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Veterinarian Technician November 2007 (Vol 28, No 11)

Performing an Equine Ophthalmic Examination

by Pamela Kirby, RVT

    CETEST This course is approved for 0.5 CE credits

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    Key Points

    • Technicians should have an understanding of the anatomy of the horse's eye before performing an ophthalmic examination.
    • Bacterial and fungal cultures should be obtained for any horse with an ophthalmic ulcer.
    • When examining a horse's eye, it is important to follow the same order of procedures each time.

    See Glossary .

    Ophthalmic examinations are an important part of equine health care; therefore, equine technicians should be familiar with performing eye examinations on horses. This article provides an introduction to examining the horse's eye for technicians who are unfamiliar with equine ophthalmology and serves as a review for technicians who already have some experience. The described examination techniques and procedures can be performed by a licensed technician or a properly trained assistant under the supervision of a veterinarian.

    Patient History

    Obtaining a thorough patient history is the first step in performing a complete ophthalmic examination. The technician should ask questions about the horse's general health, vaccination and deworming schedules, travel, housing, and activities. The answers to these questions are important because ocular problems can be caused by several factors, including systemic illness and the presence of parasites. In addition, horses can injure themselves when being transported or can scratch their eye on a splinter while standing in a stall. It is important to know what activities the horse participates in so that the veterinarian can make informed recommendations regarding the animal's continued physical activities. If a problem is suspected, the veterinarian should ask the owner specific eye-related questions. Some examples are as follows:

    • When was the problem first noticed?
    • What signs were observed initially?
    • Has the horse's condition become better or worse?
    • Is one eye affected? Or is the problem related to both eyes?
    • Have any systemic or topical medications been administered?
    • Has the horse or any other horse on the property had previous eye problems?

    By keeping detailed records of the information that is gathered, technicians can help the veterinarian thoroughly evaluate the patient's eyes.

    Anatomy of the Eye

    Before beginning an ophthalmic examination, the technician should have a basic understanding of the physical structure of the equine eye . Although the anatomy of the equine eye is similar to that of dogs and cats, there are some important differences.

    Horses' eyes are positioned laterally on the head, giving them a field of vision of more than 300°. The globe of the horse's eye, which is larger than that of any other land mammal,1 sits in a complete bony orbit and is oval in shape. The pupil is horizontally oval and has a feathery internal margin called corpora nigra (structures that are thought to act as shades).1 The main cavity behind the lens is the vitreous. The retina of the horse is thinner than that of a dog or cat. The optic disk is salmon pink to orange in color and, like the pupil, is horizontally oval; it is usually located slightly ventral in the nontapetal area. The arteries and veins radiate only a few millimeters around the optic disk.

    Other structures of the horse's eye include the medial and lateral canthi, which are the junctions where the upper and lower eyelids meet. The nasolacrimal puncta are drainage holes located near the medial canthus and in the nose. The lower punctum in horses is located in the floor of the nostril near the mucocutaneous junction. The nasolacrimal duct is connected to the puncta and carries tears from the eye to the nasal cavity. The tapetum, which is the reflective layer in the dorsal choroid, is located between the retina and the sclera. Finally, the cornea — the clear anterior portion of the outer coating — consists of four layers: the outer epithelium, the stroma, Descemet's membrane, and the inner endothelium.

    Visual Assessment

    Observation

    The first step in performing an equine ophthalmic examination is for the technician to observe the patient. The technician should observe the horse from a distance and answer the following questions:

    • How does the horse navigate around obstacles?
    • Does it shy or spook more from the left or from the right?
    • Are the horse's eyes open and pain-free?
    • What is the horse's blink rate? (The normal blink rate for a horse is five to 25 blinks/min.2)
    • Is the horse carrying its head at a normal height?
    • Are any other abnormalities observed?

    After the horse is observed at a distance, it should be placed in the stocks or cross-tied, and its head should be held with a halter and cotton lead rope so that the eyes can be examined more closely. The examiner should approach the animal slowly and calmly. While standing in front of the horse, the technician should look for symmetry of the globes, eyelids, and pupils. The position of the eyelashes can provide clues as to the horse's comfort level. In a healthy horse, the upper lashes are perpendicular to the cornea. Eyelashes pointing toward the ground are often one of the first signs of ocular pain.3

    The examiner should then stand a short distance in front of the horse to evaluate pupil symmetry with a direct ophthalmoscope. When the light is pointed directly between the horse's eyes, the examiner should be able to see the tapetal reflection of both eyes and compare the size and shape of the pupils. The size and position of the globes should also be compared when the examiner is positioned in front of the horse.

    Assessment of Visual Ability and Reflexes

    The next part of an equine ophthalmic examination involves testing the patient's vision. The palpebral reflex is first evaluated by gently touching the patient's medial and lateral canthi. The eyelids should close completely; if they do not, the horse may have an abnormality of the fifth or seventh cranial nerve. If the palpebral reflex is normal, the examiner should check the menace response, which is a crude test of vision that is conducted by covering one of the horse's eyes while gently making a "menacing" hand motion toward the other eye. The test should then be repeated on the other eye. Care must be taken to avoid pushing air toward the eye or hitting the eyelashes or vibrissae, which may cause a false-positive response.

    If the horse is cooperating and does not seem to be in pain, the pupillary light reflex (PLR) should be assessed, preferably in a dimly lit area. If the PLR cannot be safely assessed with the horse awake, the patient can be sedated. The PLR can be used to evaluate the retina, optic nerve, midbrain, oculomotor nerve, and iris sphincter muscle (see Figure).3 The normal equine PLR is sluggish and incomplete unless a very bright light is used. Light directed into one eye should result in the constriction of the pupil in that eye (direct response) and the pupil of the contralateral eye (indirect response). Optimally, the PLR test should be conducted by two individuals. The first individual should stand at one side of the patient's head and use a Finnoff halogen transilluminator (Welch Allyn, Skaneateles Falls, NY) to provoke the direct PLR, and the second individual should stand at the opposite side of the horse's head and use a direct ophthalmoscope to observe the indirect PLR. To avoid a direct response, the second individual should stand a few feet away from the horse. The second individual should use the direct ophthalmoscope to retroilluminate the tapetum, allowing him or her to see the pupil becoming smaller. The examiners should then switch places or instruments and repeat the test on the patient's other eye.

    Once the PLR test is completed, the examiner should check for a dazzle reflex. While shining a bright light in the horse's eye, the examiner should watch for any blinking, which indicates a normal dazzle reflex.

    Gross Examination

    A gross examination of the eyelids, conjunctiva, cornea, anterior chamber, and iris should be performed. A halogen transilluminator and a magnifying head loupe (Optivisor, Donegan Optical Company, Lenexa, KS) should be used to look for any abnormalities in the structures. The lids should be examined for masses, pigment changes, or alopecia. The third eyelid, or nictitating membrane, should be checked for normal movement and position and for any masses. The conjunctiva should be evaluated for any signs of inflammation, foreign bodies, masses, or ulceration (see Figure).

    In a healthy horse, the cornea should be clear and free of any vessels, opacities, or edema. The anterior chamber (the space between the lens and the cornea) also should be clear. The iris should show normal corpora nigra, color, and pupil size.

    Anterior Chamber Examination

    By using the slit beam light on the direct ophthalmoscope, the examiner can evaluate the anterior chamber, which should appear clear or black. Three abnormalities are commonly found in the anterior chamber:

    • Aqueous flare is the result of increased protein levels in the aqueous (resembles the dust particles that can be seen as light shines through a window).
    • Hypopyon appears as pus, usually in the ventral aspect of the anterior chamber.
    • Hyphema is diffuse blood or a blood clot.

    Any of these findings may be a sign of uveitis,3 an inflammation of the uvea (the iris, ciliary body, and choroid) that may also affect the sclera, cornea, and retina.

    Restraint

    Sedation

    For most equine patients, sedation is required in order to perform a thorough ophthalmic examination. Sedation may be administered after the patient's vision is assessed. The preferred agent is detomidine hydrochloride (0.02 to 0.04 mg/kg IV) because it takes effect quickly and lasts long enough for a complete ophthalmic examination to be performed. Xylazine (0.5 to 1.0 mg/kg IV) may also be used; however, this drug has some drawbacks. One disadvantage is that horses often jerk their heads while sedated with xylazine. Another disadvantage is that the sedation effects of xylazine do not last as long as those of detomidine hydrochloride. In general, xylazine lasts 15 minutes, whereas detomidine hydrochloride lasts 45 minutes. In horses with pain or in horses undergoing painful procedures, it is appropriate to administer butorphanol (0.01 to 0.02 mg/kg IV)2 in addition to detomidine hydrochloride or xylazine, although butorphanol also may cause head jerking. A topical anesthetic and nerve block can be used in addition to the sedation and pain control.

    Lid Blocks

    The auriculopalpebral block, a motor block that affects the orbicularis oculi muscle, is commonly used for ophthalmic examinations. The orbicularis oculi muscle is mainly responsible for closing the eyelid. To administer the auriculopalpebral block in an equine patient, the zygomatic arch should be palpated to locate the palpebral branch of the facial nerve (seventh cranial nerve). The palpebral branch should feel thick, stiff, and round (like a piece of undercooked spaghetti) lying just under the skin. A 25-gauge, 5/8-inch needle is inserted into the tented skin over the nerve, and 1 to 2 ml of lidocaine hydrochloride (2%) or mepivacaine (2%) is infiltrated around the nerve. The injection site should be massaged to appropriately distribute the medication. Compared with mepivacaine, lidocaine has a longer onset of action (4 to 6 min) and a shorter duration (60 to 90 min).1 Because horses that have undergone an auriculopalpebral nerve block cannot adequately blink for several hours afterward, their eyes should be protected from irritants, such as shavings and hay, during transfer rides.

    Certain procedures (e.g., placement of a subpalpebral lavage system [ see Figure ]) may also require a sensory block of the supraorbital nerve, which is responsible for the medial two-thirds of the upper eyelid. As with the auriculopalpebral block, lidocaine or mepivacaine can be used for the supraorbital block. To locate the foramen, the examiner should place a thumb and middle finger on the lateral cranial and caudal borders of the supraorbital process. The fingers should be moved medially until a widening of the bone is felt, and an index finger can be placed midway between the thumb and the middle finger; the examiner should feel a depression.4 With a 25-gauge, 5/8-inch needle, 1 to 2 ml of anesthetic should be injected around the depression.

    Diagnostic Testing

    Culture and Cytology

    Bacterial and fungal cultures should be obtained for any horse with an ophthalmic ulcer. Horses' eyes are prone to fungal growth within a corneal defect, a condition that may progress quickly.

    Cultures should be taken before any medications (i.e., proparacaine, fluorescein) are placed in the eye. Many veterinary professionals prefer to use a premoistened microtip microbiologic culture swab. The patient's eyelids are opened to prevent contamination, and the culture swab is rolled in and around the ulcer. The sample should be submitted for mycotic culture as well as aerobic bacterial culture and sensitivity analysis.

    Before cells are collected for cytologic evaluation, a topical anesthetic should be applied to the eye. The sample is obtained using a surgical blade, corneal spatula, or cytology brush and is stained with Diff-Quik to detect bacteria, fungal organisms, inflammatory cells, or neoplastic cells.1 Gram's stain is also often used to obtain additional information about bacteria.1

    Schirmer Tear Test

    The Schirmer tear test (STT) measures reflex tearing. Because deficiencies in tear production are rarely reported in horses,1 the STT is not generally part of the equine ophthalmic examination. It should, however, be conducted in horses with chronic ulcers or in horses with an eye that looks dry. The STT should be conducted before any drops are administered. The test strip is placed carefully in the temporal lower lid margin, and the rate of tearing is evaluated. In a healthy adult horse, 14 to 34 mm/min of wetting is normal.3

    Fluorescein Stain

    Sodium fluorescein and rose bengal are the most commonly used stains for equine ophthalmic examinations. Rose bengal is often used to check the integrity of tear film. Sodium fluorescein, however, is more widely used and should be applied every time a horse's eye is examined. Sodium fluorescein can be instrumental in checking for corneal ulcers because it is hydrophilic and stains the corneal stroma but not the epithelium or Descemet's membrane. This stain may also be used to test the patency of the nasolacrimal ducts. Many veterinary professionals prefer fluorescein paper strips. Others use stain diluted in a syringe or fluorescein solution in a multiuse vial; however, the diluted stain solutions may not be concentrated enough to detect small or dendritic ulcers. The multiuse solution has been shown to harbor bacteria if not used within 24 hours.5

    To stain an eye using the fluorescein strip method, the strip is first moistened with two or three drops of saline. The upper lid is lifted, and the wet strip is touched carefully to the bulbar conjunctiva. To ensure that the stain covers the entire cornea, the patient should be allowed to blink several times or the lids should be closed manually. If an excess of stain pools on the corneal surface and in the conjunctival sac, the eye should be rinsed thoroughly with saline. The examiner should use a trans­illuminator or a cobalt blue light (on the direct ophthalmoscope) to evaluate the cornea. Any spot that clearly retains fluorescein is an ulcer that requires treatment.

    Tonometry

    Using an applanation tonometer (Tono-Pen; Reichert, Inc., Depew, NY) is the most common method of measuring a horse's intraocular pressure (IOP). Not all horses have to be sedated for tonometry; however, the veterinary professional should be consistent in his or her decision to sedate an individual patient. If a horse was sedated the first time the IOP was measured, it should be sedated every time the IOP is rechecked. If a decision is made to tranquilize the patient, a sufficient level of sedation should be achieved before tonometry begins. Manipulating the eyelids to keep them open can falsely elevate the IOP; therefore, an auriculopalpebral block should be performed. The horse's head should be level, and the jugular veins should be free of pressure.

    A topical anesthetic, such as proparacaine, should be applied to the eye with a "squirter" syringe. Several drops of the solution should be placed in a 3-ml syringe. The needle part of a 25-gauge needle should be broken off, and the hub should be placed on the syringe. The solution can then be squirted onto the eye.

    Because a diseased cornea can give false readings, the IOP should be measured at the most normal part of the cornea. Three readings, with 5% error, should be taken and averaged for each eye; this number is recorded as the IOP. The average IOP of a horse is between 15 and 30 mm Hg, with a difference of 5 mm Hg or less between the two eyes. An IOP greater than 30 mm Hg may indicate glaucoma.

    Mydriasis

    If the patient's PLR and IOP readings are normal, the horse's pupils should be dilated. The mydriatic agent (tropicamide 1%) can be put into a squirter syringe, and 0.1 to 0.2 ml should be applied generously every 5 minutes for 20 minutes; it may take up to 30 minutes for the pupil to become maximally dilated. The owner should be made aware that tropicamide can keep the pupil dilated for 4 to 12 hours, depending on the horse. Because atropine acts more slowly than does tropicamide and can keep the pupil dilated for up to 2 weeks, it is reserved for therapeutic use only and is not used for examination purposes. Once the pupil is dilated, the lens, vitreous, and fundus should be carefully examined.

    Lens Examination

    The examiner should observe the lens for normal placement and clarity using transillumination and retroillumination. When retroilluminating the lens, the light should be kept at a great distance from the eye, reflecting off the tapetum, which will make lesions more obvious. Congenital cataracts, acquired or secondary cataracts, senile cataracts, and nuclear sclerosis all may occur in horses. On transillumination, nuclear sclerosis appears as a haze in the nucleus of the lens; on retroillumination, the lens appears clear and the tapetal reflection can be seen. Nuclear sclerosis occurs mainly in horses older than 7 years of age and is part of the normal aging process. Cataracts appear white on transillumination and black on retroillumination. The most common cause of cataracts in horses is recurrent uveitis.

    Vitreous Examination

    The vitreous is the clear, jelly-like structure behind the lens. On examination, the normal vitreous should be free of any opacities. In horses, the presence of "floaters" is often the result of recurrent uveitis. It is suspected that large or numerous floaters in the vitreous may cause the horse to spook.6

    Fundic Examination

    The fundus, the posterior layers of the eye, can be viewed using direct or indirect ophthalmoscopy. Each method has advantages and disadvantages. The advantage of using direct ophthalmoscopy is that it yields an image that is magnified and not rotated. However, the examiner must stand very close to the horse's head and can view only a small portion of the fundus at a time. Indirect ophthalmoscopy is performed using a transilluminator and a handheld lens. The advantages of this technique are that the examiner can see a larger part of the fundus and can stand at a safer distance from the horse's head. The disadvantage, however, is that indirect ophthalmoscopy produces a virtual image that is rotated 180°. The change in orientation must be kept in mind when interpreting lesions. Direct ophthalmoscopy is recommended for general practice. Technicians should observe several horses in order to be able to identify what is normal so that it becomes easier to spot abnormalities.

    Direct ophthalmoscopy is performed by using the direct ophthalmoscope with the large aperture and the lens set at "0" to start. The examiner should be positioned 18 to 24 inches from the patient and be able to see the tapetal reflection. The examiner should then move closer to the cornea until the internal structures are in clear view.

    Indirect ophthalmoscopy requires more coordination. The examiner should be positioned 18 to 20 inches from the patient. When the transilluminator is held next to the examiner's eye, the tapetal reflection should be seen. A 20-diopter handheld lens should then be placed in the line of vision in front of the horse's cornea. The side of the lens with the least curvature, usually marked with a silver ring, should be toward the animal. The examiner should then move the lens either toward or away from the eye so that the image of the fundus fills the entire lens.

    Therapeutic Procedures

    Nasolacrimal Duct Flush

    Flushing the nasolacrimal duct of a horse is a common procedure and may be performed by the technician. For the safety of the technician and the patient, the horse should be sedated or twitched. Flushing is usually performed retrograde, with the tip of the catheter inserted in the punctal opening on the floor of the nostril. A 12-ml syringe filled with eyewash or saline is attached to the end of the catheter, and the solution is flushed gently through the catheter while the lower punctum is occluded with digital pressure. The fluid should exit the upper punctum near the medial canthus.

    Subpalpebral Lavage System

    It is often extremely difficult to administer eye medication to an equine patient several times a day. Therefore, a subpalpebral lavage system is used. The lavage system allows the eye to be treated without the need to get close to the eye. The technique is as simple as injecting medicine through a port located near the horse's shoulder and following it with a 1-ml injection of air. To avoid having different medications interact when mixed into one syringe during administration, only one drug should be given at a time, with 5 minutes between each medication.

    Lavage kits are commercially available. Before the lavage system is placed, the horse should be sedated and given a lid block (motor) and a line block (sensory) in which the needle will be inserted through the lid. Proparacaine should be used for topical anesthesia.

    The needle is carefully placed through the upper lid, avoiding the lacrimal gland, and the tubing is threaded through the needle. On the end of the tubing that comes in commercial kits, there is a footplate that stays under the eyelid. Care must be taken to prevent the footplate from touching the cornea because this may cause an ulcer. Lavage systems are commonly secured using duct tape and staples or sutures. A duct tape butterfly can be placed around the lavage system just above the eye and in the middle of the forehead. The tape can be stapled or sutured to the skin; both methods of adhering the tape cause the same level of pain. The tubing is threaded through two or three braids in the mane. A 20-gauge catheter is fed into the end of the tubing, and an injection cap is then placed on the catheter. The injection cap should be changed daily.

    Lavage tubes may be ripped out, cause irritation to the lid or cornea, or become plugged, preventing medication from reaching the eye. Therefore, horses with lavage systems should be hospitalized or monitored frequently. Lavage systems can safely be left in place for up to 3 weeks.4 The insertion site should be checked daily, and the lavage system should be removed if there is any sign of infection at the insertion site.

    Conclusion

    Equine ophthalmology is often challenging but is very rewarding. When performing an equine ophthalmic examination, the technician should follow the same order of procedures every time to ensure that no part of the eye is missed. After evaluating several healthy eyes, abnormalities can be recognized more easily.

    See Equipment for an Ophthalmic Kit box .

    1. Turner S: Specialist ophthalmic procedures, in Veterinary Ophthalmology: A Manual for Nurses and Technicians. Philadelphia, Elsevier, 2005, pp 1690-173.

    2. Gilger BC: Equine ocular examination: Basic and advanced diagnostic techniques, in Equine Ophthalmology. St. Louis, Elsevier Saunders, 2005, pp 1-62.

    3. Brooks DE: Equine ophthalmology. Proc Vet Ophthal Tech Soc:2005.

    4. Krohne SG: Equine eye examination techniques. Vet Clin North Am Equine Pract 172-4, 1995.

    5. Cello RM, Lasmanis J: Pseudomonas infection of the eye of the dog resulting from the use of contaminated fluorescein solution. JAVMA 132:297-299, 1958.

    6. Gilger BC: Equine vision: Normal and abnormal, in Equine Ophthalmology. St. Louis, Elsevier Saunders, 2005, p 398.

    References »

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