Assessment of the efficacy of topical anesthetics using the tactile spatial resolution method

The aim of this study was to compare the purported advantages of 4% tetracaine gel (Ametop gel) and 4% liposomal lidocaine gel (LMX4 gel) with EMLA cream (eutectic mixture of 2.5% lidocaine and 2.5% prilocaine) using an objective and repeatable method. Ametop gel and LMX4 gel were administered under occlusion for 30 min and compared to EMLA cream applied for 30 and 60 min on the intact upper lip skin of 15 volunteers each. The efficacy of the anesthetics was assessed by the spatial resolution method. Measurements were conducted just after removal of the products from the skin, then 20, 40 and 60 min later. Each of the formulations, except for EMLA cream applied for 30 min, decreased tactile spatial discrimination thresholds significantly just after removal from the skin when compared to the output levels (p<0.05). Ametop gel kept significantly good skin anesthesia also 20, 40 and 60 min later (p<0.05). The efficacy of LMX4 gel and EMLA(60) cream decreased to the initial levels after 40-min application. Ametop gel anesthetized the skin in a highly homogeneous manner providing similar effect in most subjects, which was not the case in the EMLA and LMX4 groups. In conclusion, LMX4 gel and Ametop gel appeared to be faster acting than EMLA cream. Our results showed the 30-min application of LMX4 and Ametop gel under occlusion to be equivalent to 60-min administration of EMLA cream. Ametop gel, in contrast to the rest, provides very good anesthesia for up to 60 min. The application of EMLA cream under occlusion over only 30 min cannot guarantee appropriate effects.