Topical Anesthesia

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Topical Anesthesia

Definition

Topical anesthesia is a condition of temporary numbness caused by applying a substance directly to a surface of the body. Loss of feeling occurs in the specific areas touched by the anesthetic substance.

Purpose

Topical anesthesia typically either relieves existing pain from a body surface or prevents pain during medical examinations or procedures. Body surfaces include both skin and mucous membranes, which are the moist linings of areas such as the inside of the mouth or nose. Occasionally, use of a topical anesthetic may help clarify whether a patient's pain comes from a body surface, which the substance is able to touch, or from deeper structures beyond the reach of the substance. A mucous membrane, an area of skin, or areas just beneath the skin's surface temporarily lose feeling during topical anesthesia.

Precautions

Topical anesthetic medications must be selected carefully and used in proper amounts in order to prevent harmful reactions. For instance, some topical drugs should be used only on intact skin to avoid rapid absorption into the body. Therefore, it is important to choose the right type and dosage of anesthetic for each specific purpose.

Description

Most drugs in the early 2000s used to cause topical anesthesia have at least some chemical similarity to cocaine, which was recognized as a topical anesthetic in the early 1900s. These drugs work by blocking nerve impulses that carry pain messages to the brain. Commonly used, twenty-first-century drugs include benzocaine, lidocaine, prilocaine, and tetracaine, These drugs come in several application forms, such as cream, jelly, ointment, solution, and spray, are available in varying strengths, and may be used alone or in combinations.

One common use for topical anesthetics is to relieve pain from problems such as sores in the mouth, skin scrapes, and hemorrhoids. A person typically applies a cream or jelly to the affected area. Numbness begins within a few minutes, and the anesthetic effect may last an hour or more. Repeated applications are often necessary.

The other broad category of use for topical anesthetics is to prevent pain from medical examinations or procedures on areas such as the eye, nose, throat, urethra, rectum, or skin. For example, even a single drop of anesthetic may allow examination of a painfully irritated eye or removal of a speck of dirt from the eye surface. Careful inspection of a nostril or insertion of a urinary drainage tube into the urethra may be difficult or impossible without the use of topical anesthetic spray or jelly beforehand.

Many uses of topical anesthetics involve application to the skin. Physicians and patients have long hoped for a way to numb an area of skin without having to use a needle to inject anesthetic. Original topical anesthetics were not effective in this way. A newer drug preparation called eutectic mixture of local anesthetics (EMLA, lidocaine 2.5%, and prilocaine 2.5%) can be used as a cream on intact skin. When applied for about 60 minutes, EMLA numbs the skin and penetrates as far as 5 mm below the surface. EMLA may help patients by decreasing the pain of needle pricks or simple skin procedures, such as repair of small lacerations. The main drawback to EMLA is that it takes so long for the anesthetic effect to begin, thus reducing its usefulness in settings such as the emergency rooms. Other drug combinations such as tetracaine, epinephrine (adrenaline), and cocaine (TAC), or lidocaine, epinephrine, and tetracaine (LET) have their own advantages and disadvantages.

The application of ice to a body area is a primitive method of producing topical anesthesia. Chemicals such as ethyl chloride can be sprayed onto intact skin to produce a momentary freezing and numbing effect.

KEY TERMS

Anesthetic— Not having sensation; related to the loss of sensation; or, a substance that produces loss of sensation.

EMLA— Eutectic mixture of local anesthetics, a drug combination for use on intact skin.

LET— A topical anesthetic mixture containing lidocaine, epinephrine, and tetracaine.

Mucous membranes— Moist linings of body surfaces such as the inside of the mouth or nose.

Numbness— Loss of feeling or sensation.

TAC— A topical anesthetic mixture containing tetracaine, epinephrine (adrenaline), and cocaine.

Topical— For use directly on a body surface.

Covering the ointment with plastic wrap increases the absorption of the ointment through the skin. The use of plastic wrap, or other occlusive dressings, is the standard means of applying EMLA cream. Other techniques include using gentle heat or electric current to help the drugs penetrate intact skin.

Risks

Overall, topical anesthesia tends to be very safe. Minor problems might include discoloration of the skin at the application site or an uncomfortable feeling of numbness that lasts longer than expected. Repeated use of a topical anesthetic on a damaged eye surface may interfere with the normal healing process. Rarely, too much of the drug may be absorbed into the body in a short time and cause a reaction such as seizure or rapid heartbeat. Death related to topical anesthesia is extremely rare.