Overview

Many options to deliver anesthesia have developed over the last several decades. Administration of topical anesthetics to control pain associated with procedures such as laceration repair may avoid the need for infiltrative local anesthesia injections and associated pain from the injections. Topical anesthesia also avoids the risk of wound margin distortion that exists with infiltrative injection administration. Many dosage forms exist (eg, gels, sprays, creams, ointments, patches) and provide the clinician with precise options for application under various circumstances.

Mechanism of action

Topical anesthetics reversibly block nerve conduction near their site of administration, thereby producing temporary loss of sensation in a limited area. Nerve impulse conduction is blocked by decreasing nerve cell membrane permeability to sodium ions, possibly by competing with calcium-binding sites that control sodium permeability. This change in permeability results in decreased depolarization and an increased excitability threshold that, ultimately, prevents the nerve action potential from forming.

Absorption

Skin absorption is highly variable. Most anesthetic agents exist as solids and are only superficially absorbed through intact skin. Eutectic mixtures result in liquids that melt at lower temperatures than their single components. This permits higher concentrations of anesthetics, which results in superior dermal anesthesia for intact skin. Other methods of increasing skin penetration include liposomal preparations, iontophoresis, and transdermal patches.

A recent double-blind, paired study of 82 adult volunteers compared a lidocaine/tetracaine transdermal patch (Synera) with lidocaine/prilocaine cream prior to vascular access at the antecubital site. The lidocaine/tetracaine patch provided effective anesthesia in as little as 10 minutes and results showed superior anesthesia at all application times less than 60 minute

Indications

Topical anesthetics are used for various skin and mucous membrane conditions, including (but not limited to) pruritus and pain due to minor burns, skin eruptions (eg, varicella, sunburn, poison ivy, insect bites), and local analgesia on intact skin. With the exception of lidocaine-prilocaine as a eutectic mixture (EMLA), topical anesthetics are poorly absorbed through intact skin. Because of variation in systemic absorption and toxicity, the ideal choice of topical anesthetic and particular concentration depends on the intended use. EMLA has also been applied to children to minimize discomfort prior to injections or to starting an intravenous line.

Dosage guidelines and administrative techniques

  • Apply the cream, ointment, or solution to the chosen area in incremental amounts.
  • The total dose for a topical anesthetic is smaller than that used for subcutaneous infiltration.
  • EMLA cream does not require an occlusive dressing (eg, DuoDERM, Tegaderm, Saran Wrap); however, when an occlusive dressing is applied, absorption is improved and time of onset is decreased.
  • Viscous lidocaine may be used alone or in a compounded mixture as a mouthwash (ie, swish and expectorate).
  • Iontophoresis has been used to improve the effects of topical anesthesia; however, the equipment is expensive and bulky, and some patients experience discomfort from the mild electrical sensation.
  • Sequential layered application of topical lidocaine with epinephrine (TLE) has been described for small facial or scalp wounds. One-hundred patients were enrolled in a randomized controlled trial, with 50 in each group. The study group received TLE using a unique method of "sequential layered application." The control group received 2% lidocaine infiltration anesthesia. Patients rated the pain from the application of anesthesia and from suturing, using a zero-to-ten visual analog pain scale.
  • Follow-up interviews were conducted to assess for complications and to rate patients' wound repair experience. Patients in the study group reported significantly less pain from TLE application, with 66% reporting no pain vs 0% reporting no pain from the infiltration in the control group (P <0.001). No difference in pain during wound repair was noted between the 2 groups (P ~0.59). On follow-up, 95% of patients contacted in the TLE group rated their experience in regard to pain as "excellent," compared to 5% of patients in the control group (P <0.001).

Deciphering drug concentration

  • Concentrations: Drug concentration is expressed as a percentage (eg, dibucaine 1%, benzocaine 0.5%).
    • Percentage is measured in grams per 100 mL (ie, 1% is 1 g/100 mL [1000 mg/100 mL] or 10 mg/mL)
    • Calculate the mg/mL concentration quickly from the percentage by moving the decimal point 1 place to the right, as follows:
      • Tetracaine 0.5% = 5 mg/mL
      • Viscous lidocaine 2% = 20 mg/mL
      • Benzocaine 20% = 200 mg/mL

Adverse effects

Adverse effects are usually caused by high plasma concentrations of topical anesthetics that typically result from excessive exposure caused by application to abraded or torn skin. Possible adverse effects include the following:

  • Burning or stinging may occur local to the administration site.
  • Oral viscous lidocaine may cause systemic toxicity, particularly with repeated use in infants or children.
  • CNS: High plasma concentration initially produces CNS stimulation (including seizures), followed by CNS depression (including respiratory arrest). The CNS stimulatory effect may be absent in some patients, particularly when amides (eg, tetracaine) are administered. Solutions that contain epinephrine may add to the CNS stimulatory effect.
  • Cardiovascular: High plasma levels typically depress the heart and may result in bradycardia, arrhythmias, hypotension, cardiovascular collapse, and cardiac arrest. Local anesthetics that contain epinephrine may cause hypertension, tachycardia, and angina.
  • Gag-reflex suppression may occur with oral administration.
  • Other body systems can also experience adverse effects.
    • Transient burning sensation
    • Skin discoloration
    • Swelling
    • Neuritis
    • Tissue necrosis and sloughing
    • Methemoglobinemia (with prilocaine)

The United States Food and Drug Administration (FDA) has issued an advisory regarding risk of serious adverse effects with use of topical anesthetics for cosmetic procedures. Life-threatening adverse effects occurred following topical anesthetic application over large surface areas of the body; for some, a plastic occlusive was also applied to enhance absorption. Two women experienced seizures, coma, and death following applying topical anesthetics to their legs with an occlusive dressing before laser hair removal.

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