1. science
2. medicine
3. surgery

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HOW-TO SESSION
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 Ⲑ Vol. 48 Ⲑ AAEP PROCEEDINGS
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Table 1.
Ocular Surgical Procedures that May Be Done in a Standing Horse
Adnexal
●
●
●
●
●
●
2.
Corneal
Small eyelid mass removal
Eyelid mass biopsy
Eyelid lacerations—small
Conjunctival biopsy
Third eyelid laceration repair
Third eyelid mass removal or biopsy
Intraocular
●
●
●
●
●
Corneal scraping for cytology
Ulcer debridement
Grid keratotomy
Superficial keratectomy—small
Suture of non-perforating
corneal laceration
● Removal of superficial corneal
foreign body
Materials and Methods
●
●
●
●
●
Aqueocentesis
Intraocular injection (i.e., TPA)
Iris cyst laser ablation
Laser cyclophotocoagulation for glaucoma
Intraocular mass laser ablation
Palpebral and Frontal Nerve Blocks
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 HCla (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.
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 HClb 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 HClb 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*
Medications
●
●
●
●
●
2% Lidocaine HCl
1% Proparacaine
2.5% Phenylephrine
Tranquilizer
Eye lubrication ointment
*Needed depending on type of procedure.
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Table 3.
Surgical Equipment
● 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
Basic Ophthalmic Surgical Instrument Pack
Disposable Surgery Supplies
Surgical Magnifying Loupes
● 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
268
prepped, a 22-gauge, 2.5-in spinal needlec 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
2002 Ⲑ Vol. 48 Ⲑ AAEP PROCEEDINGS
Proceedings of the Annual Convention of the AAEP 2002
Reprinted in IVIS with the permission of the AAEP
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HOW-TO SESSION
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 HClb 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 HCld) and
2.5% phenylephrinee 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 Ⲑ Vol. 48 Ⲑ 2002
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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
270
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.
2002 Ⲑ Vol. 48 Ⲑ AAEP PROCEEDINGS
Proceedings of the Annual Convention of the AAEP 2002
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HOW-TO SESSION
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.
Phoenix Pharmaceuticals, St. Joseph, MO 65403.
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.
b
c
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