Topical anesthetics

Definition and Clinical Role

Topical anesthetics are pharmacological agents applied directly to the skin or mucous membranes to produce temporary loss of sensation or numbness in a localized area. Their primary function is to block nerve conduction at the site of application, providing pain relief, itch suppression, or procedural anesthesia without systemic involvement. These agents are widely used in dermatology, dentistry, minor surgical procedures, ophthalmology, and various mucosal applications, offering rapid, site-specific anesthetic action with minimal systemic absorption when used appropriately.

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Mechanism of Action

Topical anesthetics function primarily by:

• Blocking voltage-gated sodium channels in peripheral sensory neurons.

• This prevents the generation and conduction of action potentials, thereby halting the transmission of pain, temperature, touch, and pressure stimuli.

The depth and duration of anesthesia depend on the drug’s potency, lipid solubility, concentration, formulation, and the condition of the skin or mucosa at the site of application.

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Indications

Topical anesthetics are indicated for:

1. Dermatologic use:

   • Minor skin procedures (e.g., needle insertion, laser treatment, dermabrasion)

   • Relief from sunburn, insect bites, pruritus, minor burns

2. Dental/oral use:

   • Mucosal anesthesia prior to dental procedures

   • Relief of oral ulcers, teething pain

3. Ophthalmic use:

   • Diagnostic procedures (e.g., tonometry, foreign body removal)

   • Minor surgical interventions (short-term only)

4. Otic use:

   • Pain relief in otitis externa

5. Anorectal and genital use:

   • Painful hemorrhoids

   • Episiotomy preparation

   • Vaginal or penile procedures

6. Procedural use:

   • Pre-injection numbing

   • Venipuncture, IV cannulation

   • Vaccination pain management

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Commonly Used Topical Anesthetics

1. Lidocaine

   • Mechanism: Amide-type sodium channel blocker.

   • Formulations: 2–5% creams, gels, patches, sprays.

   • Brands: EMLA (with prilocaine), LMX, Xylocaine.

   • Indications: Wide use across dermatology, dentistry, and minor procedures.

   • Onset: 20–60 minutes; Duration: 30–90 minutes.

   • Maximum dose: Variable depending on formulation and area.

   • Adverse effects: Skin irritation, rare systemic toxicity.

2. Prilocaine (used in combination with lidocaine)

   • Common formulation: EMLA cream (2.5% lidocaine + 2.5% prilocaine).

   • Indications: Topical anesthesia for intact skin before needle puncture or minor dermal surgery.

   • Contraindication: G6PD deficiency (risk of methemoglobinemia).

3. Benzocaine

   • Mechanism: Ester-type anesthetic; poorly water-soluble.

   • Formulations: Lozenges, oral gels, sprays, ointments.

   • Brands: Anbesol, Orajel, Americaine.

   • Indications: Oral, pharyngeal, anorectal mucosa.

   • Caution: Risk of methemoglobinemia, especially in children and with high doses.

4. Tetracaine (Amethocaine)

   • Mechanism: Potent ester-type anesthetic.

   • Formulations: Ophthalmic drops, skin sprays, gels.

   • Brands: Pontocaine, Ametop.

   • Indications: Eye exams, minor procedures.

   • Onset: Rapid; Duration: Short.

   • Risk: Ocular toxicity with prolonged use.

5. Dibucaine (Cinchocaine)

   • Mechanism: Potent long-acting amide anesthetic.

   • Formulations: Ointments (0.25–1%).

   • Brands: Nupercainal.

   • Use: Hemorrhoid relief, rectal pain, burns.

6. Proparacaine

   • Formulation: Ophthalmic solution (0.5%).

   • Use: Anesthesia of the cornea and conjunctiva for diagnostics.

   • Duration: 10–15 minutes.

   • Caution: Repeated use may cause corneal epithelial damage.

7. EMLA Cream (Eutectic Mixture of Local Anesthetics)

   • Composition: Lidocaine 2.5% + Prilocaine 2.5%.

   • Use: Skin anesthesia prior to needle insertion, laser treatment.

   • Onset: 45–60 minutes under occlusion.

   • Duration: ~2 hours.

8. Lidocaine/Tetracaine Combination

   • Use: Fast-onset anesthesia for dermatological laser procedures.

   • Brands: Pliaglis, Synera patch (includes heating element).

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Formulations

• Creams and ointments: Skin anesthesia for surface procedures.

• Gels and oral gels: Oral and mucosal application.

• Sprays and aerosols: Rapid coverage for burns or mucosal areas.

• Patches: Extended delivery (e.g., Lidoderm patch for neuralgia).

• Solutions/drops: Ophthalmic, otic, dental uses.

• Foams and films: Fast-drying options for dermatology.

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Pharmacokinetics

• Onset: Rapid for mucosal and ocular agents (seconds to minutes); delayed for intact skin (EMLA: ~45 min).

• Duration: Ranges from 10 minutes (eye drops) to several hours (patches).

• Absorption: Enhanced through broken skin, mucosa, and with occlusive dressings.

• Metabolism:

  • Amides (e.g., lidocaine): Hepatic metabolism.

  • Esters (e.g., benzocaine, tetracaine): Hydrolyzed by plasma cholinesterases.

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Adverse Effects

1. Local Reactions:

   • Irritation

   • Redness, stinging, or burning

   • Allergic contact dermatitis (especially with esters)

2. Systemic Toxicity (rare, but serious if absorbed excessively):

   • CNS: Drowsiness, dizziness, tremor, seizures

   • Cardiovascular: Hypotension, arrhythmias, cardiac arrest

   • Methemoglobinemia: Especially with benzocaine, prilocaine

3. Ocular Effects:

   • Corneal toxicity with repeated use of anesthetic drops

   • Delayed corneal healing

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Contraindications and Precautions

• Known hypersensitivity to local anesthetics (amide vs. ester classes)

• Broken or inflamed skin (enhanced absorption risk)

• G6PD deficiency (prilocaine/benzocaine → methemoglobinemia)

• Children under 2 years (especially benzocaine-containing products)

• Avoid prolonged or repeated ophthalmic use

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Drug Interactions

• Class IA and III antiarrhythmics: Additive cardiac effects with systemic absorption

• Sulfonamides: Benzocaine may antagonize antibacterial action

• Nitrates and oxidizing agents: Increased risk of methemoglobinemia

• Beta-blockers, cimetidine: May reduce lidocaine clearance

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Application Tips

• Use minimal effective dose

• Apply on clean, dry skin or mucosa

• Avoid applying under occlusion unless directed

• Do not apply over large surface areas unless advised

• Wash hands after application (unless hands are the treatment site)

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Examples of Brand and Generic Names

Generic Name	Brand Name(s)	Formulation	Indication
Lidocaine	Xylocaine, LMX	Cream, gel, patch	Minor procedures, burns, pain relief
Lidocaine + Prilocaine	EMLA	Cream	Skin anesthesia
Benzocaine	Orajel, Anbesol	Gel, lozenge	Toothache, sore throat
Tetracaine	Ametop, Pontocaine	Gel, ophthalmic drop	Eye exams, dermal pain
Dibucaine	Nupercainal	Ointment	Hemorrhoid relief
Proparacaine	Alcaine	Eye drops	Corneal anesthesia

Clinical Recommendations

• Use EMLA for skin anesthesia before minor procedures (e.g., venipuncture).

• Use lidocaine gel for localized skin irritation, minor burns.

• Use benzocaine or lidocaine oral gels cautiously for oral ulcers or teething pain.

• Avoid long-term use of ophthalmic anesthetics due to risk of corneal injury.

• In pediatric populations, avoid benzocaine unless clearly indicated