Diagnostic lumbosacral segmental nerve blocks with local anesthetics: a prospective double-blind study on the variability and interpretation of segmental effects.

BACKGROUND AND OBJECTIVES: Selective spinal nerve infiltration blocks are used diagnostically in patients with chronic low back pain radiating into the leg. Generally, a segmental nerve block is considered successful if the pain is reduced substantially. Hypesthesia and elicited paresthesias coinciding with the presumed segmental level are used as controls. The interpretation depends on a standard dermatomal map. However, it is not clear if this interpretation is reliable enough, because standard dermatomal maps do not show the overlap of neighboring dermatomes. The goal of the present study is to establish if dissimilarities exist between areas of hypesthesia, spontaneous pain reported by the patient, pain reduction by local anesthetics, and paresthesias elicited by sensory electrostimulation. A secondary goal is to determine to what extent the interpretation is improved when the overlaps of neighboring dermatomes are taken into account.
METHODS: Patients suffering from chronic low back pain with pain radiating into the leg underwent lumbosacral segmental nerve root blocks at subsequent levels on separate days. Lidocaine (2%, 0.5 mL) mixed with radiopaque fluid (0.25 mL) was injected after verifying the target location using sensory and motor electrostimulation. Sensory changes (pinprick method), paresthesias (reported by the patient), and pain reduction (Numeric Rating Scale) were reported. Hypesthesia and paresthesias were registered in a standard dermatomal map and in an adapted map which included overlap of neighboring dermatomes. The relationships between spinal level of injection, extent of hypesthesia, location of paresthesias, and corresponding dermatome were assessed quantitatively. Comparison of the results between both dermatomal maps was done by paired t-tests.
RESULTS: After inclusion, data were processed for 40 segmental nerve blocks (L2-S1) performed in 29 patients. Pain reduction was achieved in 43%. Hypesthetic areas showed a large variability in size and location, and also in comparison to paresthesias. Mean hypesthetic area amounted 2.7 +/- 1.4 (+/- SD: range, 0 to 6; standard map) and 3.6 +/- 1.8 (0 to 6; adapted map; P <.001) dermatomes. In these cases, hypesthesia in the corresponding dermatome was found in 80% (standard map) and 88% of the cases (adapted map, not significant). Paresthesias occurring in the corresponding dermatome were found in 80% (standard map) compared with 98% (adapted map, P <.001). In 85% (standard map) and 88% (adapted map), spontaneous pain was present in the dermatome corresponding to the level of local anesthetic injection. In 55% (standard map) versus 75% (adapted map, P <.005), a combination of spontaneous pain, hypesthesia, and paresthesias was found in the corresponding dermatome.
CONCLUSIONS: Hypesthetic areas determined after lumbosacral segmental nerve blocks show a large variability in size and location compared with elicited paresthesias. Confirmation of an adequately performed segmental nerve block, determined by coexistence of hypesthesia, elicited paresthesias and pain in the presumed dermatome, is more reliable when the overlap of neighboring dermatomes is taken into account.