1.07 Topical Anaesthetics

types of anaesthetic

general - whole body

local - temporarily numbing a specific area of the body (can be administered by injection, or can diffuse across a mucous membrane (topical))

analgesia definition

pain relief without a total loss of sensation

can still move that part of the body

anaesthesia definition

pain relief with associated loss of sensation (and perhaps loss of motor control and consciousness)

uses for topical anaesthetics for optoms 

anaesthetia of the anterior ocular surface (topical) to enable diagnostic procedures 

• contact tonometry (GAT)

• gonioscopy/fundus cls (view anterior angle)

• ultrasonography (measure axial length)

• ocular blood flow measurement 

superficial foreign body removal 

impressions for scleral cls 

insertion of punctal plugs/lacrimal syringing (treating dry eye) 

increases the efficacy of other ophthalmic drugs by increasing permeability of the cornea (controversial)

legal aspects -

how many topical anaesthetics do optoms have access to

what medicine classification are they

optoms have access to 4 topical anaesthetics

they are all POMs USE only - must never be supplied to pts under any circumstances

why is the overuse of topical anaesthetics bad

slows down corneas ability to self heal

describe what gates are opening/closing at each part of the graph

1. Na gate opens

2. Na gate closes

3. K gate opens

4. K gate closes

how do topical anaesthetics work

the active agent is a weak base (it is administered in an acidic solution - hydrochloride) increases solubility and stability in water

1. uncharged molecule passes through phospholipid bilayer 

2. change in pH. molecule becomes ionised (therefore easier to pass membrane) 

3. binds to Na+ channel and blocks it. therefore no action potential (Na+ cant enter so nerve cell cant depolarise)

relationship between blockage of conduction and concentration of drug

blockage of conduction within a nerve is related to nerve diameter - higher concentration of drug required to block thicker nerves

sensory nerves of the cornea are thin (dont have a myelin sheath) so its easy to induce a blockade using a topical drug

neural block occurs in what order

• pain blockade

• touch blockade

• temperature blockade

• pressure blockade

• motor blockade (paralysis)

ideal properties of topical anesthetics

rapid onset of a depth of anaethesia sufficient to enable procedure to take place comfortably eg applanation of the cornea

sufficient duration

rapid recovery

complete recovery

no adverse reaction to the drug

cheap

easy to store

molecular structure of topical anaesthetics

a hydrophobic aromatic ring structure

and

a hydrophilic amine structure, with intermediate chain

linked by either:

(lidocaine) an amide linkage

• drug is resistant to break down

• longer duration of action than ester linked TAs

• allergic reaction much rarer

(other TAs) an ester linkage

• drug is rapidly broken down

• therefore short duration of action

• allergic reaction is common (due to products when drug is metabolised)

why do we want TA molecules to be charged or uncharged 

uncharged for passing through cornea 

charged when storing to keep it stable and so it can bind to Na+ channels 

what are the topical anaesthetics available to optoms

benzoic acid amide linkage

• lidocaine hydrochloride

benzoic acid ester linkage

• proxymetacaine hydrochloride

• oxybuprocaine hydrochloride

• tetracaine hydrochloride

what is lidocaine hydrochloride available as and the concentration

available as

minims lidocaine and fluorescein (pre mixed with fl)

• 4% lidocaine hcl with 0.25% fl in a 0.5 ml single unit

what link does lidocaine have

amide

what temperature should we store lidocaine at 

below 25 degrees c 

pros and cons of lidocaine

pros - lower risk of adverse reaction to the drug (corneal desquamation)

stings more than benoxinate and proxymetacaine

what is proxymetacaine hydrochloride available as

available as

minims proxymetacaine

• 0.5% proxymetacaine hydrochloride

what temperature should we store proxymetacaine hcl

2-8 degrees c

in the fridge

what is the link in proxymetacaine 

ester 

what is the most commonly used TA in optometry 

proxymetacaine hcl 

what is oxybuprocaine hcl available as

minims oxybuprocaine

0.4% oxybuprocaine hcl

temperature to store oxybuprocaine

below 25 degrees c

what is the link in oxybuprocaine

ester

what is tetracaine hcl available as

minims tetracaine

0.5% and 1.0% tetracaine hcl

what temperature should tetracaine hcl be stored at 

below 25 degrees c 

what link is in tetracaine hcl

ester

why is tetracaine unpopular

it stings

onset and duration of the 4 anaesthetics (slide 24)

adverse reactions to topical anaesthetics 

stored in acidic solution so causes side effects 

conjunctical hyperaemia 

corneal oedema 

punctate keratitis 

transient blurred vision 

stinging/burning sensation 

decreased tear secretion 

inhibition of blink reflex - eye vunerable to trauma 

allergic reaction - can be delayed - 6-12 hrs following instillation 

reaction at the corneal epithelium

• loss of microvilli

• localised or diffuse epithelial desquamation (accelerated loss of epithelium cells) - VA 6/60. spontaneous resolution (ocular lubricants may help)

• severe reaction (melting cornea) rarely occurs - antibiotic prophylaxis 

• delayed wound healing 

What should we measure before instillating anaesthesia

Visual acuity