Topical agents allow for pain relief through direct application of analgesics to the skin, presumably at the direct source of pain, thereby bypassing significant systemic effects.
Derived from red chili peppers, capsaicin has efficacy in reducing arthritic pain and some neuropathic pain states. The mechanism for pain relief is thought to be derived from depletion of substance P at sites of pain. Alternatively, counterirritation mechanisms (see Chapter 2, "Sensory Pathways of Pain and Acute Versus Chronic Pain," of this book) may be involved. Some patients find the burning sensations at the site of application to be too uncomfortable, precluding use.
The lidocaine patch functions to ameliorate pain when the patch is applied to intact skin surfaces affected by pain related to neuropathy or myofascial or osteoarthritis-related pain. The local anesthetic is thought to mitigate pain by influencing voltage-sensitive sodium channels to stabilize neural membranes. The patch provides approximately 12 hours of relief, requiring reapplication of additional patches for refractory and recurrent pain. The patch is generally well tolerated, but irritation and erythema may occur at the site of application.
A eutectic mixture of local anesthetics (EMLA) is a solution consisting of two local anesthetics (lidocaine and prilocaine). This topically applied anesthetic agent has marked analgesic properties (Kapelushnik et al. 1990). It is customarily employed in conditions involving skin surgery, such as curettage of skin lesions, split-thickness graft harvesting, collagen implants, removal of warts and port-wine stains, and so forth. Application of EMLA may cause localized vasoconstriction of skin. In addition, methemoglobinemia and hypoxia may also arise as a response to prilocaine contained in the EMLA solution. Edema and erythema may develop as well, particularly when EMLA is applied to diseased tissues or skin.
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