Quantitative and Selective Evaluation of Differential Sensory Nerve Block After Transdermal Lidocaine

We evaluated the effect of transdermal lidocaine on differential sensory nerve block in 15 healthy volunteers. Lidocaine 10% gel was applied topically to a forearm and covered with a plastic film. Three types of sensory nerve fibers (A&bgr;, A&dgr;, and C fibers) were evaluated with a series of 2000-, 250-, and 5-Hz stimuli using current perception threshold (CPT) testing. Sensations of touch, pinprick, cold, and warmth were also measured. These measurements were made before the topical lidocaine (baseline), 60 min after the draping (T0), and at 1-h intervals until 5 h after T0 (T1 to T5). A significant increase in CPT compared with baseline was observed until T2 at 5 Hz and T4 at 250 Hz, whereas the increase in CPT at 2000 Hz continued throughout the study period. All subjects experienced the disappearance of pinprick and cold sensations, whereas touch and warmth sensations were detectable during the study period. We conclude that when lidocaine is applied transdermally, the sensitivity of nerves to local anesthetics is proportional to the axon diameters. However, pinprick and cold sensation are affected more strongly than other sensations at receptor sites.