Diagnostic Nerve Root Block
Because of the overlap pattern of dermatomal innervation and the anatomic variants of spinal nerves, clinical history and physical examination alone are often not sufficient to accurately diagnose the segmental level of a spinal nerve lesion. In addition, current imaging studies and electrodiagnostic tests have limited sensitivity and specificity in reaching a conclusive diagnosis of radicular pain at a specific spinal level. Therefore, a diagnostic SNRB can be an important test with respect to providing a physiologic diagnosis of the level of radicular pain. By selectively depositing a limited volume of local anesthetic directly around the spinal nerve rather than in the epidural space, pain relief after the SNRB identifies the spinal nerve blocked as the involved level. A diagnostic SNRB is indicated when imaging and/or electrodiagnostic testing studies are not corroborative with the clinical findings, or these tests demonstrate multilevel pathology and the exact pain generators are unclear. Studies reveal that a diagnostic SNRB is 87% to 100% accurate when intraoperative findings are used as the gold standard (77–79). Surgery performed at the positive SNRB level had higher success rate than surgery done at a level with a negative SNRB in the lumbar spine (80). In the cervical spine, SNRB also helped guide with a high level of success the evaluation of radicular pain in the multilevel degenerative cervical spine and subsequent surgery (91).
A diagnostic SNRB is performed with the needle tip directed to the posterior lateral portion of the neuroforamen, as for a transforaminal ESI. However, for a selective spinal nerve block, the needle tip should remain outside the neuroforamen pointing at the 5 o’clock position of the pedicle above for a left-sided SNRB or at 7 o’clock for a right-sided SNRB. The needle tip should also be localized immediately lateral to the superior articular process. Special care should, therefore, be exercised not to impale or transfix the exiting spinal nerve and it is contraindicated to inject steroid or local anesthetics directly into a spinal nerve due to their neurotoxic effects. The patients should be examined before the injection to document the maneuvers and activities that produce radicular pain. The same provocative maneuvers or activities should be repeated for comparison after the diagnostic injection. Selective spinal nerve block as a diagnostic procedure is considered positive when the patient’s radicular symptoms are reproduced upon gentle needle contact with the nerve sheath, followed by relief of the radicular pain after diagnostic blockade with local anesthetic.
Despite the apparent advantages of diagnostic selective spinal nerve block, the test has several limitations. Because of the overlap of the dermatomal distribution, blockade of one segment will not necessary produce clear-cut sensory changes. Second, because the anesthetic blockade is placed at the spinal nerve, a successful block can impede pain transmission not only from the spinal nerve but also from sites distal to the spinal nerve. Furthermore, the specificity of the blockade also depends on the spread of the injectate and the exact location of the needle tip. One study demonstrated that 1 mL of injected contrast in an L4 SNRB spread onto the L5 nerve root in 46.1%, and 1 mL of injected contrast in an L5 SNRB spread onto an S1 nerve root in 57.7% of subjects (92). If the needle tip is placed too close to the neuroforamen, even 0.5 mL of injectate can spread to the adjacent nerve root level through the neuroforamen, thus compromising the specificity of the segmental test (Fig. 68-11). However, the efficacy of blockade with anesthetic less than 0.5 mL is questionable. A study that utilized multi-slice computed tomography (CT) revealed that only 0.6 mL of injectate with contrast during cervical TEIs could be accepted as being selective enough for diagnostic investigations (93). It appears that a volume somewhere between 0.5 and 1 mL of local anesthetic should be used in performing an SNRB. More randomized, controlled studies are needed to determine the optimal volume of injectate for a diagnostic selective spinal nerve block and to ascertain the true value of a diagnostic SNRB in aiding with the selection of appropriate patients for spinal decompression surgery.