A quantitative comparison of the effect of local analgesics on argon laser induced cutaneous pain and on histamine induced wheal, flare and itch.

Abstract

A quantitative comparison was made of the effect of infiltration of local analgesics and topical analgesic cream (EMLA) on laser-induced pain and histamine-induced wheal, flare and itch. Wheal and flare were quantified by planimetry and analgesia was quantified by the pricking pain threshold to argon laser stimulation. The intensity of histamine-induced itch was scored on a 4-point scale. Local analgesics had no effect on the wheal area. The flare reaction was abolished by infiltrating lignocaine, and gradually inhibited by increased application times of EMLA. Itch was abolished after local lignocaine infiltration, but not significantly reduced after EMLA cream applied for less than 120 min, although the skin was anaesthetized to laser-induced pain. The reduction of flare area correlated to the level of analgesia, which may therefore reflect the cutaneous responsiveness to neurogenic inflammation. It is suggested that itch and pricking pain are mediated by different populations of nerve fibres, as itch can be evoked even when the sensation of pricking pain is abolished. Surgery, skin prick tests and other traumatic procedures should therefore be performed under local anaesthesia to reduce neurogenic inflammation.