Diabetes mellitus increases current requirements for popliteal nerve response during nerve stimulator-aided peripheral nerve blockade

Introduction: During the administration of peripheral nerve blocks using nerve stimulators, the intensity of the stimulating current at which responses to the nerve stimulation are observed provides an indication of the distance of the needle to the nerve.' Since development of peripheral neuropathy (especially that of the lower extremities) is frequent in patients with diabetes mellitus (DM), we studied current requirements for nerve stimulation in patients with DM undergoing popliteal sciatic nerve block. Methods: The lowest current at which a motor response could be detected or observed during popliteal sciatic nerve stimulation was recorded in 9 patients with DM and in 41 patients without diabetes (controls) undergoing lower extremity surgery. Nerve stimulation was achieved using a constant current nerve stimulator (DigiStim, B. Braun Medical, Inc., Bethlehem, PA), with the anode connected to the lateral calf, and the cathode connected to a 50 mm long insulated needle Stimuplex(r), B. Braun Medical, Inc., Bethlehem, PA. Upon obtaining an initial response (dorsal flexion or plantar flexion of the foot), the current was progressively decreased to the lowest intensity at which the motor response was still observed. Then, 40 ml of mixture of 1.5% mepivacaine with epinephrine (1:200,000) was injected. The time of onset of the block and its density were determined using a pinprick method and ability to wiggle the toes. Results: A total of 50 patients (50 ±15 yrs, 27 men and 23 women) were included in this analysis. The DM patients tended to require higher current intensity for nerve stimulation than the controls (0.36 ±0.1 mA vs 0.55 ±0.3 mA, p=0.09). Although this difference did not reach statistical significance, when the current was dichotomized at a frequently used cut-off point (< 0.4 m A vs > 0.4 mA)2, seven (78%) of DM patients required currents in the higher range, while only nine (22%) of control patients required currents of this magnitude (p= 0.003). While DM patients were heavier than the controls (85.9 ±14 kg vs 70.7 ±15 kg; p= 0.008), a regression analysis that included both the diabetic status and weight identified only diabetic status as the independent predictor for higher current requirement. Despite these differences in the current requirements, the speed of onset was comparable in both groups. All blocks resulted in excellent surgical anesthesia except one in the DM group, in which the nerve stimulation could not be achieved with currents lower than 1.0 mA. While this patient required IV supplementation during surgery, he had complete postoperative analgesia lasting 11 hours. Discussion; These data suggest that higher stimulating current may be required for localization of the popliteal nerve in DM patients. This effect is possibly due to the neuropathy frequently associated with diabetes mellitus. While stimulation with the lowest current intensity achievable is routinely attempted prior to injecting local anesthetics, these data further suggest that, in this patient population a slightly higher current can be accepted as a reliable sign of intimate needle-nerve relationship.