What Is Used as Topical Anesthetic?

Topical anesthesia may come in gels, sprays, oral rinses or skin patches. It's used for minor skin and dental procedures.

Topical anesthesia is local anesthetic applied to the surface of the skin or mucous membrane (mucosa) to temporarily numb the area superficially. Topical anesthesia is applied to control pain from minor medical procedures.

Topical anesthesia produces a loss of sensation by temporarily blocking nerve conduction near the area of application. Topical anesthetic agents block the sodium channels in the nerve endings near the skin surface and prevent the inflow of sodium ions into nerve cells. This stops the nerve cells in the area from sending pain signals to the brain until the anesthetic effects wear off.

Topical anesthesia is ideal for controlling pain caused by minor procedures. The benefits of topical anesthesia include:

• Better patient comfort
• Elimination of needle use and the associated pain
• Elimination of risks associated with local infiltration local anesthesia such as:
  • Distortion of wound margin
  • Systemic absorption and toxicity

Topical anesthesia is useful in the following medical procedures:

• Laceration and wound repair
• Catheter insertion into the vein or artery
• Fiberoptic bronchoscopy
• Ophthalmologic procedures such as:
  • Intraocular lens implant
  • Minor eyelid surgery
• Vaginal examination with speculum in postmenopausal women
• Rectal examination with a probe
• Prostate biopsy
• Treatment of paraphimosis, a condition in which the foreskin cannot be pulled over the head of the penis

Use of topical anesthetic has also been found to be effective in increasing sexual endurance in men with primary premature ejaculation.

Topical anesthetic agents are available in many forms and the type of administration depends on the area that requires numbing. 

Gels, creams and ointments: Applied in multiple layers on the chosen skin area or the mucosa in the genital or anal area.

Transdermal patches: Applied on intact skin.

Aerosol and atomizer sprays: Sprayed in the throat for procedures such as bronchoscopy.

Viscous solutions: Swished in the mouth like a mouthwash for minor oral or dental procedures.

The skin forms an effective barrier against drug absorption. The following are some of the techniques in clinical trials, to improve absorption.

• Iontophoresis: Use of mild electric current to improve drug absorption by the skin.
• Laser-assisted drug delivery: Pre-treatment with laser to increase drug absorption. 
• Use of nanoparticles: The use of liposomes (particles with fat layers) and peptides that can penetrate the cell membrane to deliver the drug.

Epinephrine may be added to the anesthetic agent for longer lasting effects.

Amino amides and amino esters are the primary classes of anesthetic agents used in topical anesthesia. Following are some of the commonly used topical anesthetic agents:

Amino amides

Amino amides are stable in solution, metabolized by the liver and excreted in the urine.

• Dibucaine (Nupercainal): Ointment that has peak effect within five minutes lasting up to an hour.
• Lidocaine (Xylocaine, ELA-Max, Lidoderm): Available as ointment, viscous jelly and skin patch, has peak effect within five minutes lasting up to 45 minutes.

Amino esters

Amino esters are unstable in solution and are metabolized by an enzyme in blood plasma.

• Benzocaine (Americaine, Cetacaine, Dermoplast, Solarcaine): Available as aerosol, cream, gel, lotion, ointment and solution, has peak effect within five minutes lasting up to 45 minutes.
• Tetracaine: Available as solution or cream, with peak effect in three to eight minutes and duration of effect up to an hour.

Others

• Dyclonine (Cepacol, Sucrets): Available as aerosol and lozenge, with peak effect within 10 minutes that lasts up to an hour.
• Pramoxine (ProctoFoam, Caladryl Cream for Kids): Available as aerosol foam, pledgets and cream, with peak effect within five minutes.
• Lidocaine/prilocaine (EMLA): Available as cream and transdermal patch. Effective up to 3 cm depth in 60 minutes and 5 cm depth in 120 minutes, and the effects last for up to an hour after patch removal.
• Lidocaine/tetracaine (Synera Patch): Transdermal patch with peak effect at approximately 100 minutes.

Adverse effects of topical anesthetics are usually because of high plasma concentration due to excessive exposure when applied to broken skin or mucosa. Side effects of topical anesthetics include the following:

• Central nervous system (CNS): High concentration of anesthetic in the plasma first causes CNS stimulation (including seizures), followed by CNS depression (including respiratory arrest). Amide anesthetics may not produce a stimulatory effect on CNS, but adding epinephrine may add to the stimulatory effect.
• Cardiovascular system: High plasma levels of anesthetics depress the heart and may cause:
  • Slow heart rate (bradycardia)
  • Irregular heartbeat (arrhythmia)
  • Low blood pressure (hypotension)
  • Cardiovascular collapse
  • Cardiac arrest
• Addition of epinephrine may have the opposite effect and can lead to:
  • High blood pressure (hypertension)
  • High heart rate (tachycardia)
  • Chest pain (angina)
• Hypersensitivity reactions: Hypersensitivity to amino amide anesthetics is rare and may be caused by preservatives in the anesthetic solution. Amino ester anesthetics, when metabolized in the plasma, release an allergen known as para-aminobenzoic acid, which may cause hypersensitivity. Allergic reactions may also result from local anesthesia systemic toxicity.

Other side effects:

• Gag reflex suppression with oral administration
• Burning or stinging sensation at the application site
• Oral viscous lidocaine may cause systemic toxicity, particularly when used repeatedly
• Methemoglobinemia (reduced oxygen supply by the red cells to the tissues) from topical lidocaine before transesophageal echocardiography (TEE) is rare but a life-threatening complication. TEE is an imaging test involving an ultrasound scope inserted down the esophagus.
• Increased aspiration risk with nasal endoscopy (a test to look inside the nose, throat and voice box)