Local Anesthetics in Cosmetic Dermatology

Local anesthesia is a central component of successful interventions in cosmetic dermatology. The number of anesthetic medications and administration techniques has grown in recent years as outpatient cosmetic procedures continue to expand. Pain is a common barrier to cosmetic procedures, and alleviating the fear of painful interventions is critical to patient satisfaction and future visits. To accommodate a multitude of cosmetic interventions, it is important for clinicians to be well versed in applications of topical and regional anesthesia. In this article, we review pain management strategies for use in cosmetic practice.

Local Anesthetics

The sensation of pain is carried to the central nervous system by unmyelinated C nerve fibers. Local anesthetics (LAs) act by blocking fast voltage-gated sodium channels in the cell membrane of the nerve, thereby inhibiting downstream propagation of an action potential and the transmission of painful stimuli.¹ The chemical structure of LAs is fundamental to their mechanism of action and metabolism. Local anesthetics contain a lipophilic aromatic group, an intermediate chain, and a hydrophilic amine group. Broadly, agents are classified as amides or esters depending on the chemical group attached to the intermediate chain.² Amides (eg, lidocaine, bupivacaine, articaine, mepivacaine, prilocaine, levobupivacaine) are metabolized by the hepatic system; esters (eg, procaine, proparacaine, benzocaine, chlorprocaine, tetracaine, cocaine) are metabolized by plasma cholinesterase, which produces para-aminobenzoic acid, a potentially dangerous metabolite that has been implicated in allergic reactions.³

Lidocaine is the most prevalent LA used in dermatology practices. Importantly, lidocaine is a class IB antiarrhythmic agent used in cardiology to treat ventricular arrhythmias.⁴ As an anesthetic, a maximum dose of 4.5 mg/kg can be administered, increasing to 7.0 mg/kg when mixed with epinephrine; with higher doses, there is a risk for central nervous system and cardiovascular toxicity.⁵ Initial symptoms of lidocaine toxicity include dizziness, tinnitus, circumoral paresthesia, blurred vision, and a metallic taste in the mouth.⁶ Systemic absorption of topical anesthetics is heightened across mucosal membranes, and care should be taken when applying over large surface areas.

Allergic reactions to LAs may be local or less frequently systemic. It is important to note that LAs tend to show cross-reactivity within their class rather than across different classes.⁷ Reactions can be classified as type I or type IV. Type I (IgE-mediated) reactions evolve in minutes to hours, affecting the skin and possibly leading to respiratory and circulatory collapse. Delayed reactions to LAs have increased in recent years, with type IV contact allergy most frequently found in connection with benzocaine and lidocaine.⁸

Topical Anesthesia

Topical anesthetics are effective and easy to use and are particularly valuable in patients with needle phobia. In certain cases, these medications may be applied by the patient prior to arrival, thereby reducing visit time. Topical agents act on nerve fibers running through the dermis; therefore, efficacy is dependent on successful penetration through the stratum corneum and viable epidermis. To enhance absorption, agents may be applied under an occlusive dressing.

Topical anesthetics are most commonly used for injectable fillers, ablative and nonablative laser resurfacing, laser hair removal, and tattoo removal. The eutectic mixture of 2.5% lidocaine and 2.5% prilocaine as well as topical 4% or 5% lidocaine are the most commonly used US Food and Drug Administration–approved products for topical anesthesia. In addition, several compounded pharmacy products are available.

After 60 minutes of application of the eutectic mixture of 2.5% lidocaine and 2.5% prilocaine, a 3-mm depth of analgesia is reached, and after 120 minutes, a 4.5-mm depth is reached.⁹ It elicits a biphasic vascular response of vasoconstriction and blanching followed by vasodilation and erythema.¹⁰ Most adverse events are mild and transient, but allergic contact dermatitis and contact urticaria have been reported.^11-13 In older children and adults, the maximum application area is 200 cm², with a maximum dose of 20 g used for no longer than 4 hours.

The 4% or 5% lidocaine cream uses a liposomal delivery system, which is designed to improve cutaneous penetration and has been shown to provide longer durations of anesthesia than nonliposomal lidocaine preparations.¹⁴ Application should be performed 30 to 60 minutes prior to a procedure. In a study comparing the eutectic mixture of 2.5% lidocaine and 2.5% prilocaine versus lidocaine cream 5% for pain control during laser hair removal with a 1064-nm Nd:YAG laser, no significant differences were found.¹⁵ The maximum application area is 100 cm² in children weighing less than 20 kg. A study of healthy adults demonstrated safety with the use of 30 to 60 g of occluded liposomal lidocaine cream 4%.¹⁶

In addition to US Food and Drug Administration–approved products, several compounded pharmacy products are available for topical anesthesia. These formulations include benzocaine-lidocaine-tetracaine gel, tetracaine-adrenaline-cocaine solution, and lidocaine-epinephrine-tetracaine solution. A triple-anesthetic gel, benzocaine-lidocaine-tetracaine is widely used in cosmetic practice. The product has been shown to provide adequate anesthesia for laser resurfacing after 20 minutes without occlusion.¹⁷ Of note, compounded anesthetics lack standardization, and different pharmacies may follow their own individual protocols.

Regional Anesthesia

Regional nerve blockade is a useful option for more widespread or complex interventions. Using regional nerve blockade, effective analgesia can be delivered to a target area while avoiding the toxicity and pain associated with numerous anesthetic infiltrations. In addition, there is no distortion of the tissue architecture, allowing for improved visual evaluation during the procedure. Recently, hyaluronic acid fillers have been compounded with lidocaine as a means of reducing procedural pain.