Topical Anesthetics Notes

Topical anesthetics are increasingly used by clinicians to reduce pain during minimally invasive dermatologic procedures.
It is crucial for clinicians to understand the indications, contraindications, pharmacological mechanisms, and applications of commonly used topical anesthetics due to their growing use.

In Australia, the Therapeutic Goods Administration (TGA) regulates the use of topical anesthetics nationally, with additional regulation in South Australia under the Controlled Substance Act 1984.
All users must be aware of their legal responsibilities and potential hazards related to topical anesthetics.

Scheduled medicines registered in Australia are clearly labelled as:
- Pharmacy Medicine
- Pharmacist Only Medicine
- Prescription Medicine
Medicines approved for use in Australia and accepted by the TGA have either an AUST R number or AUST L number on the label.
The Australian Register of Therapeutic Goods (ARTG) can be consulted to verify if a product is lawfully supplied in Australia.

Schedule 2 - Pharmacy Medicines:
- Medicines labelled as "Pharmacy Medicine" are available for purchase in a pharmacy.
- Example: Numbing creams for topical use containing 10% or less lidocaine (lignocaine).
Schedule 4 - Prescription Only Medicines:
- Medicines labelled as "Prescription Only Medicines" are available from a pharmacist only with a prescription from a doctor.
- Example: Numbing creams with more than 10% lidocaine (lignocaine).

Topical anesthetics used to numb pain sensation in skin procedures fall under Schedule 2 Medicines in Australia, which implies restrictions on their use.
Any compounded anesthetic or any anesthetic containing more than 10% (combined) requires a doctor's prescription, classifying it as a Schedule 4 medication.
Tattoo artists, dermal therapists, and laser technicians are not authorized to supply Schedule 4 medicines to the public.

Scheduled medicines can only be sold or supplied to the public by persons authorized or licensed under the Controlled Substances Act 1984 and must be administered in accordance with the Act.
The illegal sale, supply, or administration of a medicine is an offense under the Controlled Substance Act 1984, carrying a maximum penalty of $10,000$ or imprisonment for 2 years.

Topical anesthetics include:
- Lidocaine (also called lignocaine)
- Prilocaine
- Tetracaine
- Benzocane

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

Variable absorption occurs superficially through intact skin.
Absorption can be increased via:
- Liposomal preparations
- Iontophoresis
- Transdermal patches
- Occlusion

Local heat can either increase or reduce the penetration of some anesthetics.
Heat can cause vasodilatation, enhancing systemic absorption and increasing toxicity.
Occlusion increases skin permeability by raising the temperature and hydration of the stratum corneum.
Other methods to increase skin penetration include liposomal preparations, iontophoresis, and transdermal patches.

Careful attention must be paid to:

Apply a thin layer approximately $2mm$ thick across the area identified for numbing.

Many topical anesthetic agents are effective and safe for dermatologic procedures, posing a low risk of adverse events.
Caution must be taken when occlusion is used or large surface areas are treated.
Particular caution is needed with compounded mixtures and non-standard doses, as they may increase the risk of adverse events and death.

Applying TA in large amounts or to large surface areas, or on irritated or damaged skin, can cause high concentrations of the anesthetic to enter the bloodstream, leading to serious side effects or toxicity.
Actual hypersensitivity is rare, accounting for less than 1% of all reactions to local anesthetics.
Allergic reactions may be attributed to other factors like acute toxicity, concurrent drug therapy (e.g., tachycardia caused by epinephrine), or preservatives like paraben or sulfites.

Usually caused by high plasma concentrations of topical anesthetics due to excessive exposure following a tear in the skin.
Potential complications:
- Burning or stinging
- Seizures
- Respiratory arrest
- Bradycardia
- Skin discoloration
- Swelling
- Arrhythmia
- Hypotension
- Cardiovascular issues
- Death

FDA Public Health Advisory Unit January 2018 and Use of Anaesthetics: https:// fda.gov.advisorycomitte.com/
Kouba, D. J., LoPiccolo, M. C., Alam, M., Bordeaux, J. S., Cohen, B., Hanke, W., Moyano, J. V. (2016, June). Guidelines for the use of local anesthesia in office‐based dermatologic surgery. Journal of the American Academy of Dermatology, 1201‐1219.
Revised prescription medicine clinical units v2.1 August 2017 www.tga.gov.au
Sobanko, J. F., Miller, C. J., & Alster, T. S. (2012, May). Topical Aesthetics for Dermatologic Procedures: A Review. (I. American Society for Dermatologic Surgery, Ed.) Dermatologic Surgery, 28(5), 709‐721. Retrieved from Washington Institute of Dermatologic Laser Surgery: https://doi.org/10.1111/j.1524‐4725.2011.02271.x