Useful for desensitizing the conjunctiva and cornea only.
Not suitable where sharp instruments will be used as the animal may move suddenly and unpredictably.
Facilitates examination of painful eyes, collection of samples, removal of foreign bodies and cannulation for flushing of the naso-lacrimal duct.
Topical anesthetic:
Should not be used repeatedly due to corneal damage and delayed healing.
Topical agents (USA) include:
Alcaine (proparacaine hydrochloride 0.5%)
AK-T-Caine (tetracaine hydrochloride 0.5%)
Long acting for prolonged desensitization - amethocaine 0.5-1% .
Infiltration of local anesthetic
Temperament of the cat may mean extra restraint required.
Small body weight requires careful dose selection to avoid inadvertent overdosage, 0.5-1% solutions are safer.
Direct injection of appropriate small volumes of lignocaine/lidocaine 0.5-1% solutions at site of proposed incision.
For minor/superficial surgery where sharp instruments will not be used next to the eye itself in debilitated or sedated animals.
Regional local analgesia
Retrobulbar block
An alternative to the use of neuromuscular blocking agents (muscle relaxants) where surgery on the eye is difficult due to retraction into the orbit.
General anesthesia is required to allow the injection of an appropriate small volume of lignocaine/lidocaine 0.5-1% solution via the conjunctival sac into the retrobulbar space.
A small bore needle is used to minimize trauma.
Do not inject intravascularly.
A beneficial fall in intraocular pressure is produced.
Causes mydriasis which is required for intraocular surgery.
Can cause an increase in intraocular pressure (IOP).
Many animals undergoing ophthalmic surgery are old and may have other disease processes to consider .
Therefore full pre-anesthetic evaluation is necessary for such animals.
Specific preoperative preparation
Consider possible interactions between drugs being used to treat the ophthalmic condition and drugs which may be used in premedication and anesthesia.
Sympathomimetics
Adrenaline/Epinephrine used to decrease production of aqueous humor in glaucoma or to control hemorrhage in intraocular surgery.
Phenylephrine used to decrease production of aqueous humor in glaucoma.
These drugs increase the likelihood of cardiac dysrhythmias when halothane is used.
Anticholinesterases
Physostigmine and ecothiopate.
Used to constrict the pupil and to increase the drainage of aqueous humor.
Prolongs the duration of action of drugs metabolized by cholinesterases, eg procaine, suxamethonium/succinylcholine .
Diuretics
Dichlorophenamide, acetazolamide .
Reduce the production of aqueous humor by carbonic anhydrase inhibition, the diuresis produced can result in hypokalemia and hypovolemia.
Monitor potassium status and fluid balance, correct imbalances prior to inducing anesthesia.
Corticosteroids
Do not withdraw suddenly prior to anesthesia and surgery as this reduces the animal's ability to produce endogenous corticosteroids as part of the stress response triggered by anesthesia and surgery.
Awareness of pathophysiology of any concurrent disease process.
Awareness that intraocular pressure results from an equilibrium between aqueous humor production and drainage, and the resistance of the fibrous sclera and cornea to pressure.
Awareness that increase intraocular pressure is contra-indicated where penetrating wounds to the eye are present, and for intraocular surgery as this leads to increased prolapse of intraocular material.
Be aware of the oculorespiratory cardiac reflex. Manipulation of the eye can lead to:
Avoid respiratory depression as hypercarbia, hypoxemia, acidosis all increase intraocular pressure.
Avoid sudden increases in arterial blood pressure and central venous pressure, eg overtransfusion of intravenous fluids, as this can increase intraocular pressure.
Avoid stimulating the larynx during intubation - intubate after use of topical lidocaine and at adequate depth of anesthesia.
Avoid corneal drying and drugs which promote it eg ketamine .
Be aware that most general anesthetic drugs eg barbiturates, inhalational agents, reduce intraocular pressure by their actions on the central nervous, respiratory and cardiovascular systems and by increasing drainage of aqueous humor. The degree of reduction in intraocular pressure is directly related to the depth of anesthesia.
Anesthetic induced rotation of the eye (ventromedially usually) limits access to the cornea. Usually accompanied by retraction into the orbit and a degree of prolapse of the nictitating membrane. Use of neuromuscular blocking agents (muscle relaxants) avoids this, so stay sutures or scleral clips are not required avoiding the trauma associated with their use. Intermittent positive pressure ventilation will be necessary.
Other
Check previous clinical and anesthetic history, including any adverse reactions.
Be prepared to stop procedure if oculorespiratory cardiac reflex causes respiratory depression and/or bradycardia. Give intravenous glycopyrronium/glycopyrrolate or atropine .
Apnea may develop so be prepared to ventilate animal if not doing so already.
Instrumentation
Monitoring equipment required will be dictated by the risk status of the individual animal.
Blood pressure monitoring is helpful. Invasive or non-invasive, latter is easier in cats.
Selection of anesthetic technique - Standard considerations
Premedication
Aims
Avoid struggling/gagging/coughing/retching/vomiting which can all increase intraocular pressure.
Use anticholinergic drugs eg glycopyrronium/glycopyrrolate or atropine to reduce salivation, bronchial secretion (feline airway easily blocked due to small diameter) and provide protection against effects of oculorespiratory cardiac reflex.
Sedatives
Phenothiazines
Pros:
Lowers intraocular pressure by increasing drainage of aqueous humor.
Anti-emetic.
Protects heart from dysrhythmias during halothane anesthesia.
Produces hypotension contributing to low intraocular pressure.
Cons:
Lowers convulsive threshold in susceptible animals, intraocular pressure raised by convulsions.
Hypotension may be undesirable in some animals eg emergency surgery for ocular trauma in shocked/hypovolemic animal.
Alpha-2 adrenoceptor agonists
Do not use, high incidence of vomiting.
Analgesics
Morphine .
Do not use, high incidence of vomiting.
Pethidine/meperidine (Demerol) .
Buprenorphine . Vomiting rare, also produces some sedation.
Butorphanol .
Antitussive, avoiding coughing helps to maintain low intraocular pressure.
Fentanyl
These opioid analgesics should be used at an appropriate dose rate with sedative drugs to avoid excitability in cats. This has the advantage of producing a neuroleptanalgesia.
Benzodiazepines
Pros:
Anticonvulsant.
Relaxation of skeletal muscle, including extraocular, therefore may help reduce IOP.
Cons:
Can cause restlessness.
Minimal sedation.
Little reduction in anesthetic dose but prolonged sleeping time which can lead to hypothermia in recovery in the cat due to high surface area in relation to body weight of small animals.
Induction
Rapid, smooth induction desirable to avoid coughing/gagging/retching associated with increasing the intraocular pressure.
Achieve this by using intravenous induction agents unless contraindicated by risk classification and physical status of animal.
Allow sufficient time for desensitization of the larynx after application of topical local anesthetic spray before attempting endotracheal intubation.
Propofol
Pros:
More effective at reducing the pressor response to endotracheal intubation than thiopentone/thiopental .
Reduces intraocular pressure.
Recovery rapid and complete without incoordination.
Cons:
Occasional muscle spasms observed, unrelated to the depth of anesthesia.
Respiratory depression can be severe if induction dose given very rapidly.
Reduce intraocular pressure by increasing drainage of aqueous humor.
Relaxation of extraocular muscles.
Cons:
Central nervous system depression.
Repeat doses prolong recovery.
Prolonged recovery (a feature of pentobarbitone/pentobarbital anesthesia) , can lead to hypothermia.
Violent recoveries possible (particularly a feature of methohexitone/methohexital anesthesia) which can raise intraocular pressure and cause intraocular hemorrhage and/or wound dehiscence.
'Saffan' - alphaxalone + alphadolone acetate combination (Not in US).
Occasionally causes retching, vomiting or laryngeal spasm on induction, therefore avoid.
Ketamine
Do not use, increases intraocular pressure due to increased tone of extraocular muscles.
Further doses of intravenous agents may be used for minor procedures of short duration eg nictitating membrane flaps.
If lavage of conjunctival and corneal surfaces is required, endotracheal intubation should be carried out to protect airway from risk of inhalation of fluid. Also outflow from the naso-lacrimal ducts may pool in the oral cavity and pharynx.
For longer procedures eg corneal surgery, conjunctival grafts, intraocular surgery, use inhalational agents.
All inhalational agents reduce intraocular pressure.
Halothane
Pros:
Suitable.
Familiarity enhances safe use.
Cons:
Cardiac dysrhythmias more likely in the presence of adrenaline/epinephrine (catecholamines) . Adrenaline/Epinephrine is used to control hemorrhage during intraocular surgery eg lens removal.
Isoflurane
Pros:
Suitable.
Does not increase likelihood of cardiac dysrhythmias in the prescence of adrenaline/epinephrine (catecholamines).
Agent of choice in many situations dictated by animal's risk classification and health status.
Rapid, smooth recovery.
Nitrous oxide
Pros:
Suitable.
Reduces required dose of volatile anesthetic agents which leads to a reduction in the side effects of these agents.
Analgesic properties.
Muscle relaxation properties.
Cons:
Ensure sufficient oxygen is supplied.
Do not use if air is to be injected into the eye to replace intraocular content, or discontinue supply at least 5 minutes before such injection and flush rebreathing circuits frequently during that time. Nitrous oxide diffuses rapidly down concentration gradients into gas filled spaces.
Neuromuscular blocking agents
Must have adequate personnel to ventilate manually for the proposed duration of the procedure or access to reliable mechanical ventilation . Equipment should be checked to ensure output (Wright' s respirometer) and that correct adaptors are in place to connect endotracheal tube to anesthetic circuit prior to inducing paralysis.
Do not use suxamethonium/succinylcholine as this drug increases intraocular pressure.
Choice of non-depolarizing neuromuscular blocking agent will depend on animal's health status eg elimination of vecuronium is slowed in hepatic impairment.
Non-depolarizing agents' effects can be reversed with neostigmine or edrophonium together with an anticholinergic to prevent bradycardia developing as a side effect of the reversing drug. Spontaneous recovery usually begins 20 minutes after the dose of neuromuscular blocking agent, tidal volume can be measured to assess the adequacy of ventilation and support given if necessary.
Avoid hypoxia and hypercarbia while re-establishing spontaneous respiration.
Monitoring
Standard monitoring
Specific monitoring
Those parameters where alteration from normal values causes increased intraocular pressure should be monitored for intraocular surgery:
Arterial hemoglobin saturation and peripheral pulse (perfusion) by means of pulse oximetry avoid hypoxemia.
Arterial blood pressure by non-invasive means if possible, and central venous pressure . Avoid hypertension.
Avoid situations which increase intraocular pressure, and can cause wound breakdown, trauma and swelling of the palpebrae, hyphema and anterior uveitis:
Avoid repeated vocalization.
Avoid vomiting/retching.
Avoid violent recovery.
Avoid hypothermia and shivering.
If necessary continue sedation into the post operative period, eg acepromazine has a long duration of action.
Specific precautions relating to the animal's health status may also need to be taken eg continued monitoring of respiratory efficiency and ventilatory support where neuromuscular blocking agents have been used.
Brunson, D B (1980) Anesthesia in ophthalmic surgery.Veterinary Clinics of North America Small Animal Practice.10, 481-495 (Overview).
Crispin, S M (1981) Anaesthesia for ophthalmic surgery.Proceedings of the Association of Veterinary Anaesthetists of Great Britain and Ireland.9, 171 (Review by a leading ophthalmologist).
Other sources of information
Hall, L W and Taylor P M (1994) Eds Anaesthesia of the Cat. London: Bailliere Tindall. pp 152, 268. ISBN 0 7020 1665 9.
Weaver, B M Q (1989) Anaesthesia for ophthalmic surgery. In Manual of Anaesthesia for Small Animal Practice. Ed A D R Hilbery. Cheltenham: British Small Animal Veterinary Association. pp 101-105. ISBN 0 905214 09 9.
Petersen-Jones S & Crispin S (2002) BSAVA Manual of Small Animal Ophthalmology. 2nd edn. British Small Animal Veterinary Association. ISBN 0 905214 54 4
Apnea may develop so be prepared to ventilate animal if not doing so already. 
