Topical skin anesthesia

During the last three decades, a variety of transdermal anesthetic preparations have been developed; however, these have been documented almost invariably to be ineffective due to insufficient concentrations of the uncharged base (the active form) of the anesthetics or to poor skin absorption secondary to the lipophilic characteristics of amide derivatives (lidocaine, benzocaine, etc.). In trying to increase the anesthetic base concentrations, organic solvents have been used, but unacceptable local irritation prevented their clinical application. Because of their poor penetration through intact skin, these solutions can be used only to anesthetize mucous membranes. Production of oil-in-water emulsion of single anesthetic agents may help in achieving relatively high concentrations of the active form with better skin penetration due to the presence of water. These preparations, however, do not produce effective skin analgesia even with high concentrations attainable.