A look at analgesia for enucleation

We cannot rely on the increasingly popular grimace scales to grade post-surgical pain level

Figure 1 (top): The maximum dose lidocaine is 1.5 cc or less. Figure 2 (bottom): Put an approximately 20-degree bend about halfway down the needle length. Photos courtesy Michael C. Petty
Figure 1 (top): The maximum dose lidocaine is 1.5 cc or less.
Figure 2 (bottom): Put an approximately 20-degree bend about halfway down the needle length.
Photos courtesy Michael C. Petty

The surgical procedure for an eye enucleation is within the reach of any reasonably skilled surgeon. However, the pain during and after a procedure can be excruciating. This is especially true when the enucleation is occurring because of a chronically painful condition, such as glaucoma; wind-up pain can lead to an acute-on-chronic scenario wherein the enucleation produces much more pain than the veterinary team might have expected.

Many times, the surgeon and nurse must rely on various methods of restraint—E-collars, boxing glove bandages—to prevent the patient from causing injury to itself.

Given the animal may have been experiencing pre-operative pain, the likelihood of the development of persistent post-operative pain is much greater than it otherwise would be. Yet, even if pain had been well controlled pre-operatively, as with any surgical procedure, uncontrolled post-operative pain can easily lead to allodynia or hyperalgesia, which can persist for the life of the animal.

As if this is not bad enough, we cannot rely on the increasingly popular grimace scales to grade post-surgical pain levels; with the surgical procedure being performed on the face, any such grimace evaluation becomes unreliable. We must rely on other signs, such as body position and direct palpation. However, if we can provide complete analgesia to the region, we can more reliably predict a good post-operative outcome of pain relief.

This article addresses both parenteral and local analgesia. The local analgesia will include a retrobulbar block, the key to performing a pain-free surgery, and the injection of Nocita, the slow-releasing bupivacaine liposomes.

Parental analgesia and anesthesia

I have always been a fan of the “doggy magic” and “kitty magic” multimodal injections. These drugs work together to provide a wide margin of analgesia and sedation, at lower doses than what you would use for any one of them alone. This provides a more complete analgesic profile at a much-improved margin of safety because of the reduced dosing of the individual medications.

A typical “magic” combination would be an opioid, dexmedetomidine, and ketamine. I also like this combination because of the reversibility of mu opioids, like morphine, hydromorphone, or fentanyl, as well as being able to reverse the alpha-2 component of the injection, should there be an adverse event requiring it to happen.

After the full effect of the “magic” has taken place, intubation is sometimes possible without using another drug like propofol or alfaxalone. The animal can then be maintained on the inhalant of your choice.

To find charts for administering these “magic” combinations, you can look at ivapm.org or vasg.org.

The retrobulbar block

Figure 3: The needle between the two red dots on the eyelid.
Figure 3: The needle between the two red dots on the eyelid.

Although scary at first, retrobulbar injections are easy to perform. And don’t forget, you are using this for the purpose of removing the eye, so if you accidentally penetrate the globe it will make for a more difficult surgery; but the eye was coming out anyway, so there is no harm in the end. The equipment required to do a retrobulbar block include a 3-cc syringe with a 1.5 in., 22- gauge needle and lidocaine. That is it!

Step 1) Draw up the lidocaine for injection, using aseptic technique as the syringe and needle will be handled by the surgeon. The maximum dose here is 1.5 cc or less, depending on the size of the animal and the calculated safe dose for that animal (Figure 1).

Step 2) The needle has to have a bend in it to accommodate the curve of the orbit. You will need to put about a 20-degree bend about halfway down the needle length (Figure 2).

Step 3) This involves getting the top of the needle to the level of the optic nerve, while avoiding the eye itself. The landmark for insertion is the midway point between the lateral canthus and the midpoint of the inferior lid. Refer to Figure 3 and observe the needle between the two red dots on the eyelid.

  • After you insert the needle at the spot described above, direct it along the floor of the orbit with the back side of the eye/optic nerve as the target. In order to do this, you will have to redirect the needle dorsally to reach the apex of the orbit (Figure 4).
  • I often skid the needle along the bony orbit to make sure I am staying away from the globe.
  • You might encounter a slight popping sensation as you advance the needle through the orbital fascia.
  • Once you have reached your target, aspirate to make sure you are not injecting into a blood vessel.
  • Slowly inject the anesthetic: There should be no resistance. If you feel resistance, you could actually be inside the optic nerve, so withdraw slightly and then try to inject again.

Step 4) Wait a few minutes for the lidocaine to take effect prior to starting the enucleation.

Complications

As mentioned earlier, the biggest risk is penetration of the globe, but as this is an enucleation, it only complicates the surgery, not the patient.

Figure 4: Redirect the needle dorsally to reach the apex of the orbit.
Figure 4: Redirect the needle dorsally to reach the apex of the orbit.

Another risk is lacerating a blood vessel on advancement of the needle. This can also complicate the surgery, but is no risk to the patient. The least likely risk but biggest concern is injection of lidocaine directly into the optic nerve as this can result in intrathecal uptake.

Nocita

Lidocaine provides analgesia during and for a few hours post-surgery. Nocita can extend the analgesia for up to three days. Nocita is a liposome-encapsulated bupivacaine product that was originally licensed for use for post-operative cruciate surgery. However, off-label use, such as here, provides opportunities to provide pain relief for many different surgical procedures, such as this enucleation.

It is suggested lidocaine and Nocita should not be used in the vicinity of one another, but as you are removing the ocular contents, and hence the lidocaine, this is not an issue for this procedure after the retrobulbar block. As an additional caution, I always wipe out the empty socket with gauze to make sure there is no residual lidocaine left behind.

Dose the Nocita based on the manufacturer’s recommendation; however, it is unlikely you will use the entire amount except in the smallest of animals. Inject the Nocita into the subcuticular layers prior to closure in a line-block technique, but injecting on both sides of the incision. Prior to suturing the layer closed, use some of the remaining Nocita as a splash block.

Post operatively

Always check for pain using direct palpation, both immediately post-operatively and at a time when you expect the lidocaine to have lost its efficacy. There is no such thing as something that works 100 percent of the time, and pain evaluation is mandatory for all surgical procedures.

Michael Petty, DVM, is a graduate of the veterinary school at Michigan State University. As the owner of Arbor Pointe Veterinary Hospital and the Animal Pain Center in Canton, Mich., he has devoted his professional life to the care and well-being of animals, especially in the area of pain management. Dr. Petty is the past president of the International Veterinary Academy of Pain Management. A frequent speaker and consultant, he has published articles in veterinary journals and serves in an advisory capacity to several pharmaceutical companies on topics of pain management. Petty has been the investigator/veterinarian in 12 FDA pilot and pivotal studies for pain management products. He has lectured both nationally and internationally on pain management topics.

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