Topical medications discussed in this chapter include:
Antimicrobial and antiparasitic agents
Antiperspirants, antipruritics, and astringents
Bleaching and keratolytic agents
Topical psoriasis and wart therapies
This chapter reviews topical therapies that are used to treat dermatologic diseases but that are not discussed elsewhere in the text. When applied to large areas of skin, particularly in the presence of skin disease, or to infants and children, all topical medications have the potential to cause systemic side effects.
Capsaicin is the active ingredient responsible for the “hotness” in chili peppers. Initial topical application results in itching, pricking, and burning as a result of activation of the transient receptor potential vanilloid-1.1 Repeated application depletes substance P from cutaneous nerve endings and leads to desensitization of epidermal nerve fibers, thereby producing hypoalgesia.
Topical capsaicin has been used to treat postherpetic neuralgia, diabetic neuropathy, reflex sympathetic dystrophy, Raynaud phenomenon, osteoarthritis, plantar warts, and diabetic neuralgia.2 A recent metaanalysis of 6 randomized controlled trials found high efficacy of topical capsaicin in the treatment of postherpetic neuralgia.3 In addition, capsaicin is also an effective antipruritic agent for localized areas of pruritus of neuropathic origin, such as in brachioradial pruritus and notalgia paresthetica.4
The chief side effect of capsaicin is irritation and an intense burning sensation. Thus, capsaicin should not be applied to a large area of the body and may not be tolerated by children.
Numerous topical anesthetic formulations are available. Eutectic mixture of local anesthetics (EMLA) cream and topical lidocaine are two of the most frequently employed topical anesthetics and are discussed in further detail below.
EUTECTIC MIXTURE OF LOCAL ANESTHETICS CREAM
EMLA cream contains the sodium channel-blocking amide anesthetics lidocaine 2.5% and prilocaine 2.5%. Application under occlusion to intact skin or genital mucous membranes for at least 1 hour before performance of a painful procedure, including debridement of venous leg ulcers,5 can provide local anesthesia that may persist for up to 2 hours. In addition, EMLA is effective in cases of postburn pruritus6 and notalgia paresthetica.
The cream may cause transient local blanching followed by transient local erythema. Like all products containing lidocaine, it should not be used in patients with hypersensitivity to amide anesthetics. Side effects to EMLA are usually limited to mild skin reactions. Rare severe complications include CNS toxicity, cardiotoxicity, and methemoglobinemia.7 The prilocaine component of EMLA has been linked to cases of methemoglobinemia in patients for whom applications exceeded the recommended dose, application area, or application time.8 Those particularly susceptible to methemoglobinemia include patients who are very young, those with glucose-6-phosphate dehydrogenase deficiency, and those taking oxidizing drugs such as sulfonamides and antimalarials.