Topical anaesthetics Flashcards
Role of topical anaesthetics
Drugs that reversibly block transmission in
sensory nerves
What type of medicines are topical anaesthetics
POM (Prescription only medicine)
What are topical anaesthetics most commonly used for
Golmann tonometry
Uses of topical anaesthetics
- Contact tonometry
- Contact lens fitting
- Foreign body removal
- Gonioscopy = anterior eye
- Lacrimal procedures e.g. insertion of punctal plugs which prevents drainage
- Never for the relief of symptoms
Why shouldn’t you use topical anesthetics for treating and relieving symptoms
- Cause healing is delayed in eye that is already compromised
- It should be treated by antibiotics or foreign body removal, shouldnt avoid the pain.
- If use anesthesia for this, can cause keratitis which can lead to nucleation
Advantages of topcial anaesthetics
- Allow certain procedures to be conducted
- Makethepatient more comfortable
- Make procedures easier for practitioner
Disadvantages of topcial anaesthetics
- Sting(due to pH)
- Delay healing because produces reduction in collagen
- Eye is more susceptible to damage
- Repeated instillations
What is pH of topical anaesthetics and eye
-Topcial anaesthetic = 3.5 - 5.5
- Eye = 7
How many drops needed for topical anaesthetic
1
How do topical anaesthetic work - mode of action
- Sensory information passes along nerve fibres via electrical impulses (action potentials)
- Nerve has negative charge at rest
- Action potential is generated by influx of sodium ions into the nerve
- Results in positive charge (depolarisation)
- Efflux of potassium ions returns nerve to resting potential i.e repolarization
- Action potential passes along nerve
- So get successive depolarizations and repolarizations of adjacent areas
Role of topical anaesthetic
Reversibly block nerve conduction by blocking sodium channels therefore action potential can’t be generated
Structural features of local anaesthetic
- Has a lipid soluble hydrophobic portion and a readily ionisable hydrophilic portion (that can switch from uncharged to charged form)
- It’s due to the fact that they can be lipophilic and hydrophobic, and charged or uncharged, that enables them to rapidly diffuse across the lipid membrane of the epithelial cells and bind to the intracellular portion of sodium channels
- The lipid soluble and hydrophilic portions are separated by an intermediate alkyl chain which contains either an ester or an amide linkage
What allows the local anaesthetic to rapidly diffsue across lipid membranes of epithelial cells and bind
Cause they can be lipophilic and hydrophobic, and charged or uncharged,
Differences between ester link and amide link
Ester link:
- Hydrolysed by cholinesterase
- Short duration = so ester linkage broken more easily than amide bond
Amide link:
- Resistant to hydrolysation
- Longer duration
Examples of ester link
- Oxybuprocaine
- Tetracaine
- Proparacaine
Examples of amide link
Lignocaine
What is common in esters
Allergic reaction
Onset of topical anaesthetic
1 minute
Duration of topical anaesthetic
Amethocaine = 20 mins
Benoxinate = 15 mins
Proxymetacaine = 15 mins
Lignocaine = 30 mins
Features of Tetracaine Hydrochloride (Amethocaine)
- Most potent
- Stings the most
- Provides deepest anaesthesia = so useful for foreign body removal
- Sensitivity is rare but occurs after repeated doses
- Associated with punctate staining due to stinging aspect of pH level
Contraindications/Cautions:
- Px’s taking sulphonomides
- Premature babies
- Known allergy
- Pregnancy/lactation
Preparations of Tetracaine Hydrochloride (Amethocaine)
– 0.5 and 1.0% in Minims
– Store below 25°C
- Protect from light cause the chemical structures are hydrolysed by light so drug will become damaged
– Solutions >1% damage cornea
Role/use of Tetracaine Hydrochloride (Amethocaine)
Removal of foreign body = possible after instillation of one drop
Features of Oxybuprocaine Hydrochloride (Benoxinate)
- Most widely used topical anaesthetic by Optometrists in UK
- Sensitivity reactions rare
-Less stinging, irritation than amethocaine
-Bactericidal properties
Contraindications/Cautions
* Known allergy
* If other eye drops with containing chlorhexidine acetate as a preservative are being used
* Pregnancy/lactation
* Premature babies
What is the most widely used topical anaesthetic by Optometrists in UK
Oxybuprocaine Hydrochloride (Benoxinate)
Preparation of Oxybuprocaine Hydrochloride (Benoxinate)
- Available in Minims 0.4%
- Store below 25°C, protect from light
How many drops needed in Oxybuprocaine Hydrochloride (Benoxinate)
Three drops instilled over five minute interval
Use of Oxybuprocaine Hydrochloride (Benoxinate)
Allows foreign body to be removed
How long does it take to recover from Oxybuprocaine Hydrochloride (Benoxinate)
Corneal sensation recovers after one hour
Features of Proxymetacaine Hydrochloride
- Stings the least
- Minimal punctate corneal staining
- Ideal for tonometry
- Least antibacterial properties
- Useful when taking conjunctival swabs
Contraindications/Cautions:
- Known allergy
- Overactive thyroid
- Heart disease
- Premature babies
- Sensitivity rare
Preparations of Proxymetacaine Hydrochloride
- Minims 0.5%
- Store 2-8C, have to keep in a
dedicated refrigerator
How many drops used in Proxymetacaine Hydrochloride
- 1 drop every 5 – 10 minutes
- 5 - 7 drops in total
Disadvantage of Proxymetacaine Hydrochloride
Very potent
Use of Proxymetacaine Hydrochloride
Can be used in cataract extraction
Features of Lidocaine Hydrochloride (Lignocaine)
- Stings less than amethocaine
- Amide link
- Used when patients sensitive to ester link
- Longer duration
Contraindications/Cautions:
- Pregnancy/lactation
- Premature babies
Preparations of Lidocaine Hydrochloride (Lignocaine)
- Available in Minims 4% combined with 0.25% fluorescein
- Store below 25°C, protect from light
Side effects of anaesthetics
-Transient stinging
- Transient blurring
- Conjunctival hyperaemia
- Mild superficial epithelial damage not uncommon, especially with tetracaine- represents local toxicity
- Punctate keratitis
- Systemic reactions rare due to the small quantities of drug involved
- Hypersensitvity reactions, though these are less likely with lidnocaine due to amide link,can range from mild transient blepharoconjunctivitis to diffuse necrotising epithelial keratitis
- Necrotising keratitis incidence ~1/1000 with ester drugs
- Repeated use = epithelial toxicity = serious keratopathy
- This can involve: epithelial loss, stromal oedema, corneal infiltrates, anterior chamber reaction
Precautions of anaesthetic drugs
- Permeability of epithelium cells increased by instillation of topical anaesthetic Enhances effect of other drugs
- Patient should be instructed not to rub eye or leave practice until anaesthetic effect has worn off
- Healing of epithelial cells reduced by use of topical anaesthetics
- Open to abuse
- Usually by health care practitioners
- Young children
