Topical Anaesthetics

What are Analgesics ?

• Give relief from pain without loss of sensation/feeling

• Paracetamol , ibuprofen

What are anaesthetics?

• Give relief from pain but also cause loss of sensation/feeling

E.g Ketamine

What are topical Anaesthetics?

• local anaesthetics acting on surface of body → skin , eyes etc

E.g Lidocaine

How do Local Anaesthetics work?

• are chemical agents which reversibly block the transmission of nerve impulses along sensory fibres

— Nerve function recovers completely

— Loss of sensation without loss of consciousness

— Dif sensations lost according to SIZE OF AXONS

• pain axons smallest and pain sensation lost first , followed by touch and temperature

• Pressure lost last if at all

POM use

• Use in practice but NEVER supply to px

What do Topical anaesthetics do ?

• Block nerve conduction

• prevent nerve impulses by blocking Na+ channels preventing transmission of action potentials

What properties do we need in an Ideal Topical anaesthetic ?

• Quick onset of action

• Reasonable duration of action

• Provide enough depth of anaesthesia

• Reversible action

• No pain at site of instillation

• No hypersensitivity or allergic reactions

• Non-toxic

• Good ocular penetration →amphiphilic

• No effect on pupil size, accommodation or IOP

• compatible with other drugs

What is the structure of local anaesthetics

• Lipophilic ring

• Intermediate chain linkage → ester or amide

• Hydrophilic group → allows agent to be ionised and water soluble

Ester or Amide linkage

• Ester linkages broken down more easily

• amide linkages last longer → longer duration of action

Ester types local aesthetics onset, duration and biotransformation

• Onset ~1 min

• Duration ~20-30 mins

• Biotransformed in most cells by esterase

Amide types onset,duartion,bio

• Onset ~1 min

• Duration ~30-60 mins

• Biotransformed in liver → transported there first

Lidocaine onset, classification, formula, storage

• takes 60 seconds to start working

• lasts up to 25 mins

• Amide , POM,Minims

• 4% Lidocaine HCl, 0.25% fluroescein

• Below 25 degrees , protect from light

Tetracaine, Proxymetacaine, Oxybuproacine

Where is Proxymetacaine stored?

• In the fridge

• let px know it feels cold on instillation

What are the indications for use of Topical Anaesthetics?

• Contact Tonometry → GAT

• Fitting CL → scleral lens impressions

• FB removal

• Gonisocopy

• Surgery (opthalmology)

Contraindications for topical anaesthetics

• Where wound healing should not be compromised

• Known hypersensitivity (ester/amide)

What are the side effects of using topical anaesthetics ?

• Sting on installation

• Warn px of risk of FB damage for approx 30 mins after instillation → no eye rubbing

• reduced blink rate

• delay wound healing - delay cell division

• may produce mild punctate keratitis

• repeated installation → corneal epithelial desquamation

• can lower IOP

• Proxymetacaine has least antibacterial action

What is the order of stinging of topical anaesthetics

Proxymetacaine → stings least

Oxybuprocaine

Lidocaine

Tetracaine → stings most

What are the Pros of using topical anaesthetics ?

• Good (short) duration of action

• localised effect

• cheap

• easy to administer

• fast onset

• very effective → cornea completely anaesthetised

Cons of topical anaesthetics

• If px rubs their eyes or FB gets in they won’t feel it

• heavily legislated → dangerous if px has constant supply → whatever causing the pain isn’t being addressed

What is the best Topical Anaesthetic for GAT

• Proxymetacaine is the gentlest option → least epithelial toxicity

• causes least stinging

• depth of anaesthesia light but sufficient for Contact Tonometry

• works fast

Best Topical anaesthetic for Gonioscopy

• Oxybuprocaine 0.4% provides slightly deeper anaesthesia and longer duration than proxymetacine which helps maintain comfort during lens manipulation

• stinging mild to moderate but usually well tolerated

• Proxymetacine alternative for px who are sensitive to stinging or anxious → most confortable on instillation and has lowest epithelial toxicity

• its shorter duration still effective for Gonio

Which Topical Anaesthetic for Foreign body removal

• Tetracaine 0.5–1% is preferred choice because it provides the deepest and most sustained anaesthesia

• allowing comfortable and safe manipulation of the cornea during foreign body removal

• However, causes significant stinging on instillation and has higher epithelial toxicity, so repeated dosing should be avoided