Pain Management Case of the Month: Patch

This case is about a dog named Patch who needed to undergo an eye enucleation. Patch is a male Australian cattle dog mix, who was born with bilateral micro ophthalmia.

Regional Analgesia for Eye Enucleation

by Michael C. Petty, DVM, CCRT, CVPP, DAAPM

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In animals we need to rely on direct palpation, distant observation of the animal, and grimace scales.

An eye enucleation is one of the more painful procedures that might be performed in private practice. Often, great lengths are taken to prevent self-mutilation after the surgical procedure: e-collars and “boxing glove” style bandages on feet are not uncommon. However, with proper pain control, no additional barriers or precautions need to be taken.

In human medicine, persistent postoperative pain is reported in 47% of patients undergoing an enucleation. No such statistics exist for our veterinary patients, but I would expect it to be as high, if not higher, because in people, pain is self-reported and subsequently controlled. In animals we need to rely on direct palpation, distant observation of the animal, and grimace scales. Additionally, I am not aware of any grimace scale that has been validated for any kind of facial surgery. In either people or our animal patients, if pain is not properly controlled in the postoperative procedure, hyperalgesia and allodynia (see sidebar) are common sequalae with a poor prognosis for resolution.

This case is about a dog named Patch who needed to undergo an enucleation. Patch is a male Australian cattle dog mix and at the time of surgery was 8 months old and weighed 15.5 kg. Patch was born with bilateral micro ophthalmia. He was completely blind in his right eye, and debris tended to accumulate within the orbit of the right eye causing discomfort unless the owner could flush it out.

Additionally, the small eye caused an entropion issue as the lids were not supported and turned into the orbit, rubbing the eye. The decision was made to remove the nonfunctioning eye to improve the comfort level for Patch. This case utilizes a retrobulbar block prior to surgery and a block using bupivacaine liposome injectable suspension (Nocita) during closure.

Hyperalgesia and Allodynia

Hyperalgesia and allodynia are abnormal conditions wherein there is an increased amount of pain secondary to neuropathic changes. Hyperalgesia is when a painful condition hurts more than expected. Allodynia is when a nonpainful stimulus causes pain to be felt. Allodynia is sometimes called light-touch pain.

Premedication, Induction, and Anesthesia

Hydromorphone 2 mg/mL was dosed at 0.9 mL and dexmedetomidine was dosed at 0.6 mL and combined in the same syringe and given intramuscularly. At 20 minutes after injection, Patch was heavily sedated, so a 22-gauge indwelling catheter was placed, and propranolol was injected to effect allowing for intubation. At that point, Patch was maintained on sevoflurane at varying concentrations of 0.5–1.5%. The following is a description on how to do a retrobulbar block followed by a Nocita block as it was done in Patch, but this procedure can be done in any dog or cat.

Retrobulbar Block

Retrobulbar injections are relatively easy to perform. No special equipment is needed: a 3cc syringe, a 1.5-inch 22-gauge needle with a minor modification, and lidocaine. Retrobulbar needles are available; however, they are an unnecessary expense.

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Figure 1: A 3cc syringe with the maximum dose of lidocaine, 1.5ccs.

 

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Figure 2: Modified needle with an approximate 20-degree bend.

The needle needs to be modified by putting an approximate 20-degree bend in it. Prior to bending the needle, you should draw up lidocaine at a maximum dose of 1.5ccs, keeping in mind the maximum may be less depending on the total body weight (Figure 1). Then, using aseptic technique, manually bend the needle to a 20-degree angle (Figure 2).

The next step is to insert the needle into the orbit, avoiding the eye, until it is positioned at the level of the optic nerve. The landmarks for the insertion point are the lateral canthus and the middle of the lower eyelid: choose a point halfway between the two of them at the level of the orbital rim. See Figure 3 and notice the needle between the two red dots.

Needle insertion and injection of anesthetic

CS2.jpgIllustration showing the needle’s path while performing a retrobulbar injection of anesthetic. (Illustration by Lauren Petty)

  • Insert the needle at this landmark and direct it along the floor of the orbit until you are past the globe of the eye, then redirect dorsally and toward the midline to reach the apex of the orbit. It is OK to “skid” the needle along the bone of the orbit to increase your confidence that you are keeping the tip of the needle away from the globe.
  • You may or may not encounter a slight “popping” sensation as the needle passes through the fascia within the orbit.
  • Aspirate to make sure you are not injecting into a blood vessel. • Slowly inject the anesthetic.
  • There should be no resistance during the injection. If you feel resistance, withdraw slightly, and try again. You want to inject around the optic nerve, not directly into it.

At this point you just need to wait a few minutes for the lidocaine to take effect prior to starting the surgical procedure.

CS5.jpgFigure 3: The red dots indicate landmarks for the retrobulbar injection insertion point.

Complications

The biggest complication is penetration of the globe. However, since you are using this for an enucleation, it only makes the surgical removal a bit more difficult with no other consequences. You can also hit a blood vessel, obscuring the surgery site if bleeding occurs. And finally, pay close attention to the amount of resistance as you don’t want to inject the drug in the optic nerve, which could allow for intrathecal uptake.

Nocita

Once the eye has been surgically removed, it is time to use Nocita. Nocita is a liposome encapsulated bupivacaine product from Elanco that allows for up to three days of local anesthesia. There is a slow release of the bupivacaine as the liposomes break down. Nocita should not be used in the same spot as lidocaine; however, this is not an issue as the injected portion of the ocular structure is removed. As an additional caution, the eye socket can be swabbed with a gauze sponge to remove any residual lidocaine.

Draw up the Nocita based on the animal’s weight and according to manufacturer’s direction. Inject the Nocita into the subcuticular layers prior to closure in the same technique you would use for a line block but on both sides of the surgical wound.

Before suturing the site closed, use some of the Nocita as a splash block. If there is any Nocita left after suturing the site closed, you can insert the needle through the sutured skin and deposit the remaining volume.

Pain Assessment

Gentle palpation of the surgical site is the best method for evaluation of pain control. It takes a few hours for Nocita to reach full effect. In the case of Patch, there was enough overlap between the lidocaine injection and when the Nocita took effect to control his pain. If need be, I could have injected additional hydromorphone and dexmedetomidine as a single bolus or as part of a constant rate infusion.

Discussion

Online Bonus Content

CS6.jpgClick here to see videos of this procedure.

In the case of Patch, these two methods were used to control the pain of an enucleation. However, these same methods can be used for any eye surgery requiring analgesia.

As simple as you might find the technique, it is best to do your first several procedures on an enucleation procedure where any “mistake” will be eliminated with the removal of the eye. If you want to read more on this procedure, I recommend the excellent text Small Animal Regional Anesthesia and Analgesia, edited by Luis Campoy, LV CertVA, DECVAA, MRCVS, and Matt Read, DVM, MVSc, DACVA (Wiley-Blackwell, 2013).

Michael Petty, DVM
Michael C. Petty, DVM, CCRT, CVPP, DAAPM, is in private practice in Canton, Michigan. He is a frequent national and international lecturer on topics related to pain management. Petty offers commentary on each Pain Case of the Month (and occasionally writes one himself). He was also a member of the task force for the 2015 AAHA/AAFP Pain Management Guidelines for Dogs and Cats.

 

Photo credits: Photos courtesy of Michael Petty, DVM; Illustration by Lauren Petty

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