Figure 1: Spinal needle in place to perform retrobulbar nerve block prior to standing enucleation. Photos courtesy Megan Williams Enucleation in horses may be indicated for treatment of many conditions, including severe ocular or periocular trauma, ocular or periocular neoplasia, corneal or intraocular infection that is not responsive to treatment or for which treatment is cost prohibitive, or other conditions resulting in severely painful and/or nonvisual eyes. This procedure may be performed on a referral basis, but can also be performed by general practitioners. Most clinicians prefer to perform this procedure under general anesthesia, which eliminates concerns about patient compliance, improves the ability to use meticulous aseptic technique, and minimizes the need for performing more technically challenging blocking techniques. However, there are cases and circumstances where performing this procedure in the standing, sedated patient would be desirable, or even preferable, to doing so under general anesthesia. Case selection Multiple studies have documented a relative increase in risk associated with general anesthesia for horses compared with other species.1,4 Ophthalmic procedures have also been associated with a relatively higher incidence of anesthetic complications and/or poorer quality recoveries versus other types of elective surgery.6 Appropriate case selection is important for successful standing enucleation. Some examples of good candidates for this procedure might include patients that pose a higher-than-average anesthetic risk, such as geriatric individuals, horses with cardiac abnormalities or other comorbidities, or horses with significant musculoskeletal abnormalities that pose challenges for anesthetic recovery. Client budget also plays a role, and, in some cases, standing enucleation may be elected primarily because it eliminates the cost associated with general anesthesia. Cases with extensive periocular neoplasia warranting exenteration, or cases with significant infection present, are poorer candidates for standing enucleation due to the more involved nature of the procedure in these cases, greater likelihood of significant hemorrhage, increased risk of surgical site contamination, and subsequent infection. Additionally, patients with behavior that does not lend well to standing surgical procedures are likely poor candidates. Be prepared Appropriate preparation is also critical for success when performing enucleation in the standing horse. The procedure should be performed in a clean, dust-free environment with adequate lighting. Patient preparation should include achieving and maintaining an appropriate plane of sedation. As this procedure can be more time-consuming than what could be completed with a single bolus dose of sedative (typically an alpha-2 agonist such as xylazine or detomidine), placement of an intravenous catheter is recommended. The horse can then receive a bolus dose of sedation with an alpha-2 agonist followed by a constant rate infusion of sedative (usually detomidine with or without butorphanol), with the rate of administration increased or decreased throughout the procedure as needed based on patient compliance and level of sedation. This affords a more consistent plane of sedation that is easily adjusted by a technician or assistant. Surgical site preparation proceeds as it would if the procedure were performed under general anesthesia. The surrounding area is clipped and aseptically prepared. Use of dilute betadine solution and saline around the eye itself is preferable as for any ophthalmic procedure, and thorough saline lavage is recommended for the cornea, conjunctiva, and third eyelid. Chlorhexidine or betadine scrub alternating with alcohol can be used to aseptically prepare the surgical site surrounding the eye. Accurate placements One of the most critical steps to ensure a smooth and successful standing enucleation is accurate placement of nerve blocks to eliminate sensation for the patient and eliminate movement of the eyelids and globe. Multiple blocks must be performed to accomplish proper sensory and motor blockade of the region. The following nerve blocks are recommended: 1) Eyelids and cornea: Auriculopalpebral (motor) Supraorbital (sensory) Lacrimal (sensory) Infratrochlear (sensory) Zygomatic (sensory) Cornea/conjunctiva topically desensitized with proparacaine hydrochloride 0.5% 2) Globe and surrounding structures: Retrobulbar block (sensory and motor (Figure 1); and/or 4-point block (sensory and motor) Blocks should be performed using lidocaine or mepivacaine, after aseptic surgical site preparation has been completed. It is important to note performing the retrobulbar block (and/or the 4-point block) is a more technically difficult block and does have potential complications. They include accidental penetration of the globe or optic nerve, hematoma or periorbital abscess formation, and in rare cases intrameningeal injection.5 Therefore, this block should be performed following meticulous aseptic preparation and with a thorough understanding of the relevant anatomic landmarks. An additional round of aseptic preparation after all blocks have been completed may be beneficial and affords time for the blocks to take effect before starting the procedure. While it is difficult to drape this area in the standing horse, use of a clean halter and/or small sterile drapes or huck towels to cover the halter beneath the eye and over the bridge of the horse’s nose may help to maintain sterility during the procedure and help avoid dragging suture across contaminated surfaces. Figure 2: Performing dissection around and toward the back of the globe prior to transection of canthal ligaments and extraocular musculature. Surgical technique for a standing enucleation does not differ from when the procedure is performed under general anesthesia. The transpalpebral enucleation technique is ideal in this scenario because it can be rapidly performed and is simpler to perform than alternate methods. The basic steps of this procedure are as follows: 1) Suture eyelids closed using 0 or 2-0 suture. 2) Make an elliptical, full thickness skin incision adjacent to your suture line. 3) Dissect toward the back of the globe, avoiding accidental entry of the conjunctival sac (Figure 2). 4) Transect medial and lateral canthal ligaments and extraocular muscles. 5) Transect the retractor bulbi muscle and optic nerve (many surgeons will place a clamp on the nerve prior to transection). 6) Temporary placement of laparotomy sponge as needed to control hemorrhage. 7) Lavage surgical site with sterile saline. 8) Close subcutaneous tissues using synthetic absorbable suture, suggested size 2-0. 9) Close skin using cruciate, simple interrupted, or other preferred pattern, suggested size 0. Peri- and post-operative care Peri- and post-operative care for standing enucleation generally does not differ from when this procedure is performed under general anesthesia. Administration of tetanus prophylaxis is recommended if the horse has not been vaccinated for tetanus in the past four to six months. Duration of administering perioperative antimicrobials is dependent on individual surgeon preference, and on the condition of the eye at the time of removal (infected tissues warrant a longer course of antibiotics). One retrospective study evaluating risk factors associated with surgical site infection following enucleation in horses found performing this procedure standing was associated with an increased risk of infection.3 However, other studies have indicated very low risk for development of a surgical site infection following standing enucleation.2,8 Analgesics should be administered prior to the start of the procedure as well, typically consisting of either phenylbutazone or flunixin meglumine, and should be continued for two to three days post-operatively. A temporary bandage can be placed over the surgical site, taking care not to irritate or cover the opposite eye. Alternatively, a clean fly mask can be used to protect the surgical site until suture removal at 12 to 14 days post-surgery. While the concept of standing enucleation can be intimidating to inexperienced surgeons, the procedure is generally straightforward and can be completed quickly. Most horses tolerate the procedure surprisingly well, provided they are adequately blocked and sedated. Potential complications associated with the procedure include intraoperative hemorrhage and post-operative surgical site infection, however these complications can also be encountered when the procedure is performed in the anesthetized horse. Proper patient selection and preparation, maintenance of a good plane of sedation, accurate performance of nerve blocks to provide optimal perioperative analgesia, and appropriate aseptic and surgical technique can result in an excellent outcome. Megan Williams, DVM, DACVS (large animal) is an assistant professor of equine surgery at Oklahoma State University. She completed her surgical residency training at Michigan State University and worked as an equine surgeon in private practice for three years before coming to Oklahoma State in 2017. Dr. Williams can be contacted at megan.williams12@okstate.edu. References Bidwell LA, Bramlage LR, Rood WA. Equine perioperative fatalities associated with general anesthesia at a private practice – a retrospective case series. Vet Anaesth Analg. 2007; 34: 23-30. https://pubmed.ncbi.nlm.nih.gov/17238959/ Hewes CA, Keoughan GC, Gutierrez-Nibeyro S. Standing enucleation in the horse: A report of 5 cases. The Canadian Veterinary Journal. 2007 48.5: 512. https://pubmed.ncbi.nlm.nih.gov/17542371/ Huppes T, Hermans H, Ensink JM. A retrospective analysis of the risk factors for surgical site infections and long-term follow-up after transpalpebral enucleation in horses. BMC Vet Res. 2017; 13: 155. https://bmcvetres.biomedcentral.com/articles/10.1186/s12917-017-1069-5 Jones RS. Comparative mortality in anaesthesia. Br J Anaesth. 2001; 87: 813-815. https://pubmed.ncbi.nlm.nih.gov/11878679/ Labelle AL, Clark-Price SC. Anesthesia for ophthalmic procedures in the standing horse. Veterinary Clinics: Equine Practice. 2013; 29: 179-191. https://pubmed.ncbi.nlm.nih.gov/23498052/ Parviainen AK, Trim CM. Complications associated with anaesthesia for ocular surgery: a retrospective study 1989-1996. Equine Vet J. 1996; 32: 555-559. https://pubmed.ncbi.nlm.nih.gov/11093632/ Pollock PJ, Russell T, Hughes TK, et al. Transpalpebral eye enucleation in 40 standing horses. Vet Surg. 2008; 37: 306-309. https://pubmed.ncbi.nlm.nih.gov/18394080/ Townsend WM. How to perform a standing enucleation. Am Assoc Eq Pract Proc. 2013; 59: 187-190. https://aaep.org/sites/default/files/issues/OphthalTownsend.pdf
