Anesthetic patches of lidocaine in acid mantle cream have been used for cutaneous anesthesia. ¹¹ However, when we performed in vitro penetration studies we found that only a small amount of lidocaine mixed in acid mantle cream was absorbed percutaneously compared with that mixed in DMSO. Similarly, although EMLA is a popular anesthetic for reducing superficial pain,⁹ it delivered very little total anesthetic drug within two hours of application.

Tetracaine base 33% in 100% DMSO was used to achieve dermal anesthesia. ¹² Local anesthetic bases are readily soluble in DMSO compared with their salts. Dimethyl sulfoxide has been used to increase penetration of chemicals applied to the skin in percutaneous absorption studies, however, these formulations have not received Food and Drug Administration approval for commercial use. The major disadvantage of DMSO is the whealing response, which is strongly influenced by body region and dose. ¹³ It was necessary for us to outline the lesion with a ball-point pen prior to applying the solution so that we could tell where the vascular lesion stopped.

Various methods to increase percutaneous absorption of medicines have been used. lontophoresis, the electrical enhancement of ionic transport, increases penetration of topical agents but requires special equipment. ¹⁴ We were interested in finding an easy topical method of achieving anesthesia without special equipment.

Side effects of topical anesthetics can include mild stinging and itching at the site of application. We had no patients who complained of this. Initial transient local redness was seen in all patients, but disappeared by the time of laser treatment.

Major toxic effects of lidocaine involve the heart and central nervous system (CNS). ¹⁵ Cardiac effects occur because of direct action causing heart block in patients with preexisting bundle branch disease. The CNS effects can occur when the serum lidocaine concentration is 5 mg/L or greater. Dizziness, drowsiness, perioral paresthesias, and tinnitus may be noted early. More toxic effects are delirium, disorientation, convulsions, and coma. The amount of drug absorbed percutaneously in our preparation using six drops or fewer was far below the toxic range. The maximum amount of lidocaine that could be absorbed was 75 mg over 24 hours (based on the time course of penetration shown in Figure 1), assuming that none of the applied dose was rubbed or washed off during or after the clinic visit. Our dose of topical lidocaine, even if totally absorbed, was far less than what is recommended for maximum intralesional use, recommended at 4.5 mg/kg of body weight for children or adults. We saw no evidence of ECG anomalies, cardiac, or CNS disturbances.

Measurement of pain or anesthesia is difficult to assess and is highly subjective. Visual analog scales have a higher sensitivity in adults than the traditional simple descriptive verbal scale. ¹⁶ We used both in our study, mainly to confirm that patients understood the procedure and the scale. Because many of our patients were young, we also wanted to verify their understanding of the scale.

We found 25% lidocaine base mixed in 70% DMSO/ethanol to be an effective and well-tolerated topical anesthetic that can be applied in the office prior to treatment with the pulsed dye laser. This approach did not produce total anesthesia, but was sufficient to allow the patient to complete a treatment with minimum discomfort as opposed to having to come back several times for therapy. The alternatives, general anesthesia and conscious sedation, are not accepted by many patients. One word of caution should be mentioned. As in the case of injectable lidocaine, limits as to the total amount of topical lidocalne used should be established. Large areas should not be freely painted with this mixture, nor should it be used on infants without considering systemic absorption.

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^aDivision of Dermatology, Washington University School of Medicine, St. Louis, Missouri.

^bDepartment of Dermatology, University of Arkansas for Medical Sciences, Little Rock, Arkansas.

Address correspondence to: Thomas J. Franz, MD, Department of Dermatology, MS 576, University of Arkansas for Medical Sciences, 4301 W. Markham Street, Little Rock, AR 72205- 7199. No reprints available.

*This article appeared in Pediatric Dermatology, Volume 10, Number 4, pp. 370-375.

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