Excerpt from Anesthesia, Topical
Synonyms, Key Words, and Related Terms: topical anesthesia, anesthetics, local anesthetic, drug concentration, drug dilution, epinephrine, onset of action, duration of action, local absorption, topical absorption, lidocaine, viscous lidocaine, plasma concentration, TAC solution, LET solution, TAC, LET
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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.1, 2, 3
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.1, 3
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.2, 3
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.2, 3
- Viscous lidocaine may be used alone or in a compounded mixture as a mouthwash (ie, swish and expectorate).3
- 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.1, 3
- 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). 4
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.2, 5 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 (Click here to complete a Medscape CME activity on the diagnosis and treatment of optic 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. For more information, see the FDA Medwatch Public Health Advisory.
Warnings regarding cocaine as a topical anesthetic
Various anesthetic mixtures that contain cocaine have been used to anesthetize minor skin lacerations, especially on the face or scalp. One such combination that is extemporaneously prepared by hospital pharmacies includes tetracaine 0.5%, epinephrine (Adrenaline) 1:2000, and cocaine 11.8%. This combination is commonly referred to as TAC solution. Its use results in decreased pain on application and may provide better patient tolerance of suturing, particularly in those who are unable to tolerate injections or who have difficulty following instructions or sitting still (eg, children, individuals with mental disabilities). However, serious toxic effects (eg, seizures, cardiac death) have been described following topical cocaine application, particularly in infants and children.2, 5 Because of increased toxicity, expense, and federal regulatory issues, cocaine is no longer recommended for topical anesthesia.
Compounded mixtures such as lidocaine, epinephrine, tetracaine (LET) solutions, have replaced cocaine with lidocaine 4% because of their superior safety when applied to nonintact skin. Still, these solutions should not be applied to wounds with end-arteriolar blood supply.
Allergic reaction to local anesthetics
Actual hypersensitivity is rare and accounts for less than 1% of all reactions to local anesthetics. Allergic reactions may be attributed to other factors such as acute toxicity, concurrent drug therapy (eg, tachycardia caused by epinephrine), or preservatives, such as paraben or sulfites, that the product may contain.2, 5
Local anesthetics with a para-amino benzoic acid (PABA) ester-type structure seem to cause most anesthesia-related allergic reactions. Consequently, use esters (eg, tetracaine, benzocaine) with caution or use a topical anesthetic from the amide class (eg, dibucaine, lidocaine). Documented cross-sensitivity has been exhibited within the ester-based local anesthetics and structurally related compounds (eg, paraben preservatives). Hypersensitivity to the amide local anesthetics is rare.1, 3 To quickly determine whether an anesthetic agent is an ester or an amide, note whether the generic name contains the letter i once or twice. Esters contain the letter i once in their generic names, whereas the generic names for amides contain the letter i twice.
Physiochemical variables
Onset of action, anesthesia depth, and duration of action are determined by the pKa level, pH level, lipid solubility, protein binding, and vasodilatory effects of the specific local anesthetic. These factors also depend on the area of the skin to which the anesthetic is applied, the vascularity of tissues, the surface area, and anesthesia technique. Duration of application is important to improve topical anesthetic properties.3
Table 2. Topical Anesthesia6, 1, 2, 3, 5
Anesthetic Class
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Common Topical Anesthetics Generic Name (Trade Name)
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Available/Recommended Concentration(s), %*
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Dosage Form(s)
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Maximum Adult Topical Dose, mg
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Maximum Adult Mucosal Dose, mg
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Maximum Pediatric Mucosal Dose, mg/kg
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Peak Effect, Minutes
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Duration, Minutes
| | Amides | Dibucaine (Nupercainal) | 1 | Cream, ointment | 25 | -- | -- | <5 | 30-60 |
| Lidocaine (Xylocaine, ELA-Max, Lidoderm) | 2-5 | Viscous jelly, patch, ointment, liposomes | Variable depending on dosage form | 250-300 | 3-4 | 2-5 Liposome: 30 | 15-45 | | Esters | Benzocaine
(Americaine, Cetacaine, Dermoplast, Solarcaine) | 0.5-20 | Aerosol, cream, gels, lotion, ointment, solution | -- | -- | -- | <5 | 15-45 |
| Cocaine† | 4-10 | Solution | 200 | 1 mg/kg | 1 | 1-5 | 30-60 |
| Tetracaine
(Pontocaine) | 0.5-2 | Solution, gel, cream | 50 | 20 | 0.75 | 3-8 | 30-60 | | Miscellaneous | Dyclonine
(Cepacol, Sucrets) | 0.5-1 | Aerosol, lozenge | -- | 50 | -- | <10 | <60 |
| Pramoxine
(ProctoFoam, Caladryl Cream for Kids) | 1 | Aerosol foam, pledgets, cream, ointment | 200 | -- | -- | 3-5 | -- |
| Lidocaine/Prilocaine
(EMLA) | 2.5/2.5 | Cream,
transdermal patch | 1 g/10 cm2‡ | -- | -- | 60: 3 cm depth
120: 5 cm depth | 30-60 after removal |
| Lidocaine 70 mg and tetracaine 70 mg (Synera Patch) | -- | Transdermal
patch | Adults or children: Apply 1 patch 20-30 min before superficial dermatological procedure | -- | -- | 100 | -- |
*Use lower concentrations for children or patients who are elderly or debilitated.
†Cocaine is generally not recommended for topical or mucosal application (see warnings above).
‡This dose also applies to the pediatric population (maximum application time in children aged <3 mo = 1 h; >3 mo = 4 h).
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