Tetracaine

Tetracaine, also known as amethocaine, is a potent, long-acting ester local anaesthetic. It is used as a topical gel for skin anaesthesia before venepuncture, as a topical agent for the eye and for mucosal surfaces, and rarely for spinal anaesthesia. It is more potent and more toxic than procaine, and absorption from mucous membranes is rapid.

Class. Ester local anaesthetic (long-acting, topical).

Mechanism. Reversible blockade of voltage-gated sodium channels.

Pharmacokinetics. Topical gel onset 30 to 45 minutes on intact skin; effect lasts 4 to 6 hours after removal. Ocular onset within 1 minute, duration 15 to 20 minutes. Spinal onset 3 to 5 minutes, duration 2 to 3 hours (longer with adrenaline). Protein binding about 76%. Hydrolysed slowly by plasma pseudocholinesterase to para-aminobenzoic acid. Systemic absorption from mucosal surfaces is rapid and can produce high plasma levels.

Indications are topical anaesthesia of intact skin before venepuncture or cannulation, topical anaesthesia of the cornea and conjunctiva for short procedures, mucosal anaesthesia (limited use), and spinal anaesthesia where the preparation is available.

Adults and children over 1 month (gel 4%)

• Venepuncture: apply 1 tube (about 1.5 g) under an occlusive dressing for 45 minutes. Cannulation: 30 minutes. Remove the gel and wipe the area before the procedure.
• Local anaesthesia lasts about 4 to 6 hours after removal.
• Keep to the labelled quantity of gel and area; use the smallest area that serves the procedure.

Ocular (0.5% drops)

• Adults: 1 to 2 drops in the conjunctival sac immediately before a procedure; repeat for longer procedures at intervals.
• Single use in the clinic only. Not for self-administration.

Spinal (specialist, where licensed)

• Hyperbaric 0.5%: 5 to 10 mg for perineal surgery, 10 to 15 mg for lower-limb and lower abdominal surgery, 15 to 20 mg for upper abdominal surgery; reduce in older adults and in pregnancy.
• Adding adrenaline lengthens the block to about 3 hours.

Children. Gel from 1 month (term infants). Not used spinally in routine practice.

Contraindications. Hypersensitivity to ester anaesthetics or para-aminobenzoic acid; application to broken skin, mucous membranes or near the eye (gel); preterm infants and infants under 1 month (gel); pseudocholinesterase deficiency; large mucosal doses.

Boxed-warning-level risks. Systemic toxicity with seizures and cardiovascular collapse after absorption from mucosal surfaces or overdose; corneal damage with repeated ocular use.

Adverse effects. Common: erythema, oedema and itching at the application site, transient blanching. Serious: contact dermatitis, severe local reaction on repeated application, corneal epithelial damage with prolonged ocular use, allergic reactions, local anaesthetic systemic toxicity, methaemoglobinaemia (rare).

Interactions. Para-aminobenzoic acid antagonises sulphonamides. Cholinesterase inhibitors slow hydrolysis and add toxicity. Other local anaesthetics add systemic toxicity. Adrenaline in spinal solutions interacts with tricyclic antidepressants and monoamine oxidase inhibitors.

Common prescribing errors. Applying gel to broken skin or mucosa; leaving gel on longer than the labelled time, especially in children; using ocular drops repeatedly for pain relief; applying near an open wound; applying over too large an area; omitting to warn that sensation is absent for hours.

Special groups

• Pregnancy: acceptable for topical use at usual doses; limited data for spinal.
• Breastfeeding: compatible at usual doses.
• Older adults: usual topical doses; reduce spinal doses.
• Kidney impairment: no dose change.
• Liver impairment: no dose change for topical use; caution if repeated doses.
• Regional: confirm the preparation (gel, drops, spinal), as concentrations and strengths vary.

Formulations. Gel 4% (tubes of 1.5 g), ophthalmic solution 0.5%, hyperbaric spinal solution 0.5% in some markets.

Administration. Apply the gel to intact skin with a gloved finger or applicator. Cover with an occlusive dressing. Remove the gel and wipe the site before the procedure. Use ophthalmic drops under direct supervision and protect the eye from injury while it is anaesthetised.

Baseline. Allergy history (ester, para-aminobenzoic acid), skin integrity at the site, pseudocholinesterase deficiency history, age, and for spinal use coagulation and cardiac status.

Monitoring. Skin reaction at the site, effectiveness before procedure, corneal protection after ocular use, and for spinal use sensory and motor block and haemodynamics.

Therapeutic drug monitoring. Not used.

Dose reduction and stopping thresholds. Remove the gel and wash the site for irritation, blistering or systemic symptoms. Stop ocular drops if the corneal epithelium shows damage.

Referral and escalation. Seizures or collapse: stop the exposure, give oxygen and airway support, benzodiazepine for seizures, and 20% lipid emulsion 1.5 mL/kg over 1 minute then 15 mL/kg/hour (maximum cumulative 12 mL/kg). Corneal injury needs ophthalmology review. Anaphylaxis needs intramuscular adrenaline.