5. topical anesthetics

what are some uses of topical anaesthetics for optometrist?

• Applanation tonometry

• corneal rigid lens fitting and impression haptic fitting (scleral lens fitting)

• foreign body removal

• enhancing cycloplegia and mydriasis, particularly in cases of high iris/skin/hair pigmentation

• ocular dimension measurements - Traditional contact ultrasound measurements require TAs

• gonioscopy- TAs make lens insertion more comfortable

• Lacrimal Patency Procedures: TAs may facilitate punctual plug fitting

• Investigative Techniques: TAs can assist techniques requiring sustained fixation, such as confocal microscopy and multifocal electroretinography

Why re disposable tips used in contact tonometry?

the risk of vCJD (i.e. variant Creutzfeld-Jacob Disease, a rare neurodegenerative disorder) transmission with contact devices.

what should TA not be used for

• Symptom Management: TAs should not be used for managing general ocular symptoms

• Pre-term Neonates: TAs should be avoided in pre-term neonates due to their immature enzyme systems

What is water for irrigation (wfi) and how do you do it after remocing reusable opthalmic devices to avoic vCJD transmisison?

wfi is saline BP not domestic tapwater

1. Rinse in water for irrigation (WFI, i.e., saline BP, not tap water) for at least 30 seconds5 .

2. Clean all surfaces with soap/detergent, then rinse in WFI for at least 30 seconds

3. Immerse in a fresh preparation of 1% sodium hypochlorite (1000 ppm Cl) for 10 minutes

4. Rinse in 3 changes of WFI for at least 10 minutes

5. Shake, dry with a disposable tissue, and store dry in a suitable container

6. Follow with conventional disinfection

What is the mode of action for topical anaesthetics?

• they block the initiation and propagation of action potentials in nerve fibres by preventing the voltage-dependent increase in Na+ conductance principally by plugging Na+ channels

• Sensory information is transmitted along nerve fibres via action potentials generated by the influx of positively charged sodium ions (Na+) and the efflux of positively charged potassium ions (K+)

• The nerve membrane is lipoidal and resists the passage of cations (= positively-charged ions) except at pores known as transmembrane sodium channels. TAs block sodium channels on the inner aspect of the nerve cell membrane,

• This results in physical occlusion of the channels which bars sodium entry and hence prevents axon depolarization

What is the time course of TAs ?

onset is very rapid (normally around 30 secs to maximum anaesthetic effect - although lidocaine (aka lignocaine) can be up to 60 secs) with recovery between 20 and 30 mins

how do you check recovery with TA

with a silver of tissue (Kleenex Mediwipe will suffice) lightly touched on the cornea

why may TA;s cause adverse reactions

All TAs affect transmembrane ionic flow which inevitably has consequences for the physiology of corneal epithelial cells e.g. inhibition of epithelial cell growth

What ocular adverse reactions to TAs have been reported?

• reduced tear flow (and possibly a reduction in tear stability),

• reduced uptake of oxygen by corneal epithelial cells (and possibly delay in wound healing)

• a slight reduction in intraocular pressure

• a slight increase in corneal thickness

What systemic adverse reactions have been reported with TAs?

• Lightheadedness and tinnitus (a ringing in one or both ears).

• On occasion fainting

Why can TAs make other topical drugs more effective?

increase permeability of the cornea to subsequent drugs as mitosis and cellular migration are inhibited owing to the reduction in oxygen uptake.

why should you not repeat instillation of TAs

The rate of TA drug absorption across mucous membranes can be high so repeated instillation of TAs should be avoided as these may inadvertently cause desquamation of the corneal epithelium.

How long should px wait until re-inserting CL?

25 mins

Why do TAs sting on insertion?

acidic (pH ~5) an

which TA will sting less

proxymetacaine as its less acidic due to having a higher pKa

How do TAs impact blink rate and why?

The normal blink rate of 10-15/min is reduced by ~60% at maximum corneal anaesthesia because they inhibit the protective blink reflex so the eye is more susceptible to foreign bodies

What TAs are available to optomotrists?

Lidocaine, Proxymetacaine, Oxybuprocaine, Tetracaine

what may occur in transitory superficial corneal epithelial lesions (super rare 1 in 1000)

• a localized or diffuse corneal desquamation (also referred to as a necrotising keratitis which has the appearance of a ‘melting cornea’) may occur (but is less common with amides).

• Visual acuity can be very significantly affected and reduced to 6/60 Snellen acuity or even less.

what should you check before TAs

Vas, corneal integrity, note any reported previous problems that px may report of allergy to anesthetics

what kind of TAs are more likely to cause allergic hypersensitivity

TAs with ester linkages (e.g. proxymetacaine (aka proparacaine), oxybuprocaine (aka benoxinate) and tetracaine (aka amethocaine)

than amide linkages (e.g lidocaine aka lignocaine)

proxymetacaine HCL

1. %?

2. ester/amide?

3. used for?

4. how to store?

5. why is better than other TAS

1. 0.5%, POM

2. m-benzoic acid ester

3. contact tonometry in combination with a saline-moistened fluroscein paper strip

4. needs to be stored in a refrigerator between 2 and 8 degrees

5. stings significantly less

Oxybuprocaine

1. %?

2. ester/amide?

3. used for?

1. 0.4% POM

2. p- aminobenzoic acid ester

3. Used for contact tonometry in combination with a saline moistened fluorescein paper strip.

Lidocaine

1. %?

2. ester/amide?

3. used for?

4. when would you consider it?

1. 4% in Minims® with fluorescein (0.25%), POM

2. is a benzoic acid amide.

3. Ophthalmological use: lidocaine, with or without adrenaline, may also be injected into the eyelids for minor surgery while subconjunctival, retrobulbar or peribulbar injections are used for surgery of the globe.

4. Lidocaine might be considered where there has been a previous history of allergy/hypersensitivity to ester TAs

Tetracaine HCL

1. %?

2. ester/amide?

3. used for?

4. when would you consider it?

1. 0.5 and 1.0%, POM

2. It is an ester TA (like benoxinate)

3. Not used commonly in general optometric practice. Ophthalmological use: tetracaine produces a more profound anesthesia and is suitable for use before minor surgical procedures, such as the removal of corneal sutures.

which TA is the only one not available in minims

lidocaine