Local Anaesthetics Flashcards
What is the order of blockade (which feelings are blocked first)?
Pain
Touch
Temperature
Pressure
Motor
What is the mechanism of action for local anaesthetics?
‘Blockade’ of conduction - Action potential is stopped by blocking the sodium influx into the nerve cell, meaning that depolarisation cannot occur and so no pain signal travels up the nerve.
What is the basic structure of a local anaesthetic?
Hydrophobic aromatic ring and hydrophilic amine with an intermediate chain, linked by either an amide or ester-linkage
What are the characteristics of an amide linkage?
Break down resistant (done in liver)
Longer duration of action (30-60 mins)
Quick onset (~1 min)
e.g. Lidocaine
What are the characteristics of an ester linkage?
Quicker to break down (done by esterases in cells)
Shorter duration of action (15-30 mins)
Quick onset (~1 min)
e.g. proxymetacaine, oxybuprocaine, tetracaine
What are the indications for using a local anaesthetic?
Contact tonometry
Gonioscopy
Superficial foreign body removal
Scleral CL impressions
RGP fitting
Some Schirmer tests
Lacrimal syringing
Punctal plug insertion
What are the minor ADRs from local anaesthetics?
Conjunctival redness
Transient blurred vision
Stinging
Mild punctate keratitis
Reduced tear secretion (dryness)
Reduced blinking (dryness)
Reduced IOP
What are the allergic reactions to local anaesthetics?
Usually to ester forms, come on 6-12 hours after instillation
Epithelial stippling
Stromal oedema
Increased conjunctival redness
Lid oedema
Increased lacrimation
Severe pain
What are the more serious ADRs for local anaesthetics?
Corneal epithelial desquamation (resolves quickly due to quick healing)
Melting cornea
Delayed cell division so delayed wound healing
Tell me all you know about Lidocaine
AKA Lignocaine
Available as minims - 4% lidocaine with 0.25% fluorescein
Amine-linkage
Store under 25 degrees
Has a lower risk of ADRs
Tell me all you know about Tetracaine
AKA Amethocaine
Available as minims - 0.5% or 0.1%
Ester-linkage
Store under 25 degrees
May get sensitivity reaction with repeated use
Most stingy
Tell me all you know about proxymetacaine
Available as minims - 0.5% or 0.5% with 0.25% fluorescein
Ester-linkage
Store in fridge (2-8 degrees)
Least stingy local anaesthetic
Tell me all you know about oxybuprocaine
AKA Benoxinate
Available as minims 0.4%
Ester-linkage
Store under 25 degrees
Has bactericidal properties
