Week 1.11 Topical Anaesthetics Flashcards
What does the term analgesia and anaesthesia mean
Analgesia: Pain relief without a total loss of sensation
Anaesthesia: pain relief with associated loss of sensation (and perhaps loss of motor control and consciousness)
What do we use topical anaesthetics for
Contact tonometry
Gonioscopy
Ultrasonography - measure length of eye
Ocular blood flow measurement
Superficial foreign body removal
Impressions for scleral contact lenses
Insertion of punctal plus
Increases efficacy of other ophthalmic drugs as it increases the permeability of cornea
How are topical anaesthetics stored
Stored in acidic solutions
Active agent is a weak base
Administered in acidic solution
Following administration pH rises and active agent released
What are some ideal properties of TA
- Reversible
- Rapid onset of a depth of anaesthesia sufficient to enable procedure to take place comfortably
- Sufficient duration of anaesthesia
- Rapid recovery
- Complete recovery
- No adverse reaction to the drug
- Cheap, easy to store etc
What is the structure of a TA
Hydrophobic aromatic ring structure
And
Hydrophilic amine structure, with intermediate chain
Linked either by amide-linkage
Or ester linkage
What are the properties of an amide linked TA
Drugs is resistant to break down - easy to store
Longer duration of action than ester-linked TAs
Allergic reaction much rarer
What are the properties of ester linked TA
- drug rapidly breaks down
- shorter duration of action
- allergic reaction not uncommon
What are the names of TA available to optometrists and are they amide or ester linked
Amide linked:
- lidocaine hydrochloride
Ester-linked:
- proxymetacaine hydrochloride
- oxybuprocaine hydrochloride
- tetracaine hydrochloride
Lidocaine
Minims lidocaine 4% and Fluorescein 0.25%
Useful for applanation tonometry
Amide - linked topical anaesthetic
Store: below 25ºc
Stings more than oxybuprocaine and proxymetacaine
Lower risk of adverse reaction to the drug
Proxymetacaine
Most commonly used TA
0.5%
Store: 2-8ºc in fridge
Ester linked
Oxybuprocaine
Aka benoxinate
0.4%
Applanation tonometry
Stored: between 25ºc
Ester linked
Tetracaine
Stings the most
Not widely used
0.5% and 1%
Stored below 25ºc
Ester linked TA
Aka amethocaine
What is the duration of the TAs
Tetracaine - 20mins
Oxybuprocaine - 15mins
Proxymetacaine - 15mins
Lidocaine - 30 mins
Adverse reactions to TA
Minor side effects:
Conj hyperaemia
Corneal oedema
Punctate keratitis
Blurred vision
Stinging/burning sensation
Decreased tear secretion
Inhibition of blink reflex
Allergic reaction
Delayed wound healing
