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