Stains & Anaesthetics

Question 1

Which ocular stains are used by optoms and what are the formulations?

Answer 1

NaFL (corneal epithelium) ~ 1/2% Minims, 0.25% + 0.5% proxymetacaine/4% Lidocaine HCL, 1.0mg impregnated strips

RB (conjunctival) ~ 1.3mg 1% impregnated strip
LG (conjunctival) ~ 1.5mg impregnated strip

Question 2

Why are stains only used as single-use rather than multi-use?

Answer 2

Most preservatives not effective against pseudomonas aeruginosa and multi-use increase infection risk

Question 3

Explain the properties of fluoroscein

Answer 3

Stains via surface pooling, ingress around cells (uptake by cells)

pH dependent, pKa = 6.1 (ionised well near 7.4, dissolves in tears)

Examines cornea/conjunctiva

Question 4

Explain the properties of RB and LG stains

Answer 4

Stains live, damaged dead cells and mucous strands, lipophilic)

Question 5

Explain the mechanisms of action for the 3 stains

Answer 5

NaFL: reflects/transmits Orange with cobalt blue filter
RB: reflects/transmits Magenta with no filter
LG: reflects/transmits blue/green with no filter

Question 6

Explain the topical/systemic side effects of the ocular stains

Answer 6

NaFL: topical include mild irritation, allergy, stinging, systemic include nausea/vomiting/cardiovascular problems

Topical
RB: mild reactions/irritation
LG: less irritation than RB

Question 7

Explain how you would investigate corneal integrity

Answer 7

NaFL: dry eye disease
Corneal staining/Tear film stability
Punctate epithelia erosions (severe dry eye)

RB: dry eye disease
Mucus/AQ tear deficiencies

Question 8

Which 4 LAs are available for optoms?

Answer 8

All single use (Minims)
Tretracaine HCL (0.5/1 % conc.)
Proxymetacaine (0.5% conc.)
Oxybuprocaine HCL (0.4% conc.)
Lidocaine HCL (4% + 0.25% NaFL) - only amide rest are esters

Question 9

What are the physicochemical properties of LAs?

Answer 9

All weak bases in (partly) & ionised form (depends in pka/pH)

Unionised crosses lipid membrane
Ionised blocks voltage gated ion channel

Question 10

Explain the MoA of LAs

Answer 10

Block nerve conduction
Blocks voltage gated Na+ channels reduced increase of Na permeability and action potential firing/propagation

Question 11

What are the favourable conditions of LA binding?

Answer 11

Low pKa provides more non-ionised LA for faster/better absorption

Need ionised LA to bloc. Volt gate ion channels for better LA effect

Question 12

Which factors influence LA activity?

Answer 12

Lipid solubility (non ionised penetrates nerve - faster)
Tissue pH (low pKa is faster acting v.v)
Ester or Amide metabolism
Nerve myelination
Concentration

Question 13

Explain the metabolism of ester/amide LAs

Answer 13

Ester: well, fast hydrolysed by pseudo-cholinesterases in plasma to PABA (hypersensitivity reactions),
shorter 1/2 life than amide

Amide: resistant to hydrolysis, metabolised in liver by CYP450 enzymes (slower), longer 1/2 life, less allergic reaction

Question 14

What are the ideal LA characteristics?

Answer 14

Quick onset (fibre diameter, conc. pH/pKa/ionisation
Good duration/Rate of clearance
No effect on pupil size/accom./IOP
No interference with other drugs
No stinging/local toxicity