TOPICAL ANESTHETIC
A number of local anesthetic formulations are available for topical anesthesia , lidocaine, dibucaine, tetracaine, and benzocaine being the drugs used most commonly. In general, these preparations provide effective but relatively short durations of analgesia when applied to mucous membranes or abraded skin. Their efficacy is determined by drug form, melting point, concentration, and skin permeability. Lidocaine and tetracaine sprays are commonly used for endotracheal anesthesia before intubation or for mucosal analgesia for bronchoscopy or esophagoscopy.
A variety of topical local anesthetic formulations have been developed to penetrate intact skin. EMLA, which is a eutectic mixture of 2.5% lidocaine base and 2.5% prilocaine base, is widely used for venipuncture, intravenous cannulation, skin grafting, and a range of other uses, including circumcision. This preparation must be applied under an occlusive bandage for 45 to 60 minutes to obtain effective cutaneous anesthesia; longer application times increase the depth and reliability of skin analgesia. EMLA appears to be quite safe in neonates, and methemoglobinemia with the use of prilocaine is exceedingly uncommon. EMLA is more effective for newborn circumcision than placebo is but less effective than dorsal penile nerve block. Several alternative topical local anesthetic formulations also are in use, including tetracaine gel and liposomal lidocaine.
Physical methods to accelerate local anesthetic transit across skin, including iontophoresis, local heating, electroporation, and a variety of forms of needleless pressure injection, may lead to more rapid onset of cutaneous analgesia. Synera (originally studied as S-Caine) is a formulation of lidocaine and tetracaine that was developed with a heating element (activated by opening the package to initiate an oxygen-dependent exothermic reaction). This formulation has a rapid onset and evokes vasodilatation.
Topical anesthesia through cut skin is commonly used in pediatric emergency departments for liquid application into lacerations that require suturing. Historically, this had been provided by a mixture of tetracaine, epinephrine (adrenaline), and cocaine, known as TAC. TAC is ineffective through intact skin; in contrast, its rapid absorption from mucosal surfaces can lead to toxic, even fatal reactions. Another potential substance is ELA-max, a liposomal formulation of lidocaine which is useful for cuts or abrasions. Lastly, Lidoderm patches have been in use for the topical treatment of postherpetic neuralgia.
Because of concerns regarding cocaine toxicity and the potential for diversion and abuse, cocaine-free topical preparations are strongly recommended, and alternatives such as the combination of an α1-adrenergic agonist (oxymetazoline or phenylephrine) and a local anesthetic such as 2% to 4% lidocaine should be used, with more dilute solutions being recommended for infants and children.
