The goals for anesthesia in the ophthalmic patient include providing appropriate analgesia for the particular procedure
or condition, maintaining or lowering intraocular pressure (IOP), preventing activation of the oculo-cardiac reflex (OCR),
and providing a level of anesthesia that will allow surgical manipulation of eye and surrounding structures while maintaining
normal cardiovascular function
Intra-ocular pressure (IOP)
Normal IOP in the dog ranges from 10-26 mmHg. In the cat it ranges from 12-32 mmHg. Significant increases in intraocular pressure
can lead to globe prolapse or rupture, damage to the optic nerve, or retinal detachment. There are many factors that can affect
IOP in the peri-anesthetic period. Restraint, neck leads, jugular compression for blood draws, coughing, gagging or vomiting
can result in increased IOP. Endotracheal intubation, as well as many anesthetic drugs can also cause increased pressure.
Even physiologic factors such as sudden increases in blood pressure and hypercapnia from hypoventilation can increase IOP
and potentially lead to damage or blindness.
The oculo-cardiac reflex (OCR)
The oculo-cardiac reflex or OCR refers to a sudden drop in heart rate associated with traction on the eye and/or surrounding
structures. Traction on the eye puts pressure on the trigeminal / vagus nerves of the parasympathetic nervous system leading
to bradycardia and brady-arrhythmias such as atrioventicular block or sinus arrest. Certain patients are more likely to have
this drop in heart rate than others. Pediatric/neonatal patients and brachycephalic patients tend to have high vagal tone
and are pre-disposed to activation of the OCR. Anti-cholinergic drugs, such as glycopyrrolate, can be administered as needed
or as part of the premedication to prevent or treat this physiologic response.
The goals of premedication in the ophthalmic patient are to minimize stress, to decrease or maintain IOP and to prevent stimulation
of the OCR. Sedation can be provided by a tranquilizer such as acepromazine in many patients. This drug provides excellent
anxiolysis and can lower blood pressure. This drug should be used with care in pediatric or geriatric patients and avoided
in patients with hepatic disease. In some patients a benzodiazepine tranquilizer, such as midazolam or diazepam, is preferable.
Sedation is generally adequate in patients that are debilitated in some way. Analgesia is often provided by the use of opioid
drugs as they provide excellent pain control as well as sedation. One drawback to opioid use is that full agonist opioid drugs
(morphine, hydromorphone) can cause vomiting after administration and thus can increase IOP. The pre-emptive administration
an anti-cholinergic drug should be considered based on the patient and the preferences of the clinician and/or anesthetist.
An anticholinergic drug should always at least be available and can be given as needed.
Induction of patients with intraocular disease or those that are undergoing intraocular surgery can be achieved with thiopental,
propofol or etomidate depending on the needs of the individual. Ketamine and ketamine combinations should be avoided since
this drug increases IOP. Mask and box inductions should also be avoided since excess struggling often occurs. A patient undergoing
extraocular surgery where IOP is of less concern may receive any standard induction drug or drug combo and ketamine may actually
be helpful as it provides additional analgesia for painful surgeries.
Maintenance of anesthesia is usually with volatile inhalant anesthetics such as isoflurane or sevoflurane. Nitrous oxide can
also be used but is not recommended for intraocular procedures as nitrous oxide diffuses into closed, air-filled spaces and
can increase IOP if air bubbles are present. Injectable agents may also be used for certain procedures. Propofol can be given
as a constant rate infusion or intermittently as needed. For short extra-ocular procedures patients can be sedated with alpha-2
combinations. Dexmedetomidine/butorphanol (dogs) or ketamine/ dexmedetomidine/ butorphanol (cats) can be helpful combinations.