Anaesthetics Flashcards
indications for use
- contact tonometry
- cl fitting
- foreign body removal
- gonioscopy
- lacrimal procedures
- never for relief of symptoms - healing is delayed in an eye that is compromised - can elicit a severe type of keratitis
advantages of local anaesthetic
- allow certain procedures to be conducted
- make th epx more comfortable
- make procedures easier for practitioner
disadvantages of local anaesthetic
- sting (due to pH)
- delay healing - reduces collagen production
- eye is more susceptible to damage
- repeated instillations
mode of action
- sensory info 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 chage (depolarisation)
- efflux of potassium ions returns nerve to resting potential
- repolarisation
- action potential passes along nerve - successive depolarisations and repolarisatin of adjacent areas
- topical anaesthetics reversibly block nerve conduction by blocking sodium channels therefore ap can’t be generated
the significance of local anaesthetics having hydrophobic and hydrophilic portions
- lipid soluble hydrophobic portion and a readily ionisable ydrophilic portion ( that can switch from uncharged to charged form)
- its due to the fact that thye can be lipophilic and hydrophobic, and charged or uncharged, that enables them to rapidly diffuse across the lipid membrane of epithelial cells and bid to the intracellular portion of sodium channels
- the lipid soluble and hydrophillic portions are separated by an intermediate alkyl chain which contains either an ester or an amide linkage
chemical structure of anaesthetics - ester link
- hydrolysed by cholinesterase
- short duration
- oxybuprocaine, tetracaine, proparacaine
- ester linkage broken more easily than amide bond
- allergic reactions common in esters
chemical structure of anaesthetics - amide link
- resistant to hysrolysation
- longer duration
- lignocaine only
different topical anaesthetics, onset and duration
amethocaine (tetrocaine) - onset 1 min, duration 20 mins
benoxinate (oxybuprocaine) - onset 1 min, duration 15 mins
proxymetacaine (proparacaine) - onset 1 min, duration 15 mins
lignocaine (lidocaine) - onset 1 min, duration 30 mins
tetracaine hydrochloride key features and cautions/contraindications
- most potent
- stings the msot
- provides deepest anaesthesia - foreign body removal
- sensitivity is rare, occurs after repeated doses
- associated with punctate staining
- contraindications/causes: sulphonomides, prem babies, kown allergy, pregnancy/lactation
tetracaine hydrochloride preparations
- 0.5 and 1% in minims
- store below 25 degrees, protect from light
- hydrolysed by light - drug becomes damaged
- solutions > 1% damage cornea
oxybuprocaine hydrochloride
- most widely used by optoms in uk
- sensitivity reactions rare
- less stingin, irritation then amethocaine
- bactericidal properties - eg in hosp enviro and do goldmann before corneal culture taken
contraindications/cautions: known allergy, if other eye drops with containing chlorhexidine acetate as a preservative are being used, pregnancy/lactation, prem babies
side effects
- trainsieny stinging and blurring
- conjunctival hyperaemia
- mild superficial epi damage not uncommin, esp with tetracaine - represents local toxicity
- punctate keratitis
- systemic reactions rare due to small quantities of drug involbed
- hypersensitivity reactions, thoguh these are less likely with lidnocaine, 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: epi loss, stromal oedema, corneal infiltrates, ant chamber reaction
precautions
- permeability of epi cells increased by instilation of topical anaesthetics - enhances efefcts of other drugs
- px should be instructed not to rub eye or leave practice until anaesthetic effect has worn off
- healing of epi cells reduced by use of topic anaesthetics
- open to abuse - usually by health care practitioners
- young children
oxybuprocaine hydrochloride (benoxinate)
preparation - available in minims 0.4%, store below 25 degrees, protect from light
- can be used in opthalmology: three drops instilled over 5 min interval, allows foreign body to be removed, corneal sensation recovers after one hour
proxymetacaine hydrochloride
- stings the least
- minimal punctate corneal staining - ideal for tonometry
- least antibacterial properties - useful when taking conj swabs
- contraindications/ cautions : known allergy, overactive thyroid, heart disease, prem babies, pregnancy/lactation
