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Analgesic and anti-inflammatory effects of lignocaine-prilocaine (EMLA) cream in human burn injury
British Journal of Anaesthesia 1996; 76: 806–810
Analgesic and anti-inflammatory effects of lignocaine–prilocaine (EMLA) cream in human burn injury J. L. PEDERSEN, T. CALLESEN, S. MØINICHE AND H. KEHLET
Summary Pain relief may be improved by reducing sensitization of nociceptive pathways caused by tissue injury. Such a reduction depends mainly on inhibition of local inflammatory changes and the relation between duration of nociceptive block and nociceptive input. In this study we examined if prolonged topical treatment with local anaesthetics could reduce late hyperalgesia and local inflammation after burn injury in healthy volunteers. The effects of EMLA treatment for 8 h after burn on hyperalgesia, inflammation and wound healing were compared with the contralateral placebotreated leg for 48 h after bilateral burn injuries (15 25 mm, 49 ⬚C for 5 min) in a double-blind, randomized study in 12 healthy volunteers. Wound healing was studied 1 and 2 weeks after injury. Neither mechanical nor thermal primary hyperalgesia were affected significantly by prolonged EMLA treatment. Secondary hyperalgesia and skin erythema were also not changed. Seven of 12 placebo-treated legs developed blisters, in contrast with four of 12 EMLA-treated legs. Wound healing showed no apparent differences. Our data suggest that prolonged, topical treatment with local anaesthetics did not reduce local inflammation and late hyperalgesia. (Br. J. Anaesth. 1996; 76: 806–810) Key words Burns. Anaesthetics local, EMLA. Pain, mechanism.
Injury causes inflammation and may lead to prolonged sensitization of nociceptive pathways. Primary and secondary hyperalgesia are well established aspects of this process. Hyperalgesia describes the phenomenon of increased pain intensity in response to normally painful stimuli [1], primary hyperalgesia refers to the changes in sensation within the injury, while secondary hyperalgesia refers to sensory changes in the undamaged tissue surrounding the injury. Several inflammatory mediators promote hyperalgesia by sensitization of nociceptors [2]. Many in vitro and animal studies have shown antiinflammatory effects of local anaesthetics and these effects may potentiate the analgesic effect of these agents. The anti-inflammatory effects may impair the host defence, but may also improve conditions with destructive inflammation. In contrast, there are only few human reports on the anti-inflammatory effects of local anaesthetics and we have shown previously that brief (3.5 h) topical treatment with
local anaesthetics before and after injury did not significantly affect inflammation after burn [3]. The aim of the study was therefore to examine if prolonged and intense treatment with lignocaine– prilocaine cream (EMLA) could reduce late hyperalgesia and other markers of local inflammation after burn injury in healthy volunteers. The treatment was initiated after the burn, as this mimics the clinical situation.
Subjects and methods We studied 12 healthy volunteers, aged 22–47 yr (all male), after obtaining informed consent and approval from the local Ethics Committee and the Danish National Health Board. Subjects were not medically knowledgeable and were not aware of the purpose of the study. Baseline measurements, which included assessment of skin colour, mechanical pain detection threshold (MPDT) and heat pain detection threshold (HPDT), were performed on the medial calf in both legs. After obtaining baseline measurements, thermal injury was induced on each leg. All measurements, that is skin erythema, blister formation, MPDT, secondary hyperalgesia and HPDT, were made in this order, 16, 20, 24 and 48 h after the injuries. Immediately after the injury, EMLA or placebo was applied, double-blind and randomized, to the skin and covered under occlusive dressing for 8 h after the injury. Experiments were performed in a quiet room, temperature 20–22 ⬚C, with subjects in a relaxed and reclined position. Subjects were instructed to keep their eyes closed during all measurements so that they had no impression of the results of the measurements. EMLA TREATMENT
EMLA cream 2 g (2.5 % prilocaine and 2.5 % lignocaine eutectic mixture, Astra, Södertälje, Sweden) or placebo (Astra; Södertälje, Sweden) was applied in a double-blind, randomized manner to the skin and covered under occlusive dressing (Tegaderm, 3M Health Care Ltd, Leicestershire, UK) for 8 h after the injury. The cream was renewed JURI L. PEDERSEN, MD, TORBEN CALLESEN, MD, HENRIK KEHLET, MD, PHD (Department of Surgical Gastroenterology); STEEN MØINICHE, MD (Department of Anaesthesiology); Hvidovre Hospital, University of Copenhagen, Kettegaard Allé 30, 2650 Hvidovre, Denmark. Accepted for publication: January 24, 1996. Correspondence to J.L.P.
Effect of EMLA in human burns 3 and 5.5 h after initial application, and re-covered. Subjects were instructed to avoid physical activities that could displace the thick cream layer from the injury. The burns were performed in the afternoon and the cream was removed 8 h later (late evening). Subjects attended next morning for the first measurement, 16 h after the injury. THERMAL INJURY
Thermal injuries were produced on the medial surface of the right and left calf with a 1525-mm Peltier thermode (Thermotest, Somedic A/B, Stockholm, Sweden). The thermode (49 ⬚C) was applied to the skin for 5 min under standardized pressure (34 mm Hg), causing first- and superficial second-degree burn injuries [4, 5]. The burn caused immediate intense stinging pain, which was followed by a moderate burning pain with a more diffuse quality during the remainder of the stimulation period. Spontaneous pain after the injury has not been observed in the model. PRIMARY HYPERALGESIA
Mechanical pain detection threshold (MPDT) within the injured area was determined by pinprick with nine progressively rigid von Frey hairs (Somedic A/B, Stockholm, Sweden). We examined the force produced by each hair by pressing it against a balance and measured the weight it produced when it was slightly flexed. We confirmed that the steps from 1 to 9 represented logarithmic increases in force (hair no. 1 : 3 mN, 2 : 9 mN, 3 : 11 mN, 4 : 18 mN, 5 : 29 mN, 6 : 49 mN, 7 : 98 mN, 8 : 98 mN, 9 : 175 mN). MPDT was defined as the lowest force (pinprick) which produced a definite sensation of pain or discomfort. Pinprick was performed with each hair from the thinnest, 8–10 times in the area of the injury, until some of the 8–10 stimulations were reported to be painful or unpleasant. The number of the hair producing pain was recorded and the procedure was repeated three times at each measurement point. The median of three threshold measurements was reported as the MPDT. If hair No. 9 did not produce any pain or discomfort, we assigned this observation the value 10. Thermal thresholds were determined with the computerized Thermotest (Somedic A/B, Stockholm, Sweden) in the area of the injury. Heat pain detection threshold (HPDT) was the lowest temperature perceived as painful. Subjects were instructed to press a button as soon as the specified sensation was reached. Thermal thresholds were determined from a baseline of 32 ⬚C with a rate of change of 1 ⬚C s91. The upper cut-off limit was 52 ⬚C. HPDT was calculated as the average of three measurements, with intervals of 9 s between each stimulation. SECONDARY HYPERALGESIA
The area of mechanical hyperalgesia that developed around the burn injury was assessed with von Frey hair No. 9 (175 mN). In most cases pinprick with
807 this hair is a painful or unpleasant stimulus in distinct spots of the skin, although it is not universally painful. Thus areas of hyperalgesia were measured, not areas of allodynia. Borders of hyperalgesia were determined by stimulating along eight radial linear paths arranged from the centre of injury. Stimulation along each path began well outside the hyperalgesic area where slight or no pain was perceived and continued slowly towards the centre of the burn injury until the subject reported a definite change in sensation, most often to a more intense pricking with a burning after-sensation. This point was marked and the eight marks were later traced onto a clear acetate sheet. The area of secondary hyperalgesia was calculated from the eight marks using a vector algorithm. INFLAMMATION
In order to estimate the severity of inflammation, the intensity of erythema inside the injury was assessed with a hand-held skin reflectance spectrophotometer (Dermaspectrometer, Cortex Technology, Hadsund, Denmark) [6, 7]. The spectrophotometer provided a skin erythema index based on the absorption characteristics of green and red light in the skin. Measurements were performed in six spots within the burn injury. All values were recorded and the mean calculated. The development of blisters was recorded as present or absent. Healing of the burns was documented using a questionnaire, 1 and 2 weeks after the injury.
STATISTICAL ANALYSIS
Distribution of data was evaluated with the Shapiro Wilk test [8]. Primary mechanical hyperalgesia and secondary hyperalgesia in EMLA- and placebotreated legs were evaluated at separate times with multiple Wilcoxon tests for paired observation with Bonferroni correction and area under the curves (AUC) for the whole observation period with Student’s t test for differences in a paired design [9]. Evaluation of primary thermal hyperalgesia and erythema was based on AUC values but analysed also using parametric two-way analysis of variance (ANOVA) for repeated measurements [8]. Minimal differences between treatments detectable in the present study with a power of 90 % and a type I error of 5 % were evaluated with Student’s t test for differences in a paired design [8]. P 0.05 was considered statistically significant.
Results The area of hyperalgesia surrounding the injury was not significantly altered by EMLA treatment compared with placebo (fig. 1 A). The effect of EMLA was evaluated at all times (16 h, P : 0.68; 20 h, P : 0.37; 24 h, P : 0.14; and 48 h, P : 0.07; Wilcoxon test). The level of significance was reduced to 1 % in these comparisons with Bonferroni correction, because of the multiple comparisons. The area under the curve (AUC) for hyperalgesia was applied as a
808
British Journal of Anaesthesia
Figure 1 A: Secondary hyperalgesia, that is area of hyperalgesia to von Frey pinprick (175 mN) outside the injury, after burn injury in healthy volunteers treated with EMLA (. . . .) or placebo (——). Time 0 h : measurements before thermal injury. Values are medians. Comparison of AUC values revealed no significant difference between secondary hyperalgesia in EMLA- vs placebo-treated legs (P : 0.18; Student’s paired t test, n : 12). B: Primary mechanical hyperalgesia, that is mechanical pain detection threshold to von Frey pinprick, in the area of the burn injury. Values are medians. Comparison of AUC values revealed no significant difference between primary mechanical hyperalgesia in EMLA- (. . . .) vs placebo- (——) treated legs (P : 0.19; Student’s paired t test, n : 12). Von Frey hair no. (1–10) represents a rank scale, where 1 indicates a force of about 3 mN and 9 about 175 mN, whereas 10 indicates that 175 mN did not produce pain or discomfort. The steps from 1 to 9 are logarithmic increases in force (see text). C: Primary thermal hyperalgesia, that is heat pain detection threshold (HPDT) in the area of the burn injury. Values are means. Comparison revealed no significant difference between primary thermal hyperalgesia in EMLA (. . . .)- vs placebo (——)-treated legs (P : 0.61; parametric twoway analysis of variance for repeated measurements, n : 12). D: Skin erythema, that is the intensity of erythema inside the burn injury assessed with a hand-held skin reflectance spectrophotometer. Values are means. Comparison revealed no significant difference between erythema in EMLA (. . . .)- vs placebo (——)-treated legs (P : 0.34; parametric two-way analysis of variance for repeated measurements, n : 12). Table 1 Positive answers out of 12, in a questionnaire (yes/no) completed by the 12 subjects
Changes in injury
Placebo (1 week)
EMLA (1 week)
Placebo EMLA (2 weeks) (2 weeks)
Tenderness Irritated by touch Itch Any colour change Crusts Blister Skin erythema
1 2 3 7 3 2 6
0 0 2 9 3 2 6
0 0 3 6 3 1 4
0 0 1 5 1 0 0
summary measure of the total hyperalgesic response. AUC for secondary hyperalgesia after EMLA treatment was not significantly different from that after placebo (P : 0.18; Student’s paired t test). The effect of EMLA on primary mechanical hyperalgesia (fig. 1 B) was analysed in the same way and there was no significant differences between EMLA and placebo (0 h, P : 0.20; 16 h, P : 0.28; 20 h, P : 0.29; 24 h, P : 1.00; and 48 h, P : 0.25; Wilcoxon test). Comparison of AUC values confirmed this finding (P : 0.19; Student’s paired t test). Data for primary thermal hyperalgesia (fig. 1 C) and skin erythema index (fig. 1 D) were analysed with both AUC summaries and parametric two-way ANOVA for repeated measurements. There were no significant differences between EMLA and placebo with both methods (primary thermal hyperalgesia: AUC, P : 0.80 and ANOVA, P : 0.61; and ery-
Table 2 Minimum differences between treatments detectable with a power of 90 % (type II error of 10 %) and a type I error of 5 %. Differences are presented as the weighted mean for the observation period (AUC divided by 48 h)
Secondary hyperalgesia (cm2)
Primary mechanical hyperalgesia (von Frey No.)
Primary thermal hyperalgesia (°C)
Erythema index (arb. unit)
13
0.9
2.4
1.3
thema: AUC, P : 0.54 and ANOVA, P : 0.34). Seven of 12 placebo-treated legs developed blisters in contrast with four of 12 EMLA-treated legs. With binomial data (yes/no), these small numbers precluded a sensible statistical analysis. There were no apparent differences in wound healing between EMLA and placebo treatment (table 1). Minimal differences between treatments detectable in this study, with a power of 90 % and a type I error of 5 %, are presented in table 2. Differences are presented as the weighted mean for the observation period (AUC divided by 48 h).
Discussion We examined if prolonged and intensive topical treatment with local anaesthetics reduced late hyperalgesia, blister formation or erythema after thermal injury in healthy volunteers in a double-blind, randomized study. We found that the local anaesthetics neither reduced local inflammation and late
Effect of EMLA in human burns hyperalgesia nor produced apparent changes in wound healing, although the sample size does not exclude minor differences. This result may seem surprising, considering the known anti-inflammatory effects of local anaesthetics. Several in vitro studies have demonstrated reduced production of free oxygen radicals from polymorphonuclear leucocytes after treatment with local anaesthetics [10–14], and reduced adherence and migration of leucocytes have been documented in different in vivo models [15–18]. Furthermore, local anaesthetics inhibit movement of leucocytes and phagocytosis in vitro [14, 19–21]. Local anaesthetics have also been shown to reduce albumin extravasation in rats after burn injury and chemical peritonitis [22, 23] and to modify the inflammatory response to experimental colitis [24] and irradiation [25]. Local anaesthetics may alter release of inflammatory mediators, for example histamine release from mast cells in vitro [26], leukotriene B-4 and interleukin-1 release from polymorphonuclear granulocytes and mononuclear cells in vitro, respectively [27]. The flare reaction induced by trauma and injection of inflammatory mediators is diminished by local anaesthetics, in contrast with the weal and local red reaction [28–30]. Thus local anaesthetics block axon reflexes and sensory functions of nociceptors. Inhibition of axon reflexes reduces local, indirect release of inflammatory neuropeptides, but the clinical importance of diminished neurogenic inflammation is unclear. It is not known if the local effector function of directly stimulated nociceptors is affected by local anaesthetics [31]. Furthermore, local anaesthetics have been shown to inhibit substance P receptor binding [32]. Thus several anti-inflammatory effects of local anaesthetics have been demonstrated in vitro and in animal models, but evidence of effects in human studies is, to our knowledge, restricted to one uncontrolled study and one case report, where lignocaine improved ulcerative proctitis and severe interstitial cystitis, respectively [33, 34]. Topical lignocaine has been tested several times in asthmatic subjects with conflicting, but mainly negative results [35–37]. Topical lignocaine had no effects on allergeninduced nasal secretion, blood flow or obstruction, in allergic individuals [38]. We have shown earlier that short-lived topical application, or subcutaneous infiltration with local anaesthetics, did not affect inflammation after burn [3]. This study, with prolonged and intensive treatment, supports this conclusion. Our findings, and the lack of other clinical evidence, may therefore question the relevance of the anti-inflammatory effects of local anaesthetics. There may be different explanations for our results. First, the response involves several inflammatory mediators and cell types, which may not be affected by local anaesthetics. Second, the timing of the treatment (before compared with after injury) may be important in modulating inflammatory responses, as early mediators release cascades of secondary mediators. In this study we preferred treatment after injury, as this mimics most clinical situations, while treatment was initiated before
809 injury in our first study [3]. Apparently, this did not affect the results. Third, the effects of prolonged topical treatment were either too transient or incapable of altering the course of the inflammatory response. Although the analgesic effects did not last into the observation period (16–48 h after injury), we do not question the effectiveness of EMLA cream in pain relief, but effects on hyperalgesia should not be expected for prolonged periods (more than 8 h) after removal of the cream. The potential reduction of leukotriene B-4, interleukin-1, histamine, free oxygen radicals and activated leucocytes by local anaesthetics [27] should attenuate sensitization of nociceptors in the injury, but removal of EMLA cream 8 h before the first measurements may have returned the process to its natural course. Fourth, we studied only 12 subjects and we may have missed some real differences. The minimal differences between treatments detectable in this study, with a power of 90 %, are shown in table 2. The consequences for wound healing after treatment with local anaesthetics are controversial. The synthesis of collagen and glycosaminoglycans by fibroblasts in vitro is reduced by local anaesthetics [39], as is the tensile strength of skin wounds in rats [40]. Other studies [41], including our own, have been unable to demonstrate adverse clinical effects on host defences and wound healing, but our study is less specific in this respect than the first mentioned studies, and prolonged use of local anaesthetics may threaten tissue defences.
Acknowledgements The study was supported by grants from the John and Birte Meyer Foundation, the P. A. Messerschmidt and Wife’s Fund, Fonden til Laegevidenskabens Fremme and Astra Pain Control, Södertälje, Sweden.
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810 11. Ohsaka A, Saionji K, Sato N, Igari J. Local anesthetic lidocaine inhibits the effect of granulocyte colony-stimulating factor on human neutrophil functions. Experimental Hematology 1994; 22: 460–466. 12. Goldstein IM, Lind S, Hoffstein S, Weissmann G. Influence of local anesthetics upon human polymorphonuclear leukocyte function in vitro. Reduction of lysosomal enzyme release and superoxide anion production. Journal of Experimental Medicine 1977; 146: 483–494. 13. Kanbara T, Tomoda MK, Sato EF, Ueda W, Manabe M. Lidocaine inhibits priming and protein tyrosine phosphorylation of human peripheral neutrophils. Biochemical Pharmacology 1993; 45: 1593–1598. 14. Vandenbroucke Grauls CM, Thijssen RM, Marcelis JH, Sharma SD, Verhoef J. Effects of lysosomotropic amines on human polymorphonuclear leucocyte function. Immunology 1984; 51: 319–326. 15. Eriksson AS, Sinclair R, Cassuto J, Thomsen P. Influence of lidocaine on leukocyte function in the surgical wound. Anesthesiology 1992; 77: 74–78. 16. Giddon DB, Lindhe J. In vivo quantitation of local anesthetic suppression of leukocyte adherence. American Journal of Pathology 1972; 68: 327–338. 17. Paul H, Clayburne G, Schumacher HR. Lidocaine inhibits leukocyte migration and phagocytosis in monosodium urate crystal-induced synovitis in dogs. Journal of Rheumatology 1983; 10: 434–439. 18. MacGregor RR, Thorner RE, Wright DM. Lidocaine inhibits granulocyte adherence and prevents granulocyte delivery to inflammatory sites. Blood 1980; 56: 203–209. 19. Moudgil GC, Allan RB, Russell RJ, Wilkinson PC. Inhibition, by anaesthetic agents, of human leucocyte locomotion towards chemical attractants. British Journal of Anaesthesia 1977; 49: 97–105. 20. Hammer R, Dahlgren C, Stendahl O. Inhibition of human leucocyte metabolism and random mobility by local anaesthesia. Acta Anaesthesiologica Scandinavica 1985; 29: 520–523. 21. Cullen BF, Haschke RH. Local anesthetic inhibition of phagocytosis and metabolism of human leukocytes. Anesthesiology 1974; 40: 142–146. 22. Cassuto J, Nellgård P, Stage L, Jönsson A. Amide local anesthetics reduce albumin extravasation in burn injuries. Anesthesiology 1990; 72: 302–307. 23. Rimbäck G, Cassuto J, Wallin G, Westlander G. Inhibition of peritonitis by amide local anesthetics. Anesthesiology 1988; 69: 881–886. 24. McCafferty DM, Sharkey KA, Wallace JL. Beneficial effects of local or systemic lidocaine in experimental colitis. American Journal of Physiology 1994; 266: G560–567. 25. Ohlsén L, Evers H, Segerström K, Hagelqvist E, Graffman S. Local anaesthetics modifying the dermal response of irradiation. An experimental study. Acta Oncologica 1987; 26: 467–476.
British Journal of Anaesthesia 26. Johnson HG, Miller MD. Inhibition of histamine release and ionophore-induced calcium flux in rat mast cells by lidocaine and chlorpromazine. Agents and Actions 1979; 9: 239–243. 27. Sinclair R, Eriksson AS, Gretzer C, Cassuto J, Thomsen P. Inhibitory effects of amide local anaesthetics on stimulusinduced human leucocyte metabolic activation, LTB4 release and IL-1 secretion in vitro. Acta Anaesthesiologica Scandinavica 1993; 37: 159–165. 28. Wallengren J, Håkanson R. Effects of capsaicin, bradykinin and prostaglandin E2 in the human skin. British Journal of Dermatology 1992; 126: 111–117. 29. Harper EI, Beck JS, Spence VA. Effect of topically applied local anaesthesia on histamine flare in man measured by laser Doppler velocimetry. Agents and Actions 1989; 28: 192–197. 30. Pipkorn U, Andersson M. Topical dermal anaesthesia inhibits the flare but not the weal response to allergen and histamine in the skin-prick test. Clinical Allergy 1987; 17: 307–311. 31. Maggi CA. Tachykinins and calcitonin gene-related peptide (CGRP) as co-transmitters released from peripheral endings of sensory nerves. Progress in Neurobiology 1995; 45: 1–98. 32. Li YM, Wingrove DE, Too HP, Marnerakis M, Stimson ER, Strichartz GR, Maggio JE. Local anesthetics inhibit substance P binding and evoked increases in intracellular Ca2;. Anesthesiology 1995; 82: 166–173. 33. Björck S, Dahlström A, Johansson L, Ahlman H. Treatment of the mucosa with local anaesthetics in ulcerative colitis. Agents and Actions 1992; (Spec No): C60–C72. 34. Asklin B, Cassuto J. Intravesical lidocaine in severe interstitial cystitis. Case report. Scandinavian Journal of Urology and Nephrology 1989; 23: 311–312. 35. Caire N, Cartier A, Ghezzo H, L’Archeveque J, Malo JL. Inhaled lignocaine does not alter bronchial hyperresponsiveness to hyperventilation of dry cold air in asthmatic subjects. Clinical and Experimental Allergy 1989; 19: 65–70. 36. McAlpine LG, Thomson NC. Lidocaine-induced bronchoconstriction in asthmatic patients. Relation to histamine airway responsiveness and effect of preservative. Chest 1989; 96: 1012–1015. 37. Prakash GS, Sharma SK, Pande JN. Effect of 4 % lidocaine inhalation in bronchial asthma. Journal of Asthma 1990; 27: 81–85. 38. Holmberg K, Bake B, Pipkorn U. The effect of topical nasal anaesthesia on allergen-induced symptoms, obstruction and blood flow. Clinical and Experimental Allergy 1989; 19: 443–447. 39. Chvapil M, Hameroff SR, O’Dea K, Peacock EE jr. Local anesthetics and wound healing. Journal of Surgical Research 1979; 27: 367–371. 40. Morris T, Tracey J. Lignocaine: its effects on wound healing. British Journal of Surgery 1977; 64: 902–903. 41. Brofeldt BT, Cornwell P, Doherty D, Batra K, Gunther RA. Topical lidocaine in the treatment of partial-thickness burns. Journal of Burn Care and Rehabilitation 1989; 10: 63–68.
Analgesic and anti-inflammatory effects of lignocaine–prilocaine (EMLA) cream in human burn injury J. L. PEDERSEN, T. CALLESEN, S. MØINICHE AND H. KEHLET
Summary Pain relief may be improved by reducing sensitization of nociceptive pathways caused by tissue injury. Such a reduction depends mainly on inhibition of local inflammatory changes and the relation between duration of nociceptive block and nociceptive input. In this study we examined if prolonged topical treatment with local anaesthetics could reduce late hyperalgesia and local inflammation after burn injury in healthy volunteers. The effects of EMLA treatment for 8 h after burn on hyperalgesia, inflammation and wound healing were compared with the contralateral placebotreated leg for 48 h after bilateral burn injuries (15 25 mm, 49 ⬚C for 5 min) in a double-blind, randomized study in 12 healthy volunteers. Wound healing was studied 1 and 2 weeks after injury. Neither mechanical nor thermal primary hyperalgesia were affected significantly by prolonged EMLA treatment. Secondary hyperalgesia and skin erythema were also not changed. Seven of 12 placebo-treated legs developed blisters, in contrast with four of 12 EMLA-treated legs. Wound healing showed no apparent differences. Our data suggest that prolonged, topical treatment with local anaesthetics did not reduce local inflammation and late hyperalgesia. (Br. J. Anaesth. 1996; 76: 806–810) Key words Burns. Anaesthetics local, EMLA. Pain, mechanism.
Injury causes inflammation and may lead to prolonged sensitization of nociceptive pathways. Primary and secondary hyperalgesia are well established aspects of this process. Hyperalgesia describes the phenomenon of increased pain intensity in response to normally painful stimuli [1], primary hyperalgesia refers to the changes in sensation within the injury, while secondary hyperalgesia refers to sensory changes in the undamaged tissue surrounding the injury. Several inflammatory mediators promote hyperalgesia by sensitization of nociceptors [2]. Many in vitro and animal studies have shown antiinflammatory effects of local anaesthetics and these effects may potentiate the analgesic effect of these agents. The anti-inflammatory effects may impair the host defence, but may also improve conditions with destructive inflammation. In contrast, there are only few human reports on the anti-inflammatory effects of local anaesthetics and we have shown previously that brief (3.5 h) topical treatment with
local anaesthetics before and after injury did not significantly affect inflammation after burn [3]. The aim of the study was therefore to examine if prolonged and intense treatment with lignocaine– prilocaine cream (EMLA) could reduce late hyperalgesia and other markers of local inflammation after burn injury in healthy volunteers. The treatment was initiated after the burn, as this mimics the clinical situation.
Subjects and methods We studied 12 healthy volunteers, aged 22–47 yr (all male), after obtaining informed consent and approval from the local Ethics Committee and the Danish National Health Board. Subjects were not medically knowledgeable and were not aware of the purpose of the study. Baseline measurements, which included assessment of skin colour, mechanical pain detection threshold (MPDT) and heat pain detection threshold (HPDT), were performed on the medial calf in both legs. After obtaining baseline measurements, thermal injury was induced on each leg. All measurements, that is skin erythema, blister formation, MPDT, secondary hyperalgesia and HPDT, were made in this order, 16, 20, 24 and 48 h after the injuries. Immediately after the injury, EMLA or placebo was applied, double-blind and randomized, to the skin and covered under occlusive dressing for 8 h after the injury. Experiments were performed in a quiet room, temperature 20–22 ⬚C, with subjects in a relaxed and reclined position. Subjects were instructed to keep their eyes closed during all measurements so that they had no impression of the results of the measurements. EMLA TREATMENT
EMLA cream 2 g (2.5 % prilocaine and 2.5 % lignocaine eutectic mixture, Astra, Södertälje, Sweden) or placebo (Astra; Södertälje, Sweden) was applied in a double-blind, randomized manner to the skin and covered under occlusive dressing (Tegaderm, 3M Health Care Ltd, Leicestershire, UK) for 8 h after the injury. The cream was renewed JURI L. PEDERSEN, MD, TORBEN CALLESEN, MD, HENRIK KEHLET, MD, PHD (Department of Surgical Gastroenterology); STEEN MØINICHE, MD (Department of Anaesthesiology); Hvidovre Hospital, University of Copenhagen, Kettegaard Allé 30, 2650 Hvidovre, Denmark. Accepted for publication: January 24, 1996. Correspondence to J.L.P.
Effect of EMLA in human burns 3 and 5.5 h after initial application, and re-covered. Subjects were instructed to avoid physical activities that could displace the thick cream layer from the injury. The burns were performed in the afternoon and the cream was removed 8 h later (late evening). Subjects attended next morning for the first measurement, 16 h after the injury. THERMAL INJURY
Thermal injuries were produced on the medial surface of the right and left calf with a 1525-mm Peltier thermode (Thermotest, Somedic A/B, Stockholm, Sweden). The thermode (49 ⬚C) was applied to the skin for 5 min under standardized pressure (34 mm Hg), causing first- and superficial second-degree burn injuries [4, 5]. The burn caused immediate intense stinging pain, which was followed by a moderate burning pain with a more diffuse quality during the remainder of the stimulation period. Spontaneous pain after the injury has not been observed in the model. PRIMARY HYPERALGESIA
Mechanical pain detection threshold (MPDT) within the injured area was determined by pinprick with nine progressively rigid von Frey hairs (Somedic A/B, Stockholm, Sweden). We examined the force produced by each hair by pressing it against a balance and measured the weight it produced when it was slightly flexed. We confirmed that the steps from 1 to 9 represented logarithmic increases in force (hair no. 1 : 3 mN, 2 : 9 mN, 3 : 11 mN, 4 : 18 mN, 5 : 29 mN, 6 : 49 mN, 7 : 98 mN, 8 : 98 mN, 9 : 175 mN). MPDT was defined as the lowest force (pinprick) which produced a definite sensation of pain or discomfort. Pinprick was performed with each hair from the thinnest, 8–10 times in the area of the injury, until some of the 8–10 stimulations were reported to be painful or unpleasant. The number of the hair producing pain was recorded and the procedure was repeated three times at each measurement point. The median of three threshold measurements was reported as the MPDT. If hair No. 9 did not produce any pain or discomfort, we assigned this observation the value 10. Thermal thresholds were determined with the computerized Thermotest (Somedic A/B, Stockholm, Sweden) in the area of the injury. Heat pain detection threshold (HPDT) was the lowest temperature perceived as painful. Subjects were instructed to press a button as soon as the specified sensation was reached. Thermal thresholds were determined from a baseline of 32 ⬚C with a rate of change of 1 ⬚C s91. The upper cut-off limit was 52 ⬚C. HPDT was calculated as the average of three measurements, with intervals of 9 s between each stimulation. SECONDARY HYPERALGESIA
The area of mechanical hyperalgesia that developed around the burn injury was assessed with von Frey hair No. 9 (175 mN). In most cases pinprick with
807 this hair is a painful or unpleasant stimulus in distinct spots of the skin, although it is not universally painful. Thus areas of hyperalgesia were measured, not areas of allodynia. Borders of hyperalgesia were determined by stimulating along eight radial linear paths arranged from the centre of injury. Stimulation along each path began well outside the hyperalgesic area where slight or no pain was perceived and continued slowly towards the centre of the burn injury until the subject reported a definite change in sensation, most often to a more intense pricking with a burning after-sensation. This point was marked and the eight marks were later traced onto a clear acetate sheet. The area of secondary hyperalgesia was calculated from the eight marks using a vector algorithm. INFLAMMATION
In order to estimate the severity of inflammation, the intensity of erythema inside the injury was assessed with a hand-held skin reflectance spectrophotometer (Dermaspectrometer, Cortex Technology, Hadsund, Denmark) [6, 7]. The spectrophotometer provided a skin erythema index based on the absorption characteristics of green and red light in the skin. Measurements were performed in six spots within the burn injury. All values were recorded and the mean calculated. The development of blisters was recorded as present or absent. Healing of the burns was documented using a questionnaire, 1 and 2 weeks after the injury.
STATISTICAL ANALYSIS
Distribution of data was evaluated with the Shapiro Wilk test [8]. Primary mechanical hyperalgesia and secondary hyperalgesia in EMLA- and placebotreated legs were evaluated at separate times with multiple Wilcoxon tests for paired observation with Bonferroni correction and area under the curves (AUC) for the whole observation period with Student’s t test for differences in a paired design [9]. Evaluation of primary thermal hyperalgesia and erythema was based on AUC values but analysed also using parametric two-way analysis of variance (ANOVA) for repeated measurements [8]. Minimal differences between treatments detectable in the present study with a power of 90 % and a type I error of 5 % were evaluated with Student’s t test for differences in a paired design [8]. P 0.05 was considered statistically significant.
Results The area of hyperalgesia surrounding the injury was not significantly altered by EMLA treatment compared with placebo (fig. 1 A). The effect of EMLA was evaluated at all times (16 h, P : 0.68; 20 h, P : 0.37; 24 h, P : 0.14; and 48 h, P : 0.07; Wilcoxon test). The level of significance was reduced to 1 % in these comparisons with Bonferroni correction, because of the multiple comparisons. The area under the curve (AUC) for hyperalgesia was applied as a
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British Journal of Anaesthesia
Figure 1 A: Secondary hyperalgesia, that is area of hyperalgesia to von Frey pinprick (175 mN) outside the injury, after burn injury in healthy volunteers treated with EMLA (. . . .) or placebo (——). Time 0 h : measurements before thermal injury. Values are medians. Comparison of AUC values revealed no significant difference between secondary hyperalgesia in EMLA- vs placebo-treated legs (P : 0.18; Student’s paired t test, n : 12). B: Primary mechanical hyperalgesia, that is mechanical pain detection threshold to von Frey pinprick, in the area of the burn injury. Values are medians. Comparison of AUC values revealed no significant difference between primary mechanical hyperalgesia in EMLA- (. . . .) vs placebo- (——) treated legs (P : 0.19; Student’s paired t test, n : 12). Von Frey hair no. (1–10) represents a rank scale, where 1 indicates a force of about 3 mN and 9 about 175 mN, whereas 10 indicates that 175 mN did not produce pain or discomfort. The steps from 1 to 9 are logarithmic increases in force (see text). C: Primary thermal hyperalgesia, that is heat pain detection threshold (HPDT) in the area of the burn injury. Values are means. Comparison revealed no significant difference between primary thermal hyperalgesia in EMLA (. . . .)- vs placebo (——)-treated legs (P : 0.61; parametric twoway analysis of variance for repeated measurements, n : 12). D: Skin erythema, that is the intensity of erythema inside the burn injury assessed with a hand-held skin reflectance spectrophotometer. Values are means. Comparison revealed no significant difference between erythema in EMLA (. . . .)- vs placebo (——)-treated legs (P : 0.34; parametric two-way analysis of variance for repeated measurements, n : 12). Table 1 Positive answers out of 12, in a questionnaire (yes/no) completed by the 12 subjects
Changes in injury
Placebo (1 week)
EMLA (1 week)
Placebo EMLA (2 weeks) (2 weeks)
Tenderness Irritated by touch Itch Any colour change Crusts Blister Skin erythema
1 2 3 7 3 2 6
0 0 2 9 3 2 6
0 0 3 6 3 1 4
0 0 1 5 1 0 0
summary measure of the total hyperalgesic response. AUC for secondary hyperalgesia after EMLA treatment was not significantly different from that after placebo (P : 0.18; Student’s paired t test). The effect of EMLA on primary mechanical hyperalgesia (fig. 1 B) was analysed in the same way and there was no significant differences between EMLA and placebo (0 h, P : 0.20; 16 h, P : 0.28; 20 h, P : 0.29; 24 h, P : 1.00; and 48 h, P : 0.25; Wilcoxon test). Comparison of AUC values confirmed this finding (P : 0.19; Student’s paired t test). Data for primary thermal hyperalgesia (fig. 1 C) and skin erythema index (fig. 1 D) were analysed with both AUC summaries and parametric two-way ANOVA for repeated measurements. There were no significant differences between EMLA and placebo with both methods (primary thermal hyperalgesia: AUC, P : 0.80 and ANOVA, P : 0.61; and ery-
Table 2 Minimum differences between treatments detectable with a power of 90 % (type II error of 10 %) and a type I error of 5 %. Differences are presented as the weighted mean for the observation period (AUC divided by 48 h)
Secondary hyperalgesia (cm2)
Primary mechanical hyperalgesia (von Frey No.)
Primary thermal hyperalgesia (°C)
Erythema index (arb. unit)
13
0.9
2.4
1.3
thema: AUC, P : 0.54 and ANOVA, P : 0.34). Seven of 12 placebo-treated legs developed blisters in contrast with four of 12 EMLA-treated legs. With binomial data (yes/no), these small numbers precluded a sensible statistical analysis. There were no apparent differences in wound healing between EMLA and placebo treatment (table 1). Minimal differences between treatments detectable in this study, with a power of 90 % and a type I error of 5 %, are presented in table 2. Differences are presented as the weighted mean for the observation period (AUC divided by 48 h).
Discussion We examined if prolonged and intensive topical treatment with local anaesthetics reduced late hyperalgesia, blister formation or erythema after thermal injury in healthy volunteers in a double-blind, randomized study. We found that the local anaesthetics neither reduced local inflammation and late
Effect of EMLA in human burns hyperalgesia nor produced apparent changes in wound healing, although the sample size does not exclude minor differences. This result may seem surprising, considering the known anti-inflammatory effects of local anaesthetics. Several in vitro studies have demonstrated reduced production of free oxygen radicals from polymorphonuclear leucocytes after treatment with local anaesthetics [10–14], and reduced adherence and migration of leucocytes have been documented in different in vivo models [15–18]. Furthermore, local anaesthetics inhibit movement of leucocytes and phagocytosis in vitro [14, 19–21]. Local anaesthetics have also been shown to reduce albumin extravasation in rats after burn injury and chemical peritonitis [22, 23] and to modify the inflammatory response to experimental colitis [24] and irradiation [25]. Local anaesthetics may alter release of inflammatory mediators, for example histamine release from mast cells in vitro [26], leukotriene B-4 and interleukin-1 release from polymorphonuclear granulocytes and mononuclear cells in vitro, respectively [27]. The flare reaction induced by trauma and injection of inflammatory mediators is diminished by local anaesthetics, in contrast with the weal and local red reaction [28–30]. Thus local anaesthetics block axon reflexes and sensory functions of nociceptors. Inhibition of axon reflexes reduces local, indirect release of inflammatory neuropeptides, but the clinical importance of diminished neurogenic inflammation is unclear. It is not known if the local effector function of directly stimulated nociceptors is affected by local anaesthetics [31]. Furthermore, local anaesthetics have been shown to inhibit substance P receptor binding [32]. Thus several anti-inflammatory effects of local anaesthetics have been demonstrated in vitro and in animal models, but evidence of effects in human studies is, to our knowledge, restricted to one uncontrolled study and one case report, where lignocaine improved ulcerative proctitis and severe interstitial cystitis, respectively [33, 34]. Topical lignocaine has been tested several times in asthmatic subjects with conflicting, but mainly negative results [35–37]. Topical lignocaine had no effects on allergeninduced nasal secretion, blood flow or obstruction, in allergic individuals [38]. We have shown earlier that short-lived topical application, or subcutaneous infiltration with local anaesthetics, did not affect inflammation after burn [3]. This study, with prolonged and intensive treatment, supports this conclusion. Our findings, and the lack of other clinical evidence, may therefore question the relevance of the anti-inflammatory effects of local anaesthetics. There may be different explanations for our results. First, the response involves several inflammatory mediators and cell types, which may not be affected by local anaesthetics. Second, the timing of the treatment (before compared with after injury) may be important in modulating inflammatory responses, as early mediators release cascades of secondary mediators. In this study we preferred treatment after injury, as this mimics most clinical situations, while treatment was initiated before
809 injury in our first study [3]. Apparently, this did not affect the results. Third, the effects of prolonged topical treatment were either too transient or incapable of altering the course of the inflammatory response. Although the analgesic effects did not last into the observation period (16–48 h after injury), we do not question the effectiveness of EMLA cream in pain relief, but effects on hyperalgesia should not be expected for prolonged periods (more than 8 h) after removal of the cream. The potential reduction of leukotriene B-4, interleukin-1, histamine, free oxygen radicals and activated leucocytes by local anaesthetics [27] should attenuate sensitization of nociceptors in the injury, but removal of EMLA cream 8 h before the first measurements may have returned the process to its natural course. Fourth, we studied only 12 subjects and we may have missed some real differences. The minimal differences between treatments detectable in this study, with a power of 90 %, are shown in table 2. The consequences for wound healing after treatment with local anaesthetics are controversial. The synthesis of collagen and glycosaminoglycans by fibroblasts in vitro is reduced by local anaesthetics [39], as is the tensile strength of skin wounds in rats [40]. Other studies [41], including our own, have been unable to demonstrate adverse clinical effects on host defences and wound healing, but our study is less specific in this respect than the first mentioned studies, and prolonged use of local anaesthetics may threaten tissue defences.
Acknowledgements The study was supported by grants from the John and Birte Meyer Foundation, the P. A. Messerschmidt and Wife’s Fund, Fonden til Laegevidenskabens Fremme and Astra Pain Control, Södertälje, Sweden.
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