Topical anesthetics are applied directly to intact skin or mucousmembranes. All topical anesthetics are used to prevent or relieve minor pain.
Some injectable local anesthetics, such as lidocaine and tetra-caine, are also topically effective. In addition, some topical anes-thetics, such as lidocaine, are combined in other products.
Topical anesthetics produce little systemic absorption, except for the application of cocaine to mucous membranes. However, sys-temic absorption may occur if the patient receives frequent or high-dose applications to the eye or large areas of burned or in-jured skin.
Tetracaine and other esters are metabolized extensively in the blood and to a lesser extent in the liver. Dibucaine, lidocaine, and other amides are metabolized primarily in the liver. Both types of topical anesthetics are excreted in the urine.
Benzocaine, butacaine, cocaine, dyclonine, and pramoxine pro-duce topical anesthesia by blocking nerve impulse transmission. They accumulate in the nerve cell membrane, causing it to expand and lose its ability to depolarize, thus blocking impulse transmis-sion. Dibucaine, lidocaine, and tetracaine may also block impulse transmission across the nerve cell membranes.
The aromatic compounds, such as benzyl alcohol and clove oil, appear to stimulate nerve endings. This stimulation causes coun-terirritation that interferes with pain perception.
Ethyl chloride spray superficially freezes the tissue, stimulating the cold-sensation receptors and blocking the nerve endings in the frozen area. Menthol selectively stimulates the sensory nerve end-ings for cold, causing a cool sensation and some local pain relief.
Topical anesthetics are used to:
· relieve or prevent pain, especially minor burn pain
· relieve itching and irritation
· anesthetize an area before an injection is given
· numb mucosal surfaces before a tube, such as a urinary catheter, is inserted
· alleviate sore throat or mouth pain when used in a spray or so-lution.
Tetracaine is also used as a topical anesthetic for the eye. Benzo-caine is used with other drugs in several ear preparations.
Few interactions with other drugs occur with topical anesthetics because they aren’t absorbed well into the systemic circulation. (See Adverse reactions to topical anesthetics.)