Topical anaesthesia

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CHAPTER 3 Topical anaesthesia

Topical anaesthesia is an extremely useful adjunct to clinical practice in the ED and is underutilised, often because it is forgotten. It is used for local anaesthesia or analgesia on intact skin, mucous membranes and the eye through the application of local anaesthetic agents to the external epithelial surface. Topical agents may be used to decrease the pain and discomfort of medical procedures and for various skin and mucous membrane conditions, including pruritus and pain due to minor trauma, burns or inflammatory conditions affecting the skin (e.g. varicella, sunburn, contact dermatitis, insect bites).

Do not forget to ask patients about local anaesthetic allergies, as ester-linked agents (which more commonly provoke reactions) are often used for topical anaesthesia.

Cryoanaesthesia

Thermal anaesthesia (cryoanaesthesia) may be used for short-acting superficial anaesthesia for procedures such as venipuncture, intravenous cannula placement, and superficial abscess incision and drainage. Ice, refrigerant sprays and liquid nitrogen have been used for this purpose. This technique does not produce very good anaesthesia.

Technique

• Ice must be applied to the skin for at least 30 to 60 seconds before the procedure. Be careful to avoid frostbite.

• Ethyl chloride may be sprayed onto the skin for 5 to 10 seconds from a distance of about 150 to 200 mm. After a frost is produced on the skin, a 10- to 12-second period of anaesthesia occurs before the skin temperature and sensation return to normal. As a result of this limited time frame, an assistant often has to spray the area while the procedure is performed. Be careful to avoid freezing areas of normal skin by overuse of this method.

Dermal topical local anaesthesia

Topical anaesthesia to the skin is a very useful but underused form of local anaesthesia in the ED. If used correctly, it can dramatically decrease the pain and anxiety associated with injections and other minor procedures.

EMLA (eutectic mixture of local anaesthetics) cream is a combination of lidocaine and prilocaine which has good skin penetration and produces excellent anaesthesia of the skin. Anaesthesia to a depth of 3 mm is obtained after 60 minutes of application under an occlusive dressing, while after 120 minutes the depth of anaesthesia may reach a maximum of 5 mm. The onset of action may be more rapid in highly vascular areas (such as the face), and in damaged or inflamed skin. The maximum dose should never be exceeded. EMLA produces a biphasic vascular effect – vasoconstriction followed after 30 minutes of application by vasodilatation.

EMLA cream may be used to decrease the discomfort of vascular access, injections, lumbar puncture or superficial procedures. In children, while EMLA certainly has a role in minor procedures, it must be remembered that cooperation has as much to do with apprehension as it does with actual pain, so additional agents or methods may be needed to address this. EMLA cream may be applied under an occlusive dressing or as a pre-prepared product. EMLA should be used for small applications only, because of the toxicity of prilocaine, especially in children (principally methaemoglobinaemia). The maximum dose should not be exceeded (Table 3.1).

Table 3.1 Recommended dosages of EMLA for children

Recent studies have suggested that EMLA is safe even in neonates. At this stage, however, this would be an off-label application.

Technique

• Prepare the skin prior to application by cleaning with an alcohol swab to degrease the skin.

• Penetration of the topical anaesthetic agent can be increased in intact skin by partially removing the stratum corneum by tape-stripping: apply an adhesive tape to the skin and rip it off in a ‘waxing’ manoeuvre. Do not traumatise children by causing pain with this procedure.

• Apply EMLA directly to skin with coverage of 1.5 g/10 cm² in adults and 1 g/10 cm² in children and cover with an occlusive dressing, or use a commercially prepared self-adhesive disc.

• Wait sufficient time to achieve the desired depth of anaesthesia (60 minutes = 3 mm, 90 minutes = 4 mm, 120 minutes = 5 mm). If large amounts of cream are still on the skin surface, insufficient time has been allowed for its absorption. A shorter waiting time is required when EMLA is applied on irritated or broken skin.

TAC (tetracaine 0.5%, adrenaline [epinephrine] 0.05% and cocaine 5–12%) has been used as an anaesthetic for repair of facial and scalp lacerations in children, but today sufficient concern exists about toxic potential that it should not be used.

Other options include LAT (lidocaine 4%, adrenaline 0.1% and tetracaine 0.5%) which is prepared as a premixed gel. Where it is not available, a similar mixture can be improvised:

Ophthalmic tetracaine 1% 0.9 mL

Lidocaine 2% 0.9 mL

Adrenaline 1:1000 0.2 mL.

Because of the vasoconstrictor effects it should not be used on the fingers, penis, nose or ears. Topical anaesthesia should be used with care on broken skin or mucous membranes and the patient carefully monitored for adverse reactions.

The ophthalmological preparation of tetracaine 1% can be used for topical anaesthesia on its own, especially on mucous membranes (e.g. in the mouth prior to injection for dental anaesthesia).

Lidocaine 2% gel may provide good topical anaesthesia to open wounds and damaged skin (‘roasties’) but is of little value on intact skin. Do not exceed the maximum permitted dose of 10 mL on broken skin, and 15 mL on mucosa at any one time!

Other formulations and methods

Other preparations include topical liposomal lidocaine formulations and heat-enhanced delivery systems (usually lidocaine and tetracaine preparations). Other needle-free devices for administering local anaesthetic have shown promise and new products will continue to appear. Emergency physicians should be aware of new developments in order to make their patient’s visit as minimally unpleasant as possible.

Ophthalmological applications

The ophthalmic division of the trigeminal nerve supplies the cornea, conjunctiva and sclera, via a terminal network of nerves that lies in proximity to the external surface of the eye. Local anaesthesia is therefore very effective in the eye (cataract surgery can be performed with topical anaesthesia alone).

Topical anaesthesia of the eye is useful in the ED:

• To facilitate tonometry.

• For removal of foreign bodies in the cornea and/or conjunctiva.

• For superficial curettage of the cornea (removal of burn rings or ‘rust’ rings).

• To allow thorough eye lavage to remove chemical or mechanical foreign materials.

• In conjunction with other topical agents for procedures on the eyelids or eyelid margins.

• As a single application in photokeratitis (‘arc eyes’ or snow blindness) to reduce blepharospasm and allow comprehensive examination of the eye and administration of other agents.

Ester-linked local anaesthetics (such as tetracaine and oxybuprocaine) have traditionally been used in the eye because of their rapid absorption and onset of action within 5 to 10 seconds, and duration of action of 10 to 20 minutes. Amides such as lidocaine and bupivacaine have become more popular because of their longer duration of action, despite their slower absorption. Buffered solutions of lidocaine or bupivacaine have a significantly longer duration of anaesthesia than ester agents, with an onset of action within 1 minute of instillation, and duration of action of 45–60 minutes for lidocaine, and 60–180 minutes for bupivacaine. There is no general consensus on which topical local anaesthetic eye drop provides the best analgesia, but studies have shown that proxymetacaine causes the least discomfort on administration. Tetracaine, oxybuprocaine, proxymetacaine, lidocaine and bupivacaine have all been used successfully in a variety of different concentrations. Availability may determine the choice of agent, but it is essential that the preparation is preservative-free. Increasing the preparation pH (buffering 1 mL of local anaesthetic with 0.1 mL of sodium bicarbonate [1:10]) may alter the duration of action but is probably of little clinical benefit in general. Topical non-steroidal anti-inflammatory drugs, such as ketorolac or diclofenac, may be co-administered with local anaesthetics and prescribed for follow-on outpatient treatment for traumatic eye discomfort. Topical steroids should in general only be used in consultation with an ophthalmologist.

Topical local anaesthetic drops should never be prescribed for patients’ own use for three main reasons:

• When used excessively, topical anaesthetics can cause severe and irreversible damage to corneal tissues, with corneal ‘melting’.

• Local anaesthesia increases the likelihood of unnoticed trauma to the cornea from rubbing or foreign bodies – for this reason, every patient discharged from the ED should wear an eye patch on the eye/s that have been anaesthetised, until normal sensation in the eye returns;

• Other causes of pain in the eye (ocular or extraocular) may be masked by the use of topical anaesthesia.

Technique

• Instil drops directly into the affected eye; do not drop the local anaesthetic directly onto the cornea but rather into the inferior fornix of the conjunctiva.

• Follow up with additional drops every few minutes if deeper or prolonged anaesthesia is needed (especially with amide agents). More than five drops will seldom be needed in each eye.

Mucosal topical anaesthetic applications

Topical anaesthesia is very effective when applied to the mucous membranes of the ears, nose, throat, oral cavity, upper airway and genital mucosa because of its rapid absorption. For the same reason, the toxic potential of the local anaesthetic agent is high and the maximum dose should be strictly observed. Administration of topical anaesthetics should be performed with accuracy to ensure that a predetermined amount of drug is administered to allow for the intended effect while minimising the risk of toxicity.

Techniques

• Intraoral injections and procedures: in the oral cavity, topical anaesthesia is rapidly obtained by holding a pledget (or cotton bud) soaked in lidocaine 2% or tetracaine 1% against the area to be anaesthetised for 30 seconds.

• Nasal procedures and packing: topical anaesthesia of the nose for cauterisation, packing or foreign body removal is best obtained with a 4% solution of cocaine (or lidocaine in higher concentrations). Apply the solution to a pledget and hold it in place against the mucosa for several minutes. Excellent local anaesthesia with vasoconstriction will last for 30 to 60 minutes. Do not exceed the maximum dose of 3 mg/kg (±4 mL of the 4% solution in adults). Lidocaine spray can also be applied to the nasal mucosa for topical anaesthesia; it has the disadvantage of producing vasodilatation which may increase the risk of bleeding from the mucosa.

• Passage of nasogastric tubes; awake endotracheal intubation, or to blunt the autonomic response to intubation in high-risk patients; endoscopy and bronchoscopy: topical lidocaine spray is available as a metered dose spray (10 mg/0.1 mL spray) for application in the nose, mouth and throat. The maximum dose to be administered as a topical agent is 200 mg (20 sprays), because of the potential for rapid systemic absorption and toxicity. Benzocaine sprays have largely been replaced by lidocaine sprays because of the concern of methaemoglobinaemia with the use of benzocaine. Inhalations of nebulised lidocaine or bupivacaine are often used, but there is little published evidence to support this practice, and there are concerns about excessive and rapid systemic absorption. It is also probably not as effective as other topical applications. Pledgets soaked in local anaesthetic which are passed into the posterior nasal space above and below the inferior turbinate can be left in place for several minutes. This will result in anaesthesia to the entire nasal passage and proximal pharynx. Bilateral placement increases the success of the anaesthesia.

• Procedures involving the external ear canal: topical anaesthesia of the ear canal can be achieved with ear drops containing benzocaine or procaine (adhere to maximum doses); administer four to five drops into the ear canal and allow several minutes for anaesthesia to develop. EMLA cream has been used in the external ear canal with good effect, but the manufacturer warns that it should not be used if it can migrate into the middle ear (perforation of the tympanic membrane) as it has ototoxic effects in animal models.

• Placement of urethral catheters: lidocaine gel 2% provides rapid, excellent anaesthesia to the mucosa of the genital tract for the insertion of urethral catheters. It should always be used, especially in males. Do not exceed the maximum dose of 15 mL for mucosal application in adults.