Local Anaesthetics

Question 1

How do local anaesthetics work?

Answer 1

By inhibiting the conduction of action potentials in free nerve endings by blocking the transient increase in membrane permeability to sodium ions in a reversible reaction

Question 2

what is a topical anaesthetic?

Answer 2

its a type of local anaesthetic that is applied onto a surface such as the skin/ corneal surface

Question 3

what is the order of pain that the neural block occurs in?

Answer 3

occurs in order of size:
-pain blockade
-temperature blockade
-pressure blockade
-motor blockade (paralysis)

Question 4

why is the cornea a good surface for topical anaesthetic?

Answer 4

as the sensory nerves on the cornea are thin

Question 5

what two kinds of topical anaesthetics are there?

Answer 5

-amide linkage
-ester linkage

Question 6

what structures do all topical anaesthetics have?

Answer 6

-hydrophobic ring structure
-hydropilic amine structure

and then amides have an amide linkage and esters have an ester linkage

Question 7

how are amide and ester anaesthetics different?

Answer 7

-amides make the drug more resistant to breakdown and have a longer duration of action whereas esters are rapidly broken down and so have a shorter duration of action
-amides are biotransformed in the liver whereas esters are biotransformed in most cell esterases

Question 8

what is an amide link?

Answer 8

NH-CH2-CH2

Question 9

what is an ester link?

Answer 9

O=C-O

Question 10

name amide topical anaesthetics available for use by optoms

Answer 10

-lidocaine (lignocaine) hydrochloride

Question 11

name ester topical anaesthetics available for use by optoms

Answer 11

-proxymetacaine hydrochloride
-oxybuprocaine (benoxinate) hydrochloride
-tetracaine (amethocaine) hydrochloride

Question 12

when may you use topical anaesthesia in eyes?

Answer 12

-on anterior surface to enable diagnostic procedures e.g. contact tonometry, gonioscopy and Schirmer tests
-superficial foreign body removal
-impressions for scleral contact lenses
-insertion of punctal plugs; lacrimal syringing

Question 13

name some minor side affects of ocular use of topical anaesthetics

Answer 13

-conjunctival hyperaemia
-mild punctate keratitis
-transient blurred vision
-stinging/ burning sensation
-decreased tear secretion

Question 14

what are some adverse reactions of ocular use of topical anaesthetic?

Answer 14

-inhibition of blink reflex which could leave the eye vulnerable to trauma
-hypersensitivity which can be delayed so doesn’t occur until 6-12 hrs after instillation

Question 15

how would you know is a topical anaesthetic has caused reaction at the corneal epithelium?

Answer 15

-localised/ diffused epithelial desquamation which can reduce va to 6/60
-delayed wound healing

Question 16

how should you store lidocaine hydrochloride?

Answer 16

below 25 degrees so not in the fridge but not hotter than room temperature

Question 17

where should you store proxymetacaine?

Answer 17

in the fridge (between 2-8 degrees C)

Question 18

what is the 1st and then most common used topical anaesthetic in optometry?

Answer 18

1. proxymetacaine
2. benoxinate

Question 19

what is benoxinate used for alot?

Answer 19

applanation tonometry

Question 20

where should you store benoxinate?

Answer 20

store below 25 degrees C so at room temp

Question 21

where should you store tetracaine? what is another name for it and is it used commonly in optom?

Answer 21

-store below 25 degrees C so at room temp
-amethocaine
-not widely used no

Question 22

rank the 4 topical anaesthetics in order of stinging on instillation from least to worst

Answer 22

1. proxymetacaine
2. oxybuprocaine
3. lidocaine
4. tetracaine

Question 23

how can topical anaesthetics affect IOP?

Answer 23

they usually lower IOP so measure IOP soon after instillation - dont wait around after instilling the drops