Anesthesia for ophthalmic surgery (Proceedings)

Patients that present for ophthalmic surgery are often geriatric or pediatric, painful and anxious, and may present as emergency cases. Not uncommonly, the reason for their ocular complaint is a direct result of systemic disease, for example diabetic cataracts. Anesthetic challenges include safe choices of drugs in a patient who may be very young or very old, or in a patient with underlying disease, assuring a calm and pain-free recovery during which the patient does not traumatize their eye, minimizing increases in intra-ocular pressure, ensuring no sudden patient movement during intricate surgical steps, and, for intra-ocular procedures, a central and dilated pupil. Further anesthetic challenges (as with any patient) include maintaining good blood pressure and heart rate, good ventilation, maintaining adequate anesthetic depth and ensuring a smooth recovery without lengthening the hospital stay unnecessarily. Some of these goals may appear conflicting; for example, it will be necessary to provide excellent analgesia for the recovery phase, yet systemic analgesics such as opioids may prolong recovery and/or lengthen the hospital stay. Likewise, assuring that there is no risk of sudden patient movement during surgery may cause the anesthetist to make the animal excessively deep, which will in turn depress cardiovascular and respiratory function.

This lecture will take a case-based approach to anesthetic management of patients presenting for various ophthalmic procedures.

Entropion repair – Sharpei puppy

This is an otherwise healthy 6 month old female puppy. Anesthetic concerns would include: achieving good sedation for IV catheter placement, providing good analgesia for recovery without depressing ventilation excessively, maintaining normothermia, and maintaining good blood pressure, heart rate and ventilation during surgery. At 6 months of age, this puppy likely has relatively mature hepatic and renal function, but hypoglycemia and clearance of drugs may be a concern. Also, at this age blood pressure tends to be physiologically lower than in adults and blood pressure is more heart rate dependent. Thus, we want to prevent excessive bradycardia and we may want to choose anesthetic drugs that are either reversible or have extra-hepatic clearance.

Pre-anesthetic preparation would include a thorough physical exam and screening bloodwork, e.g. PCV, TP, glucose and BUN via azostick. Premedication choices should include a sedative and an agonist opioid for analgesia. For sedation acepromazine should only be used at very low doses because of its long duration and extensive hepatic metabolism. Other choices for sedation could include midazolam: desirable in that it is reversible and short-acting, but undesirable in that it provides little to no sedation when used alone in healthy young animals. A 3rd choice for sedation could be dexmedetomidine, which would certainly provide excellent sedation and is reversible if its effect is prolonged, but this sedative will cause bradycardia and depress ventilation. My personal choice would be a very low dose of acepromazine (0.01-0.02 mg/kg IM) combined with an effective agonist opioid. The inclusion of the opioid provides analgesia and synergistic sedation. Reasonable opioid choices could be hydromorphone (may cause panting), oxymorphone (expensive), or morphine (may cause nausea but this is offset by acepromazine to some extent). So let's say we decide to premedicate this patient with acepromazine and mid-dose morphine (0.5 mg/kg IM). Now 15-30 minutes later we have a moderately sedate animal that is amenable to IV catheterization (always a challenge in the Sharpei!). For this case the choice of induction drug is wide open, but propofol or ketamine with diazepam would be the two most likely choices. Ketamine may increase intra-ocular pressure, but this would not be a concern in an entropion repair. Induction would be followed by intubation and maintenance with inhalants (isoflurane or sevoflurane) in oxygen. Anesthetic management would include monitoring of temperature, blood pressure, heart rate, oxygenation (pulse oximetry), and ventilation (capnography or visual assessment), with occasional glucose checks to make sure this puppy is not becoming hypoglycemic. Fluid therapy would include any isotonic crystalloid at 10 ml/kg/hour. Once good blood pressure monitoring was in place, an NSAID such as carprofen could be administered for intra- and post-operative analgesia. Furthermore, lidocaine could be infiltrated into the eyelids for analgesia (Guiliano EA, Regional anesthesia as an adjunct for eyelid surgery in dogs. Topics Companion Animal Medicine, 2008 Feb; 23(1):51-6.).

For recovery of this patient, I would want to make sure that she does not paw at or traumatize her repair, and I would want to keep her temperature normal. While not truly a brachycephalic breed, we would also want to ensure that she could "protect" her airway after extubation and that she is awake enough to breath normally. For analgesia, if I have used morphine in my premedication and she has received an NSAID during surgery, she may not need any additional opioid at recovery, since morphine is relatively long acting (4-6 hours). If I used a shorter-acting opioid, however, then I would have a low dose of hydromorphone or oxymorphone on hand at extubation so that I could give a small amount quickly IV. I would not choose morphine in this instance as it can cause histamine release when given IV as a bolus.

Enucleation – Husky

This is a middle-aged male neutered malamute presenting with refractory 1o glaucoma with a painful eye that needs to be removed. The dog is otherwise healthy but has an anxious and unpredictable temperament. Anesthetic concerns would be managing the dog's temperament and personnel safety, ensuring a pain-free recovery, preventing dysphoria at recovery (a common issue with northern breed dogs), being prepared for surgical blood loss (rare for this procedure), and maintaining good temperature, heart rate, blood pressure and ventilation during surgery.

Pre-anesthetic preparation would include a thorough physical exam and screening blood work, in this case either just a PCV, TP, glucose and azostick or a full CBC and chemistry depending on the physical and history and practitioner preference. This breed of dog does not usually sedate very well with agonist opioids, so if an agonist opioid was chosen (e.g. morphine, hydromorphone or oxymorphone) it would need to be combined with either a moderate dose of acepromazine or with dexmedetomidine. This is also a surgery where a local anesthetic retrobulbar block could be used for analgesia, so large doses of opioid can be avoided. Because we need good sedation for personnel safety, my personal choice would be to sedate this dog with dexmedetomidine (3-7 mcg/kg IM) and combine it with butorphanol or methadone, both of which are less likely to cause dysphoria in this breed. Anesthetic induction choices could be propofol or ketamine with diazapam, and maintenance and anesthetic monitoring could be routine, with IV crystalloids at 10 ml/kg/hour and vigilance to blood pressure and possible blood loss. For analgesia, a retrobulbar block with bupivicaine could be easily performed. Contra-indications to this block would be neoplasia and possible seeding of neoplastic cells, which is not the case here. This technique has been described (Accola PJ, Bentley E, Smith LJ, Forrest LJ, Baumel CA, Murphy CJ. Development of a retrobulbar injection technique for ocular surgery and analgesia in dogs. J Am Vet Med Assoc, 2006 229(2):220-225; Myrna KE, Bentley E, Smith LJ, Local anesthetic retrobulbar block reduces postoperative pain following enucleation of canine eyes: a randomized, placebo controlled, double-masked clinical trial, JAVMA in press.)

For recovery of this dog, if the retrobulbar block has been performed, and if dexmedetomidine was used for preanesthetic sedation, there may be no need for additional sedation or analgesia. If the dog is anxious, agitated, or dysphoric on recovery, a low dose of dexmedetomidine (1-2 mcg/kg IV) could be administered.

Eyelid laceration – Domestic shorthair cat

This case presents as an emergency to your clinic. The repair will require general anesthesia. The cat is a 12 year old female spayed calico. There is no pertinent history, other than the cat returned home with the eyelid laceration after a night outside. You are able to perform a physical exam which is unremarkable other than the cat appears slightly dehydrated and is in poor body condition. You are not able to perform full lab work at this hour of the night, so you do a quick PCV, TP, and azostick. The PCV is 30%, TP is 7.0, and the azostick is high at 30-45. Because of the cat's age, physical exam and blood work, you suspect that she has renal insufficiency but you are not able to work it up further at this time.

Anesthetic concerns include: a geriatric cat with limited organ "reserve", likely renal insufficiency and reduced renal clearance of drugs, underlying metabolic acidosis and anemia, dehydration. Because of her poor body condition she will be more at risk for hypothermia during anesthesia. The surgery is anticipated to be moderately painful and she will need good analgesia for recovery. Because of her reduced organ reserve, particularly her renal reserve, we will need to be very vigilant that her blood pressure and oxygenation are good and we should avoid NSAIDs in this patient because of potential nephrotoxicity. We also want to minimize her stress, as the associated catecholamine release will further compromise her renal blood flow.

Things to avoid in this patient would be a mask or box induction (stress! And she could further damage her eyelid), NSAIDs, ketamine or telazol because of their renal clearance in cats, high doses of acepromazine or dexmedetomidine because of resulting hypotension and reduced organ blood flow. My choices for premedication in this patient would be midazolam (0.1 mg/kg IM) and oxymorphone (0.1 mg/kg IM), a combination that often provides light sedation in cats, particularly when they are geriatric. IF her temperament required, and I really needed to have good control and restraint for an IV catheter, I would give her a very small dose of dexmedetomidine transmucosally or IM (1-2 mcg/kg). Once an IV catheter was placed, I would try to rehydrate her over an hour's time, estimating that she is about 3% dehydrated. After rehydration I would induce her with propofol, avoiding ketamine or other dissociatives because of the renal concerns. Anesthetic maintenance could be with sevoflurane or isoflurane in oxygen, with continued crystalloids IV at 5-10 ml/kg/hour depending on what her PCV is after rehydration. Monitoring should include vigilant blood pressure measurement and attention to oxygenation with a pulse oximeter. Capnometry and maintaining her CO2 in a normal range is also very important, as hypoventilation and respiratory acidosis will further lower her pH which is also likely low from metabolic acidosis.

For recovery in this cat I would consider redosing with oxymorphone once she is extubated. I would not expect her to be extremely painful after an eyelid repair, but because we omitted NSAIDs she might need additional opioid analgesia.