Amethocaine (0.5-1%) has a similar duration of action as proxymetacaine but produces more discomfort on initial application.
Infiltration of local anesthetic
Uses : minor/superficial extraorbital surgery if sharp instruments will not be used next to the eye itself Either For very debilitated or deeply sedated patients Or In combination with neuroleptanalgesia.
Agents : lidocaine (1-2%), direct injection of small volumes (0.1-0.3 ml) at site of intended incision.
Supra-orbitalnerve block
Action : blocks sensation to upper eyelid.
Use : alternative to infiltration at the site for minor sugical procedures in the upper eyelid.
Agent : lidocaine (1-2%), bupivacaine 0.5%: inject an appropriate small volume (<0.5 ml) close to the nerve.
Sedation/precautions as for infiltration above.
Retrobulbar
Action :
Decreases intra-ocular pressure - beneficial.
Causes midriasis - requirement for intra-ocular surgery.
Use : alternative to neuro-muscular blocking agents (muscle relaxants) if eye surgery is difficult due to retraction into orbit.
Agent : general anesthesia is required as an injection of an appropriate small volume of lidocaine (1-2%) is made via the conjunctival sac into the retrobulbar space.
Small bore needle required, eg 25 g to minimize trauma. Do not inject intravascularly.
Auriculo-palpebral
Action : blocks a branch of the facial motor nerve → prevents blepharospasm.
Uses :
Prevents tightly closed lids putting pressure on the eye in the post-operative period.
Assists in removal of foreign bodies from conjunctival sac. Not an alternative to anesthesia as motor blockade only without any analgesia.
Preparation
Risk assessment
Many patients for ophthalmic surgery are old +/- have other conditions, eg diabetes mellitus (cataract surgery).
Full pre-anesthetic evaluation is necessary.
Pre-operative considerations
Pupil dilatation may be required - give pre-op atropine 1% or sodium fluroiprofen 0.03%.
Stay sutures in sclera to rectus muscles or the use of neuromuscular blockade may help to maintain good eye position.
Specific Pre-operative preparation
Consider possible interactions between drugs being used to treat the ophthalmic condition and drugs which may be used in pre-medication and anesthesia.
Pros :
Epinephrine: decreases production of aqueous humor in glaucoma; controls hemorrhage in intra-ocular surgery.
Phenylephrine : decreases production of aqueous humor in glaucoma.
Cons :
Increase likelihood of hypertension and cardiac dysrhythmias.
Anticholinesterases
Physostigmine, ecothiopate.
Constrict pupil and increase drainage of aqueous humor. Prolong the action of drugs metabolized by cholinesterase, eg procaine , suxamethonium .
Diuretics
Dichlorophenamide, acetazolamide .
Decrease the production of aqueous humor; diuresis may → hypokalemia and hypovolemia with prolonged use. Monitor potassium status and fluid balance; correct imbalance before anesthetic induction.
Corticosteroids
If already being administered avoid sudden withdrawal - risk of decreased animal ability to cope with stress response triggered by anesthesia and surgery. May require perioperative supplementation in addition.
Monitor blood pressure. If non-invasive methods are not available place central venous catheter +/- arterial catheter at this stage.
That intra-ocular pressure = equilibrium between aqueous humor production and drainage and resistance to pressure of fibrous sclera and cornea.
That increased intra-ocular pressure is contra-indicated if penetrating wounds to eye and if intra-ocular surgery because increases prolapse of intra-ocular material.
Of oculo-respiratory cardiac reflex. Manipulation of the extrinsic muscles of the eye and pressure on the globe can → severe respiratory depression, bradycardia and cardiorespiratory arrest.
Anesthetic - need to avoid
Drugs which increase intra-ocular pressure, eg ketamine, suxamethonium.
Respiratory depression because hypercarbia, hypoxemia, acidosis, all increase intra-ocular pressure.
Sudden increases in arterial blood pressure and central venous pressure, eg poor positioning and occlusion of the jugular veins, can increase intra-ocular pressure.
Excessively deep general anesthesia. Barbiturates and inhalation anesthetics decrease intra-ocular pressure via action on central nervous, respiratory and cardiovascular systems and drainage of aqueous humor.
Corneal drying and anesthetic drugs which promote it, eg ketamine .
Eye rotation caused by anesthetic (ventro-medially usually) which limits access to the cornea. Usually concurrent retraction into orbit and prolapse of nictitating membrane. Neuromuscular blocking agents (muscle relaxants prevent this and avoid need for stay sutures or scleral clips).
Other
Check previous clinical and anesthetic history including any adverse drug reactions.
Stop procedure if oculo-respiratory cardiac reflex causes respiratory depression +/- bradycardia and give intravenous glycopyrronium (0.1 mg/kg IV) or atropine (0.015-0.4 mg/kg IV) (be prepared to ventilate if apnea develops).
Monitor for tachycardia or other dysrrhythmias.
Instrumentation
Capnography and blood pressure monitoring useful.
Monitoring equipment required depends on risk status of individual animal.
Handle carefully - do not use a leash or other collar around neck, use harness (pressure on the jugular vein can cause a dramatic rise in intraocular pressure). Talk to blind animals when approaching and handling. If hospitalized make sure they can find their water bowl in their cage.
Aims
Avoid struggling/gagging/coughing/retching/vomiting - all can increase intra-ocular pressure.
Sedatives
Phenothiazines , eg acepromazine (0.02-0.05 mg/kg SC or IM 30 min prior to surgery).
Pros :
Lower intra-ocular pressure by increasing drainage of aqueous humor.
Rapid, smooth induction → avoids increased intra-ocular pressure through coughing/gagging/retching.
Use intra-venous agents unless contra-indicated by risk and physical status of animal. Consider spraying larynx with topical anesthetic, eg lidocaine in animals with ocular trauma because coughing/gagging at intubation increases prolapsing intra-ocular content. Alternatively lidocaine (1 mg/kg) IV at induction may also reduce reaction to intubation.
Steroidsand phenols
Propofol .
Pros :
More effective at reducing the pressor response to endotracheal intubation than thiopental .
Decreases intra-ocular pressure.
Rapid and complete recovery without inco-ordination.
Cons : Occasional muscle spasm unrelated to depth of anesthesia.
Alfaxalone
Pros : similar to propofol without the occasional muscle spasm.
Barbiturates
Thiopental , methohexitone , pentobarbitone .
Pros :
Increase drainage of aqueous humor → decrease intra-ocular pressure.
Relax extra-ocular muscles.
Cons : Violent recoveries possible (particularly methohexitone).
Top-up doses of intravenous agents for minor procedures of short duration, eg incising imperforate lacrimal puncta, nictitating membrane flaps, small tumor excision.
Use inhalation agents for longer procedures, eg corneal surgery, conjunctival grafts, intra-ocular surgery.
All inhalation agents decrease intra-ocular pressure.
Halothane.
Pros : Familiarity enhances safe use.
Cons :
Concurrent adrenaline used to control intra-ocular hemorrhage, eg lens removal, may cause cardiac dysrhymias.
Difficult to obtain.
Isoflurane.
Pros :
Concurrent epinephrine does not increase likelihood of cardiac dysrhythmias as much as halothane.
Agent of choice in many situations dictated by animals risk classification and health status.
Rapid recovery.
Sevoflurane
Pros : more rapid smooth inductions and recoveries.
Cons : only licensed in dogs.
Nitrous oxide.
Pros :
Decreases required dose of volatile anesthetic agents → decreases their side effects.
Analgesic properties.
Muscle relaxant properties.
Cons : Either Do not use if air is to be injected into eye to replace intra-ocular content Or Stop using at least 5 min before injecting air and flush rebreathing anesthetic circuits frequently during that time. Nitrous oxide diffuses rapidly down concentrations into gas filled spaces. Use neuromuscular blocking gas to provide a fixed, centrally positioned cornea without need for stay sutures or scleral clips. Do not use suxamethonium as it increases intra-ocular pressure.
Monitoring
Parameters which at abnormal values cause increased intra-ocular pressure:
Arterial hemoglobin oxygen saturation and peripheral pulse (perfusion), eg pulse oxymetry to avoid hypoxemia.
Arterial blood pressure by non-invasive means if possible.
Central venous pressure to avoid raised jugular vein pressure.
Blood gas, acid base measurement to avoid acidosis.
End-tidal carbon dioxide concentration to avoid hypercarbia.
Body temperature to avoid hypothermia.
Neuro-muscular blockade (train of four, tetanic stimulation).
Other health status , eg intra-operative blood [glucose] if diabetes.
Clutton R E, Boyd C, Richards D L S & Schwink K (1988) Significance of the oculocardiac reflex during ophthalmic surgery in the dog.JSAP29 , 573-579.
Brunson D B (1980) Anesthesia in Ophthalmic Surgery.Vet Clin North Am Small Anim Pract10 , 481-495 (Overview) PubMed.
Crispin S M (1981) Anesthesia for Ophthalmic Surgery.Proc Ass Vet An GB & Ireland9 , 171 (Review by a leading ophthalmologist).
Other sources of information
Gelatt K N & Gelatt J P (2001) Anesthesia for ophthalmic surgery. In: Small Animal Ophthalmic Surgery: Practical Techniques for the Veterinarian. Chapter 3. Butterworth Heinemann.
Weaver B M Q (1989) In: Manual of Anaesthesia for Small Animal Practice, Anaesthesia for Ophthalmic surgery. British Small Animal Veterinary Association. pp 101-105.
Vetstream contributor(s)
Dr Sheilah Robertson PhD DipACVA DipECVA
Dr Jackie Brearley MA VetMB PhD DVA DipECVA MRCA MRCVS
Not suitable if sharp instruments are to be used because animal may move suddenly.
Consider possible interactions between drugs being used to treat the ophthalmic condition and drugs which may be used in pre-medication and anesthesia.