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 u sed diagnostically in patients with chronic low back pain radiating into th e leg. Generally, a segmental nerve block is considered successful if the p ain is reduced substantially. Hypesthesia and elicited paresthesias coincid ing with the presumed segmental level are used as controls. The interpretat ion 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 i s to establish if dissimilarities exist between areas of hypesthesia, spont aneous pain reported by the patient, pain reduction by local anesthetics, a nd paresthesias elicited by sensory electrostimulation. A secondary goal is to determine to what extent the interpretation is improved when the overla ps 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 subsequen t levels on separate days. Lidocaine (2%, 0.5 mL) mixed with radiopaque flu id (0.25 mt) was injected after verifying the target location using sensory and motor electrostimulation. Sensory changes (pinprick method), paresthes ias (reported by the patient), and pain reduction (Numeric Rating Scale) we re reported. Hypesthesia and paresthesias were registered in a standard der matomal map and in an adapted map which included overlap of neighboring der matomes. The relationships between spinal level of injection, extent of hyp esthesia, location of paresthesias, and corresponding dermatome were assess ed quantitatively. Comparison of the results between both dermatomal maps w as done by paired I-tests. Results: After inclusion, data were processed for 40 segmental nerve blocks (L2-S1) performed in 29 patients. Pain reduction was achieved in 43%. Hype sthetic 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 dermato me was found in 80% (standard map) and 88% of the cases (adapted map, not s ignificant). Paresthesias occurring in the corresponding dermatome were fou nd 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 d ermatome corresponding to the level of local anesthetic injection. Ln 55% ( standard map) versus 75% (adapted map, P < .005), a combination of spontane ous pain, hypesthesia, and paresthesias was found in the corresponding derm atome. Conclusions: Hypesthetic areas determined after lumbosacral segmental nerve blocks show a large variability in size and location compared with elicite d paresthesias. Confirmation of an adequately performed segmental nerve blo ck, determined by coexistence of hypesthesia, elicited paresthesias and pai n in the presumed dermatome, is more reliable when the overlap of neighbori ng dermatomes is taken into account.