Laser-induced pain for evaluation of local analgesia: a comparison of topical application (EMLA) and local injection (lidocaine).

High-energy lasers are suitable for experimental pain stimulation because they selectively activate the polymodal nociceptors. Argon laser light penetrates deep into the skin and makes this laser type preferable for simulating pain arising from surgical skin incisions. Short argon laser pulses were applied to the skin and three parameters were quantified before and during analgesia; sensory threshold, pain threshold, and the pain-related cortical response (latency and amplitude). Determination of sensory and pain thresholds made it possible to distinguish between two levels of analgesia; the pain block, where no pain was felt but other sensations were still perceived; and total sensory block, where the laser stimulus elicited no sensations of any type. The analgetic effects of cutaneous injections of lidocaine and topical application of EMLA (eutectic mixture of local anesthetics) cream were evaluated and compared by means of the introduced parameters. Lidocaine produced total sensory block almost immediately after injection, which was associated with the absence of cortical response to cutaneous laser stimulation. When the EMLA cream was applied for 15 minutes, both sensory and pain thresholds increased. During the next 30 minutes after removal of the cream, the thresholds increased further. The increase in analgetic effect after removal of the cream was studied using different times (15, 30, 60, 80, 100, and 120 minutes) for topical EMLA cream application. Total sensory block was reached 20 minutes after removal of application for 80 minutes or immediately after removal of the cream after it was applied for 100 or 120 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)