Effects of Lidocaine Patch on Intradermal Capsaicin Induced Pain and Hyperalgesia

To determine the effects of Lidocaine patch on the pain and hyperalgesia induced by intradermal capsaicin

A randomized, double-blinded, placebo controlled methodology will be conducted. At the session subjects will be exposed to placebo patch and lidocaine patch. Prior to study drug administration, a baseline neurosensory test on the volar aspect of both forearms will be performed and baseline vital signs will be measured. A placebo patch and a lidocaine patch will then be applied to the volar aspect of each forearm. The arms will be randomized to which arm receives placebo and which one receives the lidocaine patch. After four hours of application the right forearm patch will be removed the neurosensory testing will be repeated on the right forearm. After completing the testing, capsaicin (10µl, 10 mg/ml) will be injected intradermally on the volar aspect of the right forearm. Elicited and spontaneous pain scores, blood pressure, heart rate, and respiratory rate will be measured at the time of injection and every 2.5 minutes for 10 minutes. A McGill Pain Questionnaire will be administered at the time of capsaicin injection only. Ten minutes after the capsaicin injection, the hyperalgesic area will be established to von Frey hair, stroking, and heat; the flare response will be outlined; and neurosensory testing will performed halfway between the edge of this defined area and the capsaicin injection site. At the completion of the testing on the right forearm, the left forearm patch will be removed and the procedures described for the right forearm will be repeated for the left forearm.

Touch will be measured using von Frey hairs. Calibrated von Frey hairs are filaments of varying size. The filament are selected at random and 3 successive stimuli are applied for 1.5 second at 5 second intervals per filament

At the completion of the stimulation, areas of cutaneous allodynia and hyperalgesia will be mapped. The region of hyperalgesia will be established with a 5.18 von Frey hair, and the area of allodynia with a foam brush gently stroked on the skin.

Inclusion Criteria: Age 18 and above Exclusion Criteria: Pregnancy. Allergy to lidocaine Current painful condition Current use of analgesics for the treatment of pain Lack of ability to understand the experimental protocol and to adequately communicate in English. The neurosensory testing we plan to perform requires the complete cooperation and understanding of the subject. It would be impossible to perform these studies on patients who do not adequately communicate in English.

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