Laser-Induced Pain for Evaluation of Local Analgesia

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. Pain block was achieved 30 minutes after removal of cream applied for 30 minutes, 15 minutes after removal of cream applied for 60 minutes, and immediately after applications for 80, 100, or 120 minutes. During the period of increasing and decreasing analgesia, cortical responses were recorded. Changes in amplitude and latency reflected the changes in perception (intensity and modality) to laser stimuli. In some cases a sensation of local warmth was perceived when pain perception was absent. The sensation of local warmth was accompanied by cortical deflections with a latency of 600–800 ms from stimulus onset, which was 200–300 ms later then the cortical deflections related to pain perception. Thresholds and cortical responses to argon laser stimuli were affected by minor changes in sensitivity of the nociceptors in human skin and were found to be suitable parameters for quantitative evaluations and comparisons of dermal analgesics.