Chapter 124. Topical Anesthesia

Invasive procedures can cause significant anxiety in patients both young and old, much of which is related to fear of the associated pain. Topical anesthesia has been shown to decrease pain and anxiety surrounding procedures such as lumbar puncture, intravenous access, and laceration repair.1 This chapter will discuss topical anesthetic agents and the range of techniques that are available for delivery of these agents.

Mechanical, thermal, and chemical stimuli are detected by nerve endings called nociceptors. These pain receptors are located in the skin, specifically in the dermis and the epidermis, below the stratum corneum (Figure 124-1). Nociceptors are free nerve endings that have their cell bodies outside the spinal column in the dorsal root ganglia. The intact stratum corneum, the outer layer of cornified epithelial cells of the skin, is an effective barrier to the outside environment. Local anesthetics must transverse the stratum corneum to be delivered to the terminals of cutaneous sensory nerve fibers. The three methods by which the stratum corneum can be bypassed to deliver a local anesthetic are direct injection, passive diffusion, and needle-free drug delivery strategies. Refer to Chapter 123 for the complete details regarding the direct injection of local anesthetic solution. This chapter will discuss techniques for enhancing passive diffusion and needle-free administration of local anesthetic agents, collectively known as topical anesthesia.

Topical anesthesia is commonly utilized in the Emergency Department in two situations. The first is the patient experiencing pain from an injury such as a laceration, abrasion, or contusion. The second is in the patient who will undergo a painful procedure such as venipuncture, lumbar puncture, abscess incision and drainage, or laceration repair.

Topical anesthetic agents offer several potential advantages over local infiltration anesthesia. They are less painful to apply, do not distort the wound margins, and decrease the need for sedation.2 The major limitations of topical anesthesia have been the extended time required to achieve anesthesia and the lack of sufficient analgesia in many clinical situations that often requires supplemental infiltration anesthesia. These constraints have limited the use of topical anesthesia in the Emergency Department. Several new agents and delivery techniques have addressed these limitations with some success, offering the Emergency Physician more options for providing anesthesia.

There are very few contraindications to the use of topical anesthetics. The topical anesthetic agent is typically not systemically absorbed to any significant degree. However, they can cause local adverse reactions and should not be used in patients who are allergic to the medication or its components (such as preservatives). Use care when applying topical anesthetics to mucous membranes, as absorption is typically more rapid and efficient than through skin and more substantial systemic absorption can occur. A relative contraindication to using topical anesthetic agents are patients taking Class 1 antiarrhythmics such as mexiletine and tocainide, as they can produce additive and possibly toxic effects. Many of the topical anesthetics are contraindicated or require special ophthalmic formulations for use in the eye. When using topical anesthetics in neonatal patients, special care and attention to dosing is imperative to avoid toxicity. Specific contraindications and concerns for each topical agent and application technique are addressed later in this chapter as well as in Chapter 123.

General Supplies

• Alcohol swabs
• Povidone iodine or chlorhexidine solution
• Gauze squares, 2 × 2 and 4 × 4
• Gloves

Topical Cream, Gel, or Liquid

• TAC (0.5% tetracaine, 1:2,000 epinephrine, and 11.8% cocaine)
• LET (0.5% tetracaine, 1:2,000 epinephrine, and 4% lidocaine)
• EMLA (2.5% lidocaine and 2.5% prilocaine)
• LMX-4 or LMX-5 (4% or 5% liposomal lidocaine)
• Topicaine (4% lidocaine gel)

Iontophoresis

• 2% lidocaine hydrochloride containing 1:100,000 epinephrine
• Device to administer iontophoresis (e.g., Phoresor II or Dupel)
• Iontophoresis System

Ultrasound-assisted Local Anesthetic Delivery

• Ultrasound device (e.g., SonoPrep)
• Topical local anesthetic agent

Powdered Anesthetics

• Needle-free powder lidocaine delivery system

Lidocaine Needle-free Injection

• Needle-free injection system, prefilled or fillable
• 1% or 2% lidocaine

Heat-enhanced Diffusion

• Lidocaine–tetracaine thermal patch

Laser-assisted Transdermal Passage

• Cutaneous resurfacing laser
• Topical anesthetic agent

Topical Vapocoolant Sprays

• Topical vapocoolant spray

Discuss the procedure, its risks, benefits, complications, and alternatives with the patient and/or their representative. Take precautions to prevent further injury or pain to the patient. Cleanse the skin of any dirt and debris. Apply povidone iodine or chlorhexidine solution to the skin and allow it to dry. Consider administering a supplemental anxiolytic or procedural sedation if required.

Topical local anesthetic agents are often applied in the form of a gel or cream. There are numerous combinations of anesthetic agents to use topically on wounds. TAC (tetracaine, adrenaline, and cocaine), LET (lidocaine, epinephrine, and tetracaine), and EMLA (eutectic mixtures of lidocaine and prilocaine) are three commonly applied combinations (Table 124-1). Other topical local anesthetic agents include lidocaine gel and liposomal lidocaine.

TAC

Many Emergency Departments use TAC for topical anesthesia. It is a combination of 0.5% tetracaine, 1:2000 or 0.05% adrenalin (epinephrine), and 11.8% cocaine. TAC can be purchased or Hospital Pharmacists can compound these agents into a gel or liquid. It can be partitioned into individual use vials for easy application. The degree of anesthesia achieved with TAC is comparable to that of local infiltration with lidocaine for wounds on the face and scalp.3 The effects are less profound, however, for wounds on the trunk and extremities.

The cocaine component in TAC is a powerful and effective local anesthetic agent as well as a vasoconstrictive agent. Unfortunately, TAC is a controlled substance. This makes its storage, utilization, and monitoring subject to stringent regulation and documentation requirements. This may limit the availability and use of TAC in some institutions.

Dose recommendations generally call for 5 mL of TAC for lacerations smaller than 3 cm in length and 10 mL for lacerations greater than 3 cm in length.3 Apply the chosen volume of TAC liquid or gel onto a 2 × 2 gauze square or a cotton ball. Invert the gauze square or cotton ball to apply TAC within the wound margins. Allow TAC to remain for up to 15 minutes or until visible skin blanching occurs. Blanching indicates the presence of vasoconstriction and adequate analgesia. The gauze pad can be held in place by the patient's gloved hand, the patient's representative's gloved hand, or with tape.

TAC is not free of complications. Always wear gloves when handling TAC to prevent percutaneous absorption. Do not apply TAC to tissues with an end-arteriole supply. This includes the fingers, toes, nose, ear, and penis. There is the possibility of vasoconstrictive ischemic injury to these tissues. The systemic absorption of cocaine as a result of TAC administration has been implicated in rare episodes of respiratory arrest, seizures, and death.4,5

LET

LET is an alternative to TAC. It is a combination of 4% lidocaine, 0.1% (1:1000) adrenaline (epinephrine), and 0.5% tetracaine. It is considered safer, similarly effective, more practical, and more cost effective to use than TAC. LET does not contain a controlled substance and has no potential for abuse. The resultant security and documentation requirements are therefore much less complicated than those required for TAC. Hospital Pharmacists can compound this agent.

Apply liquid LET by dripping it into the wound or taping a LET soaked cotton ball or 2 × 2 gauze square over the wound. The addition of methylcellulose to liquid LET makes a gel that is more adherent and can be painted on wounds. The gel form will not drip or run. Similar to TAC, it should not be placed on the digits, ear, nose, penis, or other areas that are supplied by end arteries due to the strong vasoconstrictive effect and risk of ischemia. This dogma is currently being challenged.6

EMLA Cream

The use of EMLA (eutectic mixture of local anesthetics) cream has gained significant popularity, particularly in pediatric patients. It is an emulsion of 2.5% lidocaine and 2.5% prilocaine in a 1:1 ratio by weight. Each gram of EMLA contains 25 mg of lidocaine and 25 mg of prilocaine. Apply EMLA only onto intact skin and not into open wounds. It is nonsterile and preservative free. It is also commercially available in prepackaged transdermal disks.

EMLA is indicated prior to performing venipuncture, arterial punctures, accessing indwelling ports and reservoirs, lumbar puncture, minor skin procedures, or regional nerve blockade. Apply EMLA cream to intact skin and cover it with an occlusive dressing (e.g., Tegaderm) or apply a transdermal disk. Allow at least 1 hour for the EMLA to take effect. Analgesia is usually satisfactory after 1 hour, peaks at 2 hours, and persists for about 1 hour after it is removed from the skin. The prolonged time required for anesthesia to take effect limits its practical use in the Emergency Department. EMLA should not be applied to infants less than 3 months of age due to the risk of methemoglobinemia.7

Liposomal Agents

Liposomal Lidocaine (LMX-4 or LMX-5) is a 4% or 5% lidocaine cream that is a liposome-encapsulated formulation. The encapsulation of lidocaine keeps it from being rapidly metabolized. The lipid content of the liposomes allows for better drug penetration of the stratum corneum. The LMX-4 cream has several advantages over other topical anesthetic agents. This includes a more rapid onset of action and not requiring an occlusive dressing.1 The most frequent adverse reaction is local erythema. Although no serious side effects have been reported with the use of LMX-4, it is recommended that it be applied to an area less than 100 cm2 in patients who weigh less than 20 kg.8 When compared to a placebo, intravenous cannulation after the application of LMX-4 showed improved success rates on the first attempt.9 LMX-4 is as effective as EMLA.10 However, buffered lidocaine injection decreased the pain associated with intravenous catheter insertion to a greater extent than lidocaine cream.1

Topicaine

Topicaine is a 4% lidocaine gel that is used for a number of procedures in the Emergency Department including placement of Foley catheters and nasogastric tubes. It is readily available and easy to use. Patients can develop a contact dermatitis, particularly if they have an allergy to amide-type local anesthetic agents. Apply Topicaine for 30 minutes prior to a procedure for maximum efficacy.

An alternative to either the injection of local anesthetic solution or the use of topical creams and gels are needle-free drug delivery systems. These techniques utilize various strategies designed to speed the delivery of the local anesthetic agent across the stratum corneum.

Iontophoresis

Iontophoresis is a process by which direct electrical current facilitates dermal penetration of positively charged lidocaine molecules when placed under a positive electrode. The dose is calculated by multiplying the duration of delivery with the current used to deliver the local anesthetic agent. Studies evaluating this method typically use 20 to 30 mA and 2% lidocaine hydrochloride with 1:100,000 epinephrine.11,12 It takes approximately 10 to 20 minutes to obtain adequate analgesia for intravenous catheter placement. The side effects of iontophoresis include skin blanching, erythema, tingling, itching, and burning sensations. Burns have been reported to rarely occur during iontophoresis with an incidence of 1 per 15,000 to 20,000 treatments.13 Iontophoresis has been successfully used prior to intravenous catheter placement and lumbar puncture.14

Ultrasound-Assisted Local Anesthetic Delivery

Ultrasound has been utilized to accelerate the delivery of topical anesthetic agents. Ultrasound disrupts the layers of the stratum corneum forming temporary pores that facilitate the distribution of topical anesthetic agents below the epidermis.15 Focal ultrasound followed by a 5 minute application of 4% liposomal lidocaine provided more effective anesthesia when compared with a placebo or standard care.16,17 Ultrasound reduces the time to achieve anesthesia with EMLA cream from 60 minutes to 5 minutes.18 The use of ultrasound prior to a 5 minute application of liposomal lidocaine produced the same amount of pain relief as the typical 30 minute application of liposomal lidocaine.19 Ultrasound-assisted delivery is effective for intravenous catheter placement and phlebotomy.20

Powdered Anesthetics

Another alternative to the injection of lidocaine solution is the transepithelial injection of powdered lidocaine using a specialized delivery system. The delivery system is a prefilled, disposable, single use device that is designed to produce rapid analgesia for venipuncture and intravenous catheter placement. The system is applied 3 minutes prior to venipuncture or intravenous cannulation. The device works by releasing pressurized helium, which in turn ruptures a cassette of lidocaine powder. The particles of lidocaine powder are accelerated to a velocity that is sufficient to penetrate the stratum corneum and become deposited in the subepithelial layer. The onset of anesthesia is approximately 1 to 3 minutes after the injection.21 When compared with a placebo, this delivery system results in a significant reduction in pain.22 The potential adverse effects of powdered lidocaine administration include local skin contusion, application site bleeding, and a subsequent cellulitis.

Jet Lidocaine

Another form of needle-free lidocaine injection is commonly referred to as jet lidocaine. The device uses pressurized CO2 to inject up to 0.5 mL of aqueous lidocaine into the subcutaneous tissue. To use the device, the lidocaine is drawn up into the reservoir and it is then held firmly against the skin where the proposed procedure is to occur. The CO2 cartridge is activated, forcing the microaerosolized lidocaine thru the stratum corneum, depositing it the subcutaneous tissues. The onset of anesthesia is approximately 1 to 3 minutes after the injection. Approximately 19.5% of patients in a randomized trial experienced pain from administration of the system.23 Other complications include device failure 10% of the time and minor local bleeding. In studies comparing jet lidocaine to EMLA or liposomal lidocaine, jet lidocaine was found to provide more effective anesthesia.24,25 Jet lidocaine has been found to be more effective than placebo.26,27 Interestingly, a study compared jet lidocaine to jet placebo (saline).28 It found jet lidocaine to be no more effective at providing local anesthesia, suggesting that the injection itself may play a role in providing anesthesia.

Heat-Enhanced Diffusion

The application of heat improves the dermal absorption of topical anesthetics. The lidocaine–tetracaine topical patch consists of 70 mg of each local anesthesia agent, combined with an air-activated heating system incorporated into an adhesive patch. It is specifically designed to warm the skin which, in turn, accelerates the absorption of the local anesthetic agents. The heating component of the patch is automatically activated when the patch is removed from its packaging. The lidocaine–tetracaine patch can be used for minor invasive procedures such as lumbar puncture, intravenous catheter placement, and arterial puncture. The lidocaine–tetracaine patch can provide effective anesthesia after 10 minutes. It was more effective than EMLA or a placebo at all application times shorter than 60 minutes.29,30 Common side effects include skin erythema, blanching, and edema.

Laser-Assisted Transdermal Passage

Another technique to break down the stratum corneum skin barrier involves the use of a laser.31 A cutaneous resurfacing laser is used to ablate the outer layer of skin, generally an area about 6 mm in diameter. Topical anesthetic is then applied to this area and is able to rapidly penetrate through the ablated skin to the subcutaneous pain receptors. The ablation procedure is typically painless. Anesthesia occurs approximately 5 minutes after the local anesthetic application. The amount of energy used for the ablation varies but typically is in the range of 2.0 to 3.5 J/cm2. Side effects may include mild pain, erythema, and itching at the ablation site. In addition, patients may experience temporary hyperpigmentation or hypopigmentation at the treatment site. The biggest drawback to this technique is the cost of the handheld laser.

Vapocoolant sprays, also known as vapocoolants and refrigerant sprays, are an alternative to topical local anesthetic agents. Vapocoolant sprays are a good choice for patients requiring brief procedures or with allergies to the medications used in topical anesthetics. Vapocoolant sprays work immediately, are easy to use, cost-effective, and may be repeatedly applied without the risk of methemoglobinemia or systemic toxicity. Vapocoolant sprays are sterile liquids that vaporize upon contact with the skin. As the liquid vaporizes, it cools the skin surface and provides brief topical anesthesia. Commonly used vapocoolants include ethyl chloride, fluorohydrocarbons, and alkane mixtures.

They are commercially available in containers with a tip that directs a precise stream of liquid to the desired area. The vaporization of the liquid lowers the skin temperature in the area of contact to approximately −20°C (−4°F) as it vaporizes. The skin becomes temporarily cold then frozen. Vapocoolant sprays are convenient, effective, “needle-less”, and provide immediate anesthesia.32 Their use is limited by a very brief duration of action of approximately 30 seconds. These sprays only provide superficial anesthesia. Vapocoolant sprays can be used prior to accessing indwelling ports and reservoirs, arterial puncture, venipuncture, intravenous catheter placement, intramuscular injections, local anesthetic injection, lumbar puncture, and minor skin procedures.33

Using the vapocoolant spray is very simple. Assemble and gather all the equipment required for the procedure to be performed. Clean and prep the skin. Apply sterile drapes if applicable. Hold the inverted container of refrigerant spray 10 to 15 cm above the skin surface. Spray the liquid onto the skin until a white frost appears and the skin turns white. This usually takes 7 to 12 seconds. Immediately perform the procedure to ensure an adequate anesthetic effect.

Avoid spraying the liquid onto the skin for prolonged periods as it may result in frostbite. Do not use vapocoolant sprays on mucous membranes. The vapocoolants are significantly absorbed through mucous membranes and can result in adverse reactions and toxicity. These agents are highly volatile and must be used in well-ventilated areas. Never spray around open flames or an electrocautery unit as some vapocoolant sprays are highly flammable. Do not use vapocoolant spray on open wounds as it may delay wound healing. The application of ice packs work just as well but takes longer to achieve anesthesia.

Mucous membranes warrant a special note as the systemic absorption from mucous membranes is much more rapid and effective than absorption through the skin and results in higher blood levels of the anesthetic agent. Numerous agents may be used to provide anesthesia to mucous membranes. This commonly includes benzocaine, cocaine, lidocaine, and tetracaine. These local anesthetic agents produce only superficial anesthesia. They do not provide for any pain relief that originates submucosally or deeper. The total dose applied should be one-third to one-half the dose used for infiltration. Use these agents cautiously on the oral mucosa as they can suppress the gag reflex and increase the risk for aspiration.

Benzocaine is a commonly used mucous membrane anesthetic. It is available in spray, liquid, and gel form in a concentration of 14% to 20% (e.g., Cetacaine, Hurricaine, Americaine). It is nontoxic when applied to intact mucous membranes due to its poor water solubility. It provides brief analgesia. Overuse of benzocaine has the potential to produce methemoglobinemia.34,35

Cocaine is supplied in solution at concentrations of 4% and 10%. It is an extremely effective mucous membrane anesthetic with significant vasoconstrictive properties. Systemic absorption is enhanced when it is applied to inflamed mucous membranes. The maximum dose is 3 mg/kg. This dose still has the potential to result in toxicity and serious complications. Do not use cocaine in patients with hypertension, cardiomyopathy, known or suspected coronary artery disease, or patients sensitive to exogenous catecholamines.

Lidocaine is available in jelly and liquid form with a concentration of 2% to 10%. The 2% and 4% viscous solutions are often used in the Emergency Department. Instruct the patient to swish the solution in their mouth for 30 to 60 seconds then spit it out. It can result in significant systemic absorption if swallowed. The maximal dose is 300 mg in adults (15 mL of 2% or 7.5 mL of 4%) and 3 mg/kg in children. Use extreme caution when sending patients home with viscous lidocaine for intraoral use. Frequent use and swallowing can both result in elevated blood levels of the parent drug and its metabolites, both of which can result in potential adverse and toxic effects.36,37

Tetracaine is available in liquid and aerosol form with a concentration of 0.25% to 1.0%. It has significant cardiotoxic effects. It is therefore not often used for mucous membrane anesthesia. Tetracaine should not be used for mucous membrane anesthesia in the Emergency Department.

Several alternative topical anesthetic techniques have been used to provide analgesia. S-Caine Peel (Zars Pharma) has been developed as a local anesthetic peel. It is applied to an area for 20 to 30 minutes. It dries into a flexible membrane that can be peeled off after the application period. This agent is easily applied to non-flat body surfaces. Its primary use has been for laser vein treatments and laser tattoo removal.38–40 Acupressure has been used for hundreds of years to relieve pain. It has been recently used in acute painful conditions.41 The ShotBlocker (Bionix, Toledo, Ohio) is a plastic device that is pressed against the skin just before and during an injection.42 It may be used to decrease the pain of local anesthetic injections. These techniques all have the potential to decrease the pain due to procedures. More information and trials are required before these can be used in the Emergency Department.

It is important to wait long enough after the topical agent is administered to allow the onset of adequate anesthesia before the planned procedure is started. The period of time necessary for the onset of anesthesia is highly variable and dependent on the topical anesthetic used and the delivery technique utilized. A simple examination of the area being anesthetized with fine touch and pinprick is important to insure adequate anesthesia prior to the procedure. It may be necessary to apply additional anesthetic or use an alternative anesthetic technique in areas of continued sensitivity, keeping in mind the total dose applied to avoid toxicity.

The Emergency Physician should observe the patient for a minimum of 15 minutes following the use of topical anesthesia to ensure no evidence of an adverse reaction. The patient need not wait in the Emergency Department for normal sensation to return prior to discharge.

There are case reports of patients developing CNS toxicity after the application of topical anesthetics.43 There have been cases of methemoglobinemia reported in patients, particularly with the use of benzocaine.34 Topical anesthetics in patients taking Class 1 antiarrhythmics such as mexiletine and tocainide can produce additive and possibly toxic effects. EMLA has been reported to cause ulceration of the gingival mucosa and thus should not be used for mucosal anesthesia.44 Many of the topical anesthetics are contraindicated or require special ophthalmic formulations for use in the eye. When using topical anesthetics in neonatal patients, special care and attention to dosing are imperative to avoid toxicity.

There are a number of effective techniques available to decrease the pain and anxiety associated with invasive procedures. Topical anesthetic agents, particularly those with fast acting properties, are excellent adjuncts for use in the Emergency Department. Topical anesthetic creams, active medication delivery, or alternative techniques should be considered prior to performing a painful invasive procedure.