normal anatomy or function may be distorted. Local anesthetic blocks can also corroborate or define sites of pain in dorsal facet joints and in torsional disc capsule tears, where sites of multiple imaging or negative imaging findings make for difficult clinical diagnosis. In this context, they can also allow additional injection of deposteroid preparations, extending the procedure to a longer term therapeutic measure. Less specific anatomic information can be developed with respect to contributions from afferent and efferent responses, and in peripheral as compared with central mechanisms subserving the total clinical pain. For disorders of neuropathic and sensitization pain states, the spinal segmental level of signal processing from which conscious projection is derived can be well defined. Planned use of diagnostic blocking techniques is not a shortcut to sophisticated imaging or neurophysiologic studies but readily complements such investigations when conclusions are equivocal. Diagnostic blocks are only as good as the detailed clinical examinations which precede and follow the block itself.