Describe EMLA & Topical Anaesthesia in detail.

Definition

Topical (surface) local anaesthesia is the application of LA to skin or mucous membranes to numb the surface. EMLA (‘eutectic mixture of local anaesthetics’) is a cream of lignocaine and prilocaine applied to intact skin — most familiarly to numb the skin before venous cannulation, especially in children.

Uses & Points

EMLA must be applied under an occlusive dressing for about 45–60 minutes to work, as it penetrates intact skin slowly. Other topical LAs are used on the airway (before awake intubation/endoscopy), the eye (amethocaine drops), and the urethra (lignocaine gel). Topical airway/mucosal application can produce rapid systemic absorption, so dose limits still apply; prilocaine in EMLA can rarely cause methaemoglobinaemia in infants.

Onset Time & Cautions

The practical limitation of EMLA is its slow onset through intact skin, requiring application well ahead of the procedure under an occlusive dressing, which means it must be planned rather than used on the spur of the moment; a related preparation acts somewhat faster but the principle of advance application holds. Cautions include the theoretical risk of methaemoglobinaemia from the prilocaine component in small infants and avoidance on broken skin or mucous membranes where absorption would be rapid and unpredictable, so the cream is used on intact skin for its intended purpose of painless needle procedures.

Note

On mucous membranes and the airway, by contrast, topical local anaesthetic is absorbed quickly, so the dose is counted towards the safe maximum just as an injection would be.

Prilocaine can cause methaemoglobinaemia in infants.

• Topical LA numbs skin/mucosa; EMLA = lignocaine + prilocaine cream on intact skin.
• Chiefly for painless cannulation (children); needs 45–60 min under occlusion.
• Other topical uses: airway, eye (amethocaine), urethra (lignocaine gel).
• Mucosal absorption can be rapid — respect dose limits; rare methaemoglobinaemia (prilocaine) in infants.