How to Prepare for Ocular Surgery in the Standing Horse

How to Prepare for Ocular Surgery in the Standing Horse Brian C. Gilger, DVM, MS, Diplomate ACVO; and Michael G. Davidson, DVM, Diplomate ACVO Authors address: Equine Ophthalmology Service, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC 27606. 2002 AAEP. 1. Introduction There are many advantages to performing standing surgical procedures and avoiding general anesthesia in horses. Horses with orthopedic or other medical problems may be at higher risk to develop anesthesia-related complications and should not undergo routine general anesthesia. Even healthy horses can injure themselves on recovery from general anesthesia and are predisposed to develop colic, cecal impactions, and myositis in the post-anesthetic period. 1 3 Standing surgical procedures can also decrease the time needed for hospitalization compared with horses receiving general anesthesia. This is advantageous because hospitalized horses, in general, are predisposed to developing pneumonia, salmonellosis or other forms of colitis, and laminitis. 4,5 Furthermore, with decreased hospitalization time and fewer anesthesia-related complications, there is decreased cost to the horse owner. For the veterinarian, advantages of standing surgical procedures include less technical support required, less cost incurred, less time required, and decreased need for surgical and anesthetic equipment and facilities. Therefore, learning the correct and latest methods for performing standing ocular surgery will increase a veterinarian s ability to provide excellent service without a major outlay of infrastructure expense. Because the cornea and conjunctiva of the horse s eye are thin and delicate, ophthalmic surgery must be precise to avoid damage. The use of small instruments, needles, and sutures is generally required. In most instances, the surgeon will benefit from the use of magnification, which allows precise cutting of tissues and suture placement, but exaggerates patient movement when the horse is not anesthetized. To perform ocular microsurgery, the eye must be immobile. In general, the use of microsurgical technique and magnification during equine ocular surgery requires general anesthesia. However, with appropriate tranquilization, ocular nerve blocks (especially the retrobulbar nerve block), and restraint, many ocular surgeries can be performed adequately in standing horses. The purpose of this paper is to describe the equipment needed and technique for performing ocular surgical procedures in the standing horse. NOTES 266 2002 Vol. 48 AAEP PROCEEDINGS

Table 1. Ocular Surgical Procedures that May Be Done in a Standing Horse Adnexal Corneal Intraocular Small eyelid mass removal Corneal scraping for cytology Aqueocentesis Eyelid mass biopsy Ulcer debridement Intraocular injection (i.e., TPA) Eyelid lacerations small Grid keratotomy Iris cyst laser ablation Conjunctival biopsy Superficial keratectomy small Laser cyclophotocoagulation for glaucoma Third eyelid laceration repair Suture of non-perforating Intraocular mass laser ablation Third eyelid mass removal or biopsy corneal laceration Removal of superficial corneal foreign body 2. Materials and Methods Indications Ocular Surgical Procedures that May Be Done in a Standing Horse With appropriate tranquilization, ocular nerve blocks, and restraint, many short-duration ocular surgical procedures can be done in the standing horse (Table 1). In general, these are procedures involving the eyelids, conjunctiva, and cornea (tumor removal, keratectomy, eyelid laceration, third eyelid removal) that take less than 30 min to perform but do not include perforating lesions of the cornea (or lesions pending perforation such as corneal descemetoceles or infected deep corneal ulcers) or intraocular surgical procedures such as cataract surgery. However, minimally invasive intraocular procedures such as iris cyst laser disruption, 6 laser cyclophotocoagulation, 7 and anterior chamber centesis can be performed adequately with the horse standing. Standing surgical procedures should be performed in a specific order. Supplies should first be gathered, equipment should be set up, the horse should be tranquilized, eyelid nerve blocks performed, the retrobulbar nerve block area prepared aseptically and performed, the eye surgically prepped, topical anesthetics and vasoconstrictors applied, and finally the surgery should be performed. Supplies and Equipment Needed All supplies and equipment should be gathered and be accessible before positioning the horse. A list of supplies is provided in Tables 2 and 3. It is also recommended that the standing procedure be performed in stocks to protect the surgeon and assistants. Tranquilization The best tranquilizer for ophthalmic procedures in the horse is detomidine HCl a (0.01 0.02 mg/kg IV). This medication has a rapid onset and is effective for 30 45 min. The major advantage of this drug is that, during its effect, the horse s head remains still and does not tremor. Use of xylazine and/or butorphanol should be avoided because each causes head movement and exaggerated response to stimuli. Palpebral and Frontal Nerve Blocks To minimize eyelid movement and sensation of the upper eyelid, palpebral and frontal nerve blocks should be done. The palpebral nerve is a branch of the facial nerve that controls the motor activity of the upper orbicularis oculi muscle, which controls upper eyelid function. To adequately block the palpebral nerve, 2 ml of 2% lidocaine HCl b is injected SC along the dorsal zygomatic arch (Fig. 1). The frontal nerve is the branch of the trigeminal nerve that provides sensory innervation to the medial twothirds of the upper eyelid. To block the frontal nerve, 1 ml of 2% lidocaine HCl b is injected over the opening to the supraorbital foramen (Fig. 2), in which the frontal nerve exits. The frontal nerve block also denervates branches of the palpebral nerve and thus helps to decrease upper eyelid motor activity; therefore, a combination of both the palpebral and frontal nerve block provides an excellent denervation to the upper eyelid resulting in minimal eyelid movement. Table 2. Equipment and Supplies Needed for Standing Ophthalmic Surgical Procedures in the Horse General Equipment Syringes: 1, 3, and 12 cc 25-gauge needles 22-gauge 2.5-in spinal needle Ocular surgical preparation (Betadine scrub and solution) Eye wash Clean clippers Sterile surgical gloves Scalpel blades Sterile cotton-tipped applicators Sterile gauze Kimura spatula* Glass slides* Culture tubes and/or plates* Formalin* *Needed depending on type of procedure. Medications 2% Lidocaine HCl 1% Proparacaine 2.5% Phenylephrine Tranquilizer Eye lubrication ointment AAEP PROCEEDINGS Vol. 48 2002 267

Table 3. Basic Ophthalmic Surgical Instrument Pack Surgical Equipment Disposable Surgery Supplies Surgical Magnifying Loupes Bishop Harmon forceps 1 over 2 tissue forceps 0.8-mm teeth Derf needle holder Microsurgical needle holder Barraquer curved, without lock 9 11 mm with medium or fine jaw Stevens tenotomy scissors curved Eyelid speculum Castroviejo or Guyton-Park 14- to 16-mm blades) Beaver blade handle Bard Parker blade handle 6-0 or 7-0 Vicryl or Dexon on ophthalmic spatula needles 4-0 or 5-0 Monofilament on an ophthalmic cutting needle (nylon, surgilene, prolene, etc.) Blades: #64 Beaver blades #15 Bard-Parker blades Irrigating solution Keeler Loupes on a headmount Zeiss Prism Loupes on a headmount (4 with 450-mm working distance) Fig. 1. Palpebral nerve block in the horse. A: Location of palpebral (top white marker) and frontal nerve (bottom white marker) blocks in the horse. B: Two milliliters of 2% lidocaine HCl is injected SC along the dorsal zygomatic arch to block the palpebral nerve, which controls the motor activity of the upper orbicularis oculi muscle. Retrobulbar Nerve Block The orbital fossa above the dorsal orbital rim and zygomatic arch is first clipped and aseptically prepped with Betadine scrub and alcohol (Fig. 3). Care must be taken to avoid getting surgical scrub or alcohol on the ocular surface because severe irritation and corneal ulceration may develop. Once prepped, a 22-gauge, 2.5-in spinal needle c is placed through the skin perpendicular to the skull in the orbital fossa, just caudal to the posterior aspect of the boney dorsal orbital rim. The needle is advanced caudally to the globe until it reaches the retrobulbar orbital cone. When the needle advances to this location, the eye will have a slight 268 2002 Vol. 48 AAEP PROCEEDINGS

Fig. 2. Frontal nerve block in the horse. One milliliter of 2% lidocaine HCl is injected SC over the opening to the supraorbital foramen, from which exits the frontal nerve, a branch of the trigeminal nerve. This blocks sensory innervation to the medial two-thirds of the upper eyelid. dorsal movement as the needle passes through the fascia of the dorsal retrobulbar cone into the retrobulbar space (Fig. 3). Once positioned, 10 12 ml of 2% lidocaine HCl b is injected into the retrobulbar space. During the injection, the globe is pushed externally (i.e., slight exophthalmos), indicating an accurate placement of lidocaine. The lidocaine will take effect and anesthetize the eye in 5 10 min. The duration of effect is approximately 1 2 h. Because the eye is anesthetized, ocular sensation, blink reflex, and vision will be compromised. Therefore, stall rest and protection of the eye with lubricants are recommended for 2 4 h after anesthesia. Application of Topical Medications Once the retrobulbar block has been given, any cultures of the ocular surface should be done, if needed, before surgical preparation of the globe. Topical anesthetic (0.2 ml of 0.5% proparacaine HCl d ) and 2.5% phenylephrine e are applied to enhance ocular surface anesthesia and to constrict blood vessels in the cornea and conjunctiva to enhance hemostasis and visibility during surgery. The topical medications can be repeated every 15 20 min as needed to maintain effect. Surgical Procedures Following the above steps, the eye is now prepared for surgery. A list of possible ocular surgical procedures that can be done in the standing horse is listed in Table 1. Use of a twitch or other manual restraint is rarely required because the eye is anesthetized; however, certain horses do require additional restraint methods, such as use of a twitch or additional IV tranquilizer, especially when the preparation and procedure lasts longer than 30 40 min. Postoperative Management and Treatment Because the eye is anesthetized after the nerve blocks, sensation, blink reflex, and vision will also be compromised. Therefore, stall rest and protection of the eye with lubricants is recommended for 2 4 h after anesthesia. If the horse must be transported soon after the procedure or will not be monitored closely, a single temporary tarsorrhaphy suture can be placed laterally to protect the eye for the first 24 h after surgery. Other medications, such as topical ophthalmic antibiotics, topical atropine, and oral non-steroidal anti-inflammatory medications are prescribed frequently after ocular surgery. The frequency and duration depends on the severity and type of the ocular condition. 3. Results When the appropriate nerve blocks and procedures are done as described in the methods, the eye will be immobile during the surgery. The retractor oculi muscles will be paralyzed so that the eye cannot be retracted, and therefore the third eyelid will not move cranially. Visual and tactile stimuli will be reduced because the optic nerve and ophthalmic branch of the trigeminal nerve are anesthetized. This reduces stimuli to the horse and decreases induced head and body movements. This lack of movement of the eye allows the surgeon to adequately perform short ocular microsurgical procedures. Retrobulbar injections with lidocaine have been performed routinely to provide ocular anesthesia, with and without general anesthesia, for ocular surgery at the North Carolina State University College of Veterinary Medicine for approximately 12 yr. From 1996 to 2001, 189 retrobulbar injections were performed. Only two complications were documented. One horse had a hypersensitivity to lidocaine, which resulted in generalized hive formation and severe retrobulbar swelling after surgery. These lesions resolved with the use of systemic nonsteroidal anti-inflammatory and antihistamine medications within 3 days of surgery. The second horse developed a corneal ulcer after surgery, which was AAEP PROCEEDINGS Vol. 48 2002 269

Fig. 3. Retrobulbar nerve block in the horse. A: The orbital fossa dorsal to the eye is clipped and prepped surgically. B: A 22-gauge, 3.5-in spinal needle is inserted caudally to the orbital rim and advanced until it reaches the retrobulbar orbital cone (line drawing). When the needle advances to this location, the eye will have a slight dorsal movement as the needle passes through the fascia of the dorsal retrobulbar cone to the retrobulbar space. C: Lidocaine HCl (10 12 ml) is slowly injected into the retrobulbar space. The globe will become slightly exophthalmic, indicating proper placement of the lidocaine within the retrobulbar muscle cone. likely caused from exposure of the cornea because of poor eyelid function and corneal desensitization. 4. Discussion The sensitivity and rapid movement of the eye and third eyelid are the major impediments to performing standing ocular surgery. The retrobulbar retractor bulbi muscles in the horse are very powerful and cause rapid retraction of the eye and elevation of the third eyelid on stimulation. Stimulation during surgery can be sensation to touch (the cornea is one of the most innervated and sensitive structures in the entire body) and visual stimuli. The retrobulbar nerve block is effective in eliminating both of these stimuli by temporarily blocking the optic and trigemidal oculo-motor nerves. Although few complications have been seen after retrobulbar and eyelid nerve blocks in horses, rare problems associated with the injections can occur during or after the surgical procedure. Bacteria can be deposited in the orbit by the spinal needle if the skin surgical site was not aseptically prepared. 270 2002 Vol. 48 AAEP PROCEEDINGS

This may result in orbital abscess or cellulitis formation. Laceration of the extraocular muscles, optic nerve, sclera, or ophthalmic arteries by the spinal needle is possible during the injection. These complications can be mostly avoided by use of appropriate tranquilization, eyelid nerve blocks, and added restraint methods to restrict movement by the horse. The relatively low complication rate associated with the injection techniques far outweigh risks associated with general anesthesia in horses. References and Footnotes 1. Klein L. Anesthetic complications in the horse. Vet Clin North Am [Equine Pract] 1990;6:665 692. 2. Parviainen AK, Trim CM. Complications associated with anesthesia for ocular surgery: a retrospective study 1989 1996. Equine Vet J 2000;32:555 559. 3. Little D, Redding WR, Blikslager AT. Risk factors for reduced postoperative fecal output in horses: 37 cases (1997 1998). J Am Vet Med Assoc 2001;218:414 420. 4. Traub-Dargatz JL, Salman MD, Jones RL. Epidemiologic study of salmonellae shedding in the feces of horses and potential risk factors for development of the infection in hospitalized horses. J Am Vet Med Assoc 1990;196:1617 1622. 5. Alford P, Geller S, Richardson B, et al. A multicenter, matched case-controlled study of risk factors for laminitis. Prev Vet Med 2001;49:209 222. 6. Gilger BC, Davidson MD, Nadelstein B, et al. Treatment of cystic corpora nigra in horses with an ophthalmic ND:YAG laser: 8 cases (1988 1996). J Am Vet Med Assoc 1997;211: 341 343. 7. Miller TM, Willis AM, Wilkie DA, et al. Description of ciliary body anatomy and identification of sites for transscleral cyclophotocoagulation in the equine eye. Vet Ophthalmol 2001;4:183 190. a Dormosedan, Pfizer Animal Health, Exton, PA 10017. b Phoenix Pharmaceuticals, St. Joseph, MO 65403. c Becton Dickinson, Franklin Lakes, NJ 07417. d Alcaine, Alcon Laboratories, Fort Worth, TX 76134. e Phenylephrine Hydrochloride Ophthalmic Solution USP, Bausch & Lomb, Tampa, FL 33637. AAEP PROCEEDINGS Vol. 48 2002 271