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Anesthesia & Analgesia:
doi: 10.1213/01.ANE.0000111206.50145.47
Anesthetic Pharmacology: Research Report

Skin Sensitivity to Rocuronium and Vecuronium: A Randomized Controlled Prick-Testing Study in Healthy Volunteers

Dhonneur, Gilles MD*,; Combes, Xavier MD*,; Chassard, Didier MD†,; Merle, Jean Claude MD*

Section Editor(s): Bovill, James G. Section Editor

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Author Information

*Department of Anesthesia and Critical Care Medicine, University Hospital and Paris XII Val-de-Marne School of Medicine, Créteil, France; and

†Clinical Research Organisation, CEPHAC.ASTER Institut, Paris, France

Supported by Organon-Teknika, The Netherlands.

Accepted for publication November 18, 2003.

Address correspondence and reprint requests to Dhonneur Gilles, MD, Département d’Anesthésie et de Réanimation, CHU Henri Mondor, 94000, Créteil CEDEX, France. Address e-mail to

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Prick tests are frequently used for the authentication of neuromuscular blocking drugs (NMBDs) as causative drugs for anaphylactic reactions during anesthesia. Unfortunately, the actual threshold concentration for skin testing remains debatable for most NMBDs. We studied the flare and wheal responses to prick tests with rocuronium and vecuronium. Thirty healthy, nonatopic, anesthesia-naive male and female volunteers (14 men and 16 women) from 18 to 40 yr of age were assigned randomly to receive a total of 10 prick tests—4 ascending dilutions (1:1000, 1:100, 1:10, and 1) of rocuronium and vecuronium and 2 controls—on both forearms. An assessor blinded to the assignment monitored systemic and skin responses to NMBDs and measured wheal and flare surfaces immediately after and 15 min after prick tests. None of the volunteers experienced any immediate systemic or cutaneous responses to rocuronium or vecuronium. Although a dilution of 1:1000 of both NMBDs failed to promote any skin response at 15 min, 50% and 40% of the subjects had a positive skin reaction to undiluted rocuronium and vecuronium, respectively. We demonstrated a sex effect related to smaller threshold concentration-induced cutaneous reactions in female volunteers to both muscle relaxants. Our observation questions the reliability of prick testing with undiluted solutions of rocuronium and vecuronium for the diagnosis of allergy.

Neuromuscular blocking drugs (NMBDs), especially steroid-derived drugs, have been frequently identified as potential causative drugs (1–3) of drug-related anaphylactoid and anaphylactic reactions that occur during anesthesia. Although a sequential analysis of plasma tryptase was initially performed, it remains difficult to obtain indisputable proof that adverse clinical manifestations are true anaphylactic reactions. The allergic mechanism of a reaction is usually confirmed retrospectively by both in vitro blood tests and investigations that use the skin as an immunological window, such as intradermal and prick testing. On the basis of positive results of skin testing, numerous reports of anaphylaxis have implicated NMBDs as offending drugs, but incidences vary greatly. Unfortunately, the absence of clear internationally accepted guidelines on skin testing may explain the apparent different incidences of allergy to NMBDs among countries (4). Intradermal tests are often polluted by false-positive tests unrelated to the mast cell degranulation (5,6) that occurs at small concentrations (7). However, prick testing, the response of which depends mainly on the presence of sensitized lymphocytes, is not always reliable because the recommended threshold test concentrations have not been defined, resulting in unreliable results. This study was performed to determine the concentration-response curves for prick tests with rocuronium and vecuronium in healthy volunteers.

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The Research Ethics Committee of our university hospital approved this trial. Thirty healthy volunteers—14 men and 16 nonpregnant, non-breast-feeding women, 18 to 40 yr of age, who had given proper written informed consent—were recruited. Subjects who had previously worked in the cosmetic or food industry or had ever had general anesthesia, anaphylaxis, asthma, or personal or familial atopic diseases were excluded. Volunteers taking or suspected to have recently taken steroids, antidepressants, neuroleptics, or antihistamines (H1 or H2) and those with highly pigmented skin were not included in this study.

All volunteers received 10 prick tests (50 μL) on both forearms; the tests were performed by a single assessor who had significant experience in skin testing (Phase I clinical research organization). First, subjects were administered control substances: phenol saline glycerol (negative control; Stallergènes S.A.) and codeine phosphate 9% (positive control; Stallergènes S.A.) on the left and right forearm, respectively. Then, independently of the size of the cutaneous reactions resulting from these controls, each relaxant—rocuronium bromide 10 mg/mL (Org 9426, Esmeron®; NV Organon) and vecuronium bromide 1 mg/mL after reconstitution (Org NC45, Norcuron®; NV Organon), used as commercially available (undiluted) and diluted (chlorure de sodium 0.9%; Lavoisier) to 1:10, 1:100, and 1:1000 solutions—was tested, beginning with the left arm and starting with the smallest concentration. Assignment of rocuronium and vecuronium to the right or left forearm was performed in a randomized manner. The assessor who administered preservative-free freshly prepared study medications was blinded to the NMBD assignment. After 4 prick tests of ascending dosage of the first relaxant had been performed (10 min) without systemic reactions, the second NMBD was tested immediately under the same conditions on the right forearm. A second assessor blinded to study conditions recorded and measured skin reactions immediately after the prick tests were applied and 15 min later. A wheal was defined as a smooth, elevated pale (or slightly pink) skin area encircling the prick test. The red outermost zone surrounding a wheal characterized a flare. For each wheal and flare, the largest and smallest diameters were measured and averaged, allowing calculation of areas of skin reactions. The association of a wheal and a flare defined a positive skin reaction independently of their respective size. The circumferences of the wheal and flare were also marked out on the skin by using a black pencil. Drawings were covered with tape, stuck to a master sheet, and used as source data. Descriptive statistical analysis was performed; χ2 and Student’s t-tests were used for comparisons.

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Demographic characteristics of the volunteers are presented in Table 1. None of the subjects experienced any systemic or immediate skin reactions to either NMBDs or control solutions. Data for the subjects with positive skin reactions at the 15-min time point are shown in Table 2 and Figures 1 and 2. All volunteers demonstrated the association of a wheal and a flare with positive controls, but none reacted to negative controls. The number of subjects with skin reactions increased with increasing concentrations of rocuronium and vecuronium. Prick tests to undiluted rocuronium and vecuronium resulted in 50% and 40% positive skin reactions, respectively. None of the volunteers experienced any skin reaction to either rocuronium or vecuronium with a dilution of 1:1000. All the subjects who reacted to a given dose of rocuronium or vecuronium or both also reacted to larger doses. We constructed the dose-response curves of wheal and flare surfaces for both relaxants (Fig. 2). Mean wheal flare surfaces increased proportionally with increasing concentrations to reach 21–176 mm2 and 18–162 mm2 with undiluted rocuronium and vecuronium solutions, respectively. Because of the large interindividual variability of wheal and flare surfaces at each reactive concentration, rocuronium and vecuronium were considered to be bioequivalent in terms of skin sensitivity. More female than male volunteers significantly (P < 0.01) reacted to smaller vecuronium and rocuronium concentrations (Table 2). Positive skin reactions with a 1:100 dilution of each NMBD were seen only in female subjects, and seven of nine volunteers who reacted simultaneously to both muscle relaxants were also female subjects (Table 2).

Table 1
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Table 2
Table 2
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Figure 1
Figure 1
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Figure 2
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The results of our study show that approximately one-half of healthy, anesthesia-naive adult volunteers have skin reactions to prick tests when undiluted rocuronium and vecuronium are used. Under the conditions of this trial, rocuronium and vecuronium were bioequivalent in terms of skin sensitivity. We observed a sex effect that was related to smaller threshold concentration-induced cutaneous reactions to both rocuronium and vecuronium in female volunteers.

In recent guidelines (8) for the investigation of anesthetics as a cause of anaphylaxis during anesthesia, the Société Française d’Anesthésie et de Réanimation published recommendations concerning most NMBDs for skin tests. The French group recommended using undiluted solutions of rocuronium and vecuronium for prick tests because these concentrations are considered to be normally nonreactive in control subjects. Our observations contrast with these recommendations.

We demonstrated that almost half of the volunteers had positive skin reactions when pricked with undiluted solutions of rocuronium and vecuronium and that nearly 20% reacted to the 1:10 diluted solution of both NMBDs. The nonreactive concentration in our healthy volunteers was the 1:1000 dilution of the stock solutions. When we applied the French recommendations for prick test validity (9) to our study, most of our healthy volunteers would have fulfilled the criteria for allergy to rocuronium, vecuronium, or both (30%, 15%, and 20%, respectively). The concentrations of rocuronium and vecuronium recommended by the Société Française d’Anesthésie et de Réanimation for prick testing may be excessive, and the frequent incidence of positive cutaneous responses to rocuronium and vecuronium reported in our country probably contain some false positives. Our observations, supported by two recent studies (7,10) that demonstrated the frequent false-positive rate during skin testing in healthy volunteers, may explain the intriguingly frequent incidence of alleged allergic reactions to muscle relaxants in France. We consider that undiluted and fractional dilutions (1:10 and 1:100) of rocuronium and vecuronium are associated with frequent false-positive skin reactions in healthy volunteers. Assigning causality for allergic reactions to muscle relaxants on skin tests that are shown to be associated with frequent false positives must be questioned.

The site chosen in our study for skin testing might have influenced our results. Although this is usually performed on the anterior face of the forearm, some investigators have favored the patient’s back because this skin is less exposed to environmental modifications and has similar characteristics in all individuals. However, most reports recommend that NMBD response prick tests should be performed on the anterior face of the forearm (11–13).

Our results support other reports suggesting that steroid-derived drugs can induce positive wheal and flare responses independently of mast cell degranulation, even at small concentrations. These positive reactions are probably due to a direct effect on the inflammatory cells and the vascular system and may occur for most NMBDs at molar concentrations (5,9). Using intradermal skin tests, in which the tested compound is presented directly to skin mast cells, Levy et al. (7) tested 30 volunteers and observed a positive cutaneous reaction in almost all volunteers without evidence of mast cell degranulation at a 1:100 dilution of a stock solution of rocuronium. We evaluated prick tests in which indirect contact between the test compound and skin mast cells is supposed to filter out reactions caused by nonimmunological mast cell activation (14–16). We demonstrated positive skin reactions with undiluted rocuronium and vecuronium solutions in a large proportion of nonsensitized individuals. These reactions are probably the result of the direct vasodilating effects of rocuronium and vecuronium (17) on the skin. Using the flare surface as a marker of this dilating effect, we demonstrated an inverse relationship between dilution and skin responses with both rocuronium and vecuronium.

Moreover, we observed a sex effect, which suggests that more female than male subjects reacted to smaller rocuronium and vecuronium concentrations. Indeed, the nonreactive concentration of both rocuronium and vecuronium was 10-fold larger in male than in female volunteers. Although anaphylactoid reactions to NMBDs occur more often in women (18), such an effect in nonexposed volunteers had never been observed. The reason for this sex effect is unknown. Immune sensitization as a result of the widespread exposure to quaternary ammonium structures is unlikely, but it cannot be completely eliminated. A smaller threshold concentration for rocuronium- and vecuronium-induced direct vasodilation in more female than male subjects is also possible.

In this study we analyzed the dose (prick test-induced) skin response to increasing concentrations of rocuronium and vecuronium. We demonstrated that rocuronium and vecuronium were bioequivalent in terms of skin sensitivity; a nonreactive concentration for both muscle relaxants was the 1:1000 dilution of the stock solutions. Our observation questions the reliability of prick testing with an undiluted solution of rocuronium and vecuronium for the diagnosis of allergy. Because atopic or allergic individuals are likely to have a lower drug concentration threshold for positive skin reactions than healthy volunteers, recommendations pertaining to the dilution of NMBDs for prick-testing patients who have experienced a reaction during anesthesia remain hazardous. Finally, the entire issue of whether NMBDs are the principal cause of anaphylaxis during anesthesia is called into doubt. Indeed, the Danish Anesthesia Allergy Group (19) suggested that other drugs, such as chlorhexidine, might cause more allergic reactions than NMBDs.

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