General Comments

Examination of the equine eye is simplified by the fact that the adnexa, globe surface, and intraocular contents are easy to inspect and often easy to image. Thorough examination can be done in the field if the practitioner arrives at the call with functional portable diagnostic tools and all the common drugs and supplies needed for routine restraint and handling. Examination will be proficient if the practitioner takes the time to create an environment conducive to proper examination, chooses effective patient position and restraint, images observable abnormalities with a digital camera, and follows the methodical examination techniques outlined in Chapter 1.

Although some ophthalmic problems are chronic, many are presented as ophthalmic emergencies. Horse owners and clinic reception staff must be trained to regard swollen, painful, traumatized, or acutely discolored eyes as true emergencies and schedule visits for all these cases on a same-day basis. Clients who call in with a horse that is suffering an ophthalmic emergency should be instructed to keep the animal in a clean, dark stall until it is examined. A workspace that can be darkened for examination and well lit for treatment should be available. The client should be instructed to gather the materials needed for a bale head support (4 to 6 bales of hay or shavings and a clean cover for the top bale) prior to the arrival of the clinician (Fig. 2-1). Clients should be cautioned against administration of medication in an eye that has not been examined.

Figure 2-1 Examination, diagnostic testing, and treatment of serious eye problems in the field is aided by supporting the head of the horse with a table constructed of four to six bales of hay, shavings, or straw. Alternatively, the head may be supported by a barrel or other handy stationary object.

Horses with acute or chronic ophthalmic problems usually require ocular diagnostic testing and imaging once examination is complete. Common tests include visual maze testing, tonometry, dye tests, culture and cytology, and biopsy. Tests applicable to the problem at hand can be done in the field using simple equipment if the practitioner follows the guidelines in this text for test completion and interpretation. Practitioners are encouraged to develop expertise in digital photography so that useful ocular images can be obtained and to practice cytologic sampling so that sample analysis can be performed in house.

Examination, imaging, and diagnostic testing must lead to a diagnosis. Ocular problems like trauma or overt neoplasia are obvious, but diagnosis of other problems can be challenging. The conditions most often misdiagnosed in the field are listed in Box 2-1; these conditions merit special study by all equine practitioners. Certain conditions, particularly diffuse nonulcerative loss of corneal transparency, glaucoma, and subclinical uveitis are not completely understood, so information on differential diagnosis is sparse. Other conditions such as stromal abscesses or fungal keratitis have diagnostic elements that may be overlooked or misinterpreted. Some problems like deep corneal infections and equine recurrent uveitis may present at a stage where multiple regions of ocular anatomy are altered, causing uncertainty as to which elements are primary issues and which elements are secondary. Improvement in diagnostic acumen is dependent on experience, constant study of new information, selection of appropriate diagnostic tests, and close surveillance of case progress and therapeutic effectiveness.

Therapy for ocular problems encompasses a wide spectrum of options and skills that are discussed throughout this text. Competence can be honed by experience with the use of medical treatment aids like subpalpebral lavage systems and frequent practice of simple field surgeries. Practitioners with special interest may pursue additional expertise in procedures like enucleation, nictitans removal, and simple corneal surgeries if they have access to a facility with stocks and/or a surgery table.

Although most routine ophthalmic problems can be diagnosed and treated by general equine practitioners, some problems need referral to specialists for optimum care. Specialty practices and universities have advanced examination equipment and offer diagnostic tests that are unavailable to practitioners. They provide surgical solutions for serious problems that may spare sight. Specialists have the broadest knowledge base for assessment of confusing presentations and can take rapid action to treat serious traumas or infections. Referral centers have sufficient support staff to provide the round-the-clock care some problems require. Practitioners must cultivate a broad enough understanding of ophthalmic surgery, pathology, and pathophysiology to recognize the problems that can benefit from specialist evaluation and refer these cases early.

Excellence in field ophthalmology is within the reach of every practitioner who makes a serious effort to study the subject and hone their ophthalmic examination, testing, imaging, and treatment skill set. This chapter outlines the basic skills and knowledge needed and presents many tips for successful practice in field conditions.

Equipping the Ambulatory Vehicle

The ambulatory practitioner must be prepared to diagnose and treat a wide variety of ophthalmic problems. Box 2-2 lists the equipment and supplies used in the diagnosis and acute treatment of common field conditions. Many common drugs that are routinely dispensed are easily obtained from veterinary distributors. However, serious or chronic ophthalmic conditions often require medication with drugs that are not carried by veterinary distributors. Box 2-3 lists drugs that usually need to be obtained through retail pharmacies or ordered through compounding pharmacies.

Many practitioners devote a tote box or a drawer in the ambulatory vehicle exclusively for the storage of items used in ophthalmology. The items can then be easily gathered and brought to the patient at the visit.

Creating an “Exam Room” in the Field

Care must be taken to create a serviceable examination area in the variety of environments presented. Horse eye examinations cannot be done outdoors or in sunny sheds. Examination and care will be facilitated if:

Horses are often examined in their stalls or in barn aisles. Restraint may be provided by a trainer, groom, owner, or veterinary assistant. The handler should hold a lead rope attached to the halter. Although most horses are accustomed to having their handlers stand on their left side, equine ocular examination is facilitated if the handler stands on the side contralateral to the eye being examined and switches to the opposite side when the fellow eye is examined. Physical restraint should be minimal—attempts to fix the horse’s head in a given position using physical force or lip shanks are counterproductive. Calm, nonpainful horses may be very cooperative for basic eye examination. Judicious use of a lip twitch may assist examination or treatment of skittish individuals. The examiner may need to step on a bale of hay or a stool to examine a very tall horse, or may need to sit down on a similar aid to examine a small pony or Miniature horse. It is important that the examiner not stand directly in front of the horse and be aware of the position of the horse’s feet at all times.

Working on Painful or Fractious Animals

Flighty or very painful animals will require sedation for examination. Sedation will also be required on horses undergoing invasive or painful diagnostic or surgical procedures. Two drugs are commonly used for chemical restraint: xylazine (0.5 to 1 mg/kg) and detomidine hydrochloride (Dormosedan, 0.02 to 0.04 mg/kg). Detomidine produces more profound sedation than xylazine. Both take effect very quickly after intravenous (IV) administration. The exact dosage of drug and duration of sedation depends on the disposition of the animal, the existing level of pain, and the length of time and manipulation involved in any anticipated procedures. Variations exist among horse breeds in sedation effects: Draft breeds like Belgians and Percherons typically exhibit profound sedation at the lower end of the dosage spectrum, while high strung breeds like Arabians and Thoroughbreds may require higher doses. Please see Chapter 1 for more information on use of sedation for ophthalmic examinations.

Sedation can be minimized if the practitioner has assembled all the supplies, drugs, and examination aids needed for the situation at hand so the ocular examination can quickly be followed by any appropriate diagnostic tests, imaging, and therapy. It is better to err on the low side and add additional sedation as needed than to give so much sedative the animal collapses or becomes very ataxic. Butorphanol tartrate (Torbugesic, 0.01 to 0.02 mg/kg) is a sedative less frequently used in equine ophthalmology because effective doses are often accompanied by troublesome head tremors. However, in combination with detomidine, this drug may provide the profound sedation needed for fractious animals undergoing painful procedures.

Local and Regional Anesthesia

General examination of a comfortable eye in a cooperative horse does not require nerve blocks. However, motor and/or sensory denervation may be necessary for examination, diagnostic testing, and initial field therapy for eyes that are painful from keratitis, uveitis, or other serious conditions. Nerve blocks are essential for handling any eye in danger of rupture due to tectonic instability.

All practitioners should be skilled at performing auriculopalpebral and supraorbital nerve blocks as described in Chapter 1. Please see Chapter 1 for more information on performance of these nerve blocks.

Corneal or conjunctival topical anesthesia is necessary for many examinations and diagnostic tests. Short-term tissue desensitization is achieved by applying proparacaine onto the target surface. A dose of 0.5 mL per eye should be drawn up out of the dropper bottle into a small syringe using a 25-gauge needle. The needle should then be broken off and discarded and the anesthetic sprayed or dripped onto the corneal surface through the remaining hub. Topical anesthesia of focal areas on the conjunctiva or nictitans can be enhanced by briefly holding a sterile Dacron or cotton-tipped swab soaked in topical anesthetic against the target area. Duration of corneal desensitization may be brief in very painful conditions, so the examiner should be prepared to reapply anesthetic as needed.¹

Performing Examinations in Field Settings

Routine eye examination in the field by general practitioners involves inspection of a standing animal with simple, inexpensive equipment. Most examiners start by using a bright light source such as a penlight or (ideally) Finnoff transilluminator to inspect the head, adnexa, and ocular surfaces. The same instrument is then used for transillumination, retroillumination, and direct inspection of intraocular structures. The bright light survey is then followed by direct ophthalmoscopy, which is done both from a distance to retroilluminate focal densities against the tapetal reflection, and close to the eye to view detail directly. Many practitioners get additional information through magnification aids such as head loupes (Optivisor [Donegan Optical Co. Inc., Lenexa, KS]), handheld lenses, and digital images that have been magnified on a camera viewfinder or cropped on a viewing computer.

Detailed specifics of the ocular examination are described in Chapter 1. Several tips that may be helpful in field situations:

Figure 2-2 Hygiene is an important consideration in the field. The periocular region should be cleansed with a solution of 2% Betadine prior to any diagnostic or therapeutic procedure.

Life Stage Issues

Practitioners are responsible for the health of their patients throughout their lifespans. Surveillance for ophthalmic problems should begin shortly after birth. Ideally, ophthalmic exams should be performed regularly throughout the horse’s life. Equine practitioners should be familiar with the ocular problems commonly encountered during the various life stages listed in Box 2-4.

Most healthy foals are seen between 12 and 36 hours of age for their neonatal physical examination. During the visit, the mare is checked for postfoaling problems, and the foal is scrutinized for adequacy of colostral transfer, adjustment problems, infections, and congenital conditions. Neonatal ophthalmic examination is a very important part of the process. It should be done in a consistent, thorough manner using a focal light source and a direct ophthalmoscope.

Patient cooperation is optimized if one person holds the mare and another person holds the foal near the mare’s head. If two assistants are not available, most mares can be turned loose and will usually behave well as long as they can see the foal directly in front of them. Excessive foal restraint is inappropriate—eye examinations are best performed with the foal simply cradled by an assistant (Fig. 2-3). Rambunctious foals can be restrained with a firm arm around the chest and a tail hold. Sometimes it is helpful to position the foal in a corner or against the stall wall. The foal’s ventral mandible can be cradled with one hand while the other is used to hold the light source.

Figure 2-3 Neonatal ophthalmic examination is best done with an assistant cradling the foal’s chest and tail and the examiner steadying the head. The foal should be positioned towards the mare’s head to minimize her anxiety.

Most newborn foals are remarkably tolerant of a direct ophthalmoscope fundic exam. If the holder is calm and the examiner holds the head steady with the nonexamining hand, foals often “freeze” their head movements when the light beam engages the optic disc region.

The neonatal pupil is somewhat round in contour. The globe has a mild ventromedial orientation at birth that gives neonates a slight downward gaze. The pupil acquires a horizontal elliptical shape and becomes parallel to the lower lid over the first month of life. Pupillary light response may be sluggish in the first few days of life but should be brisk after a few days. The color of the neonatal iris may be a little grayer than the rich chocolate color commonly seen in adults (Fig. 2-4). Lens suture lines may be very prominent and should not be mistaken for cataracts. Tapetal color is variable and is correlated with coat color. The optic disc is round to oval in shape and whitish pink to salmon in color. Light gray streaks, representing bundles of axons traveling to the optic nerve, may be seen in the peripapillary area.

Figure 2-4 The appearance of the neonatal iris often has a grayish brown tint. The pupil is somewhat round in contour. Lens suture lines may be prominent.

Common findings in neonatal foals include episcleral or retinal hemorrhage, a persistent remnant of the hyaloid artery, and entropion of one or both lids. Some problems, like entropion or corneal ulcers, may necessitate immediate therapy. If the foal is a member of a breed that is at risk for eye problems (e.g., Appaloosa, Rocky Mountain horse) the neonatal examination provides an opportunity to educate the owner about known risks that might require screening later in life. If any congenital anomalies have been observed in the foal, the mare should get a thorough eye examination to see if she has similar problems.

Immature horses are at increased risk for ocular trauma due to herd interactions, training accidents, and exuberant behavior. Young mature horses, particularly Warmbloods, Appaloosas, or draft horses, may develop recurrent or insidious uveitis associated with an inherent genetic susceptibility to this immune-mediated syndrome. Pastured mature horses in temperate climates like the Northeast are susceptible to uveitis associated with acquired leptospiral infections. Horses living in tropical climates like the Southeast are prone to fungal keratitis. Mature horses living in sunny climates or high altitudes are at risk for development of actinic blepharitis or squamous cell carcinoma of the adnexa or globe, particularly draft horses, Appaloosas, and color-dilute breeds like Paints. Horses of all ages frequently present for adnexal or corneal trauma, and many practitioners report an increased incidence of ocular trauma in the late summer or fall months.

Mature horses frequently develop nuclear sclerosis as they age. This can give the lens a faint blue appearance that may be opaque on transillumination (Fig. 2-5). Geriatric horses often present with degenerative or age-related conditions of the eyes: cataracts, vitreal syneresis, asteroid hyalosis, senile retinopathy, supraorbital fat atrophy, or proliferative optic neuropathy. Horses older than 15 years are at increased risk for many kinds of neoplasia. Aged gray horses are at increased risk for melanoma and will occasionally present with ophthalmic forms of this disease.

Figure 2-5 Nuclear sclerosis gives this lens in an aged horse a faint blue appearance.

Geriatric horses may have ocular manifestations secondary to chronic conditions such as dental disease, sinus infections, and Cushing’s syndrome. Diagnosis and management of equine geriatric ophthalmic problems have been reviewed,² and more information on ocular manifestations of systemic disease can be found in Chapter 13.

Field Tips for Vision Testing

A rough assessment of vision can often be made with maze testing. This test involves the use of blinkers or a large towel to cover first one eye and then the other, observing how the horse navigates a short obstacle course with only one eye receiving visual data. The maze tests should be done under dim and normal light conditions.

Barn aisles that have been cleared of hazards are good stages for maze testing. An obstacle course is created using overturned buckets or other smooth, solid objects that are easily rearranged and safe if the horse runs into them. Stall doors and barn door openings and other paths that exit the maze area should be closed off for safety concerns.

Handlers and observers should be instructed to stay away from the maze line and avoid giving the horse any verbal cues to orientation. The horse being tested should be turned loose in the aisle. One person should stand at the end of the maze path and coax the horse to navigate the obstacles by shaking a bucket filled with grain (Fig. 2-6). Fractious horses may have to be restrained on a long lead, but the handler must avoid giving any positional cues to the horse.

Figure 2-6 Maze testing is useful in providing a rough assessment of unilateral vision and instructing an owner of a patient’s visual status. The horse is blindfolded first on one side (A) and then on the other and is coaxed to navigate an obstacle course of harmless obstacles (B) to reach a food reward (C). Most barn aisles can be set up as suitable stages for maze testing. All doors and paths that exit the maze path should be closed off.

Horses that are visual in the uncovered eye will usually walk towards the food bucket with confidence and show head and body movements that acknowledge the obstacles in the path. Horses that are blind in the uncovered eye will show a dramatic difference in their maze behavior. They will either stand still and refuse to move or walk very tentatively forward, often running into the ground obstacles.

Maze testing is very useful in instructing owners and trainers. People who work with the animal every day may not “believe” that a damaged eye is blind until they observe the asymmetric behavior in the maze.

Tips for Tonometry

Tonometry is discussed in Chapter 1. The two instruments most commonly used in the field are the Tono-Pen Vet applanation tonometer (Reichert Inc., Depew, NY <http://tonopen.com/>) and the TonoVet rebound tonometer (Icare Finland OY, Helsinki, Finland <http://www.icaretonometer.com/index.php?page=tonovet-for-animals>).

Obtaining consistent readings with either instrument can be challenging in the field. Spurious readings that are very high or very low may occur during the sampling process. Horses with substantial corneal edema may show altered applanation tonometry readings as the instrument calculations may be affected by the edema within the cornea.

Consistent technique is important for sequential readings done on different days. In general, horses should undergo light sedation, auriculopalpebral nerve blocks, topical anesthesia, and assume a consistent, normal head position for each intraocular-pressure (IOP) testing session. Ideally, each exam should be done at the same time of day.

The Tono-Pen Vet should be calibrated every day it is used. The instrument head should be cleaned often with alcohol and blown free of particulate matter with a can of compressed air. The Tono-Pen Vet will not operate well in very cold weather and should not be left in the ambulatory vehicle in cold months.

Field Tips for Ophthalmic Dye Tests

Ophthalmic dye testing procedures are discussed in Chapter 1. All eyes undergoing examination for a problem should undergo fluorescein dye staining. If eyewash is not available to mix with a test strip, sterile saline can be mixed with the torn-off paper dye strip in a syringe and sprayed onto the cornea.

A cobalt blue light is useful in assessing fluorescein dye tests. A direct ophthalmoscope can have a lens that is rarely used retrofitted with a cobalt blue filter (http://www.welchallyn.com/). Alternatively, an inexpensive cobalt blue filter that covers a standard penlight can be ordered from a human ophthalmic supply catalog (Bernell: 1-800-348-2225; Item ALPENF <www.bernell.com/product/140/2>).

Many practitioners do not appreciate the diagnostic potential of rose bengal staining of the cornea. This dye test is indicated in any suspected corneal ulceration or abrasion; it may suggest defects in the ocular tear film or the presence of viral or fungal pathogens. As with fluorescein, rose bengal test strips (Akorn Inc., Lake Forest, IL) can be mixed in a syringe barrel with saline or eyewash in advance of corneal application. Spare dye strips of both types can be stored in the ophthalmoscope case for ready access.

Performing Culture and Cytology in the Field

The most common indication for ocular culture and cytology in equine practice is a corneal ulcer that is obviously infected, unresponsive to therapy, or melting. Such cases are often critical, requiring urgent analysis for therapeutic decision making. Practitioners can process and analyze ocular samples in-house to make rapid therapeutic decisions. Additional samples may be sent to reference laboratories as appropriate.

Supplies for culture and cytology should be packed in the ambulatory vehicle at all times (Box 2-5). The sample collection process can be paired with all the other procedures that are appropriate to the case, including digital imaging, dye tests, more extensive débridement of the lesion, and instillation of a subpalpebral lavage (SPL) system. Good planning of the sequence of sampling, imaging, and treatment will allow all procedures to be done quickly with minimal sedation.

Preparation for Diagnostic Sampling in the Field

Figure 2-7 All supplies needed for culture, cytology, and any planned débridement or imaging should be assembled and set on a nearby table prior to diagnostic sampling of a corneal lesion.

Culture Sampling and Handling

If possible, obtain the culture without topical anesthetic. If the horse will not tolerate this, apply 0.5 to 1 mL of 1% proparacaine onto the cornea for desensitization.

Figure 2-8 A, The blunt end of a sterile scalpel blade can be used to obtain a corneal culture sample. After the sample is obtained, the blunt end of a second sterile scalpel blade can be used to obtain cytology samples. B, Alternatively, a Dacron swab can be used to obtain the sample. Care should be taken to touch just the target surface with the sampling tool.

Figure 2-9 The sampling blade or swab used for culture is then dropped into a screw-topped tube of thioglycollate broth for transport.

On return to the clinic, the culture sample should be placed in an incubator. Samples sitting in broth or plated on agar should be checked daily for turbidity or colony growth. If bacterial growth is seen in either medium, a smear should be prepared and Gram stained to determine morphology and staining characteristics (cocci or rods, gram-positive or gram-negative bacteria). Characterization of these bacterial properties will dictate the best type of agar plate to use for antibiotic sensitivity testing (Mueller-Hinton plain agar versus Mueller-Hinton agar with blood).

Antibiotic sensitivity testing can be referred to a reference lab or performed in-house using the Kirby-Bauer disk diffusion method of applying antibiotic impregnated disks to agar that has been plated with diluted colonies of bacteria. A sensitivity wheel should be designated for ophthalmic testing and filled with disks impregnated with the antibiotics commonly used to treat corneal infections (Fig. 2-10).

Figure 2-10 If ocular cultures are processed in-house, a Kirby-Bauer antibiotic sensitivity wheel should be stocked with antibiotic-impregnated discs that represent the spectrum of drugs commonly used to treat ocular bacterial infections.

Samples plated on Sabouraud agar should be checked daily for fungal colony growth. Plates with growth can be sent off to a reference laboratory for sensitivity analysis but results may not be received in time to influence therapy choice. Please see Chapter 5 for more information on choice of therapy based on culture results.

Cytology Sampling and Handling

All cytology sampling will require topical anesthetic, and a topical dose should be applied if it was not done prior to culture.

Figure 2-11 Cytology samples should be spread thinly onto three or four dry glass microscope slides. They should be placed in a slotted plastic transport box without any fixative.

The cytology samples can be stained with Diff-Quik stain as soon as they are air dried (Fig. 2-12). They do not need any fixative or other prestaining preparation. Diff-Quik stain is a Romanovsky stain that yields excellent detail of cellular elements and is effective for staining cocci, most large and small rods, some bipolar rods, and most fungal hyphae.

Figure 2-12 Cytology samples should be stained with Diff-Quik stain and Gram stain. These stains can easily be performed in a practice setting, and the slides can be read on the same day the samples were taken.

Samples that show bacteria on initial screening should be Gram stained. Extra slides should be saved in case submission to reference laboratories is needed. Reference laboratories can provide further analysis using other Romanovsky-type stains (Wright-Giemsa) or special stains like periodic acid-Schiff (IPAS), Gomori methenamine silver (GMS), or Cellufluor (Calcofluor white) to identify acid-fast bacteria or fungal elements.

Interpreting Ocular Cytology Samples

The goals of analyzing a corneal sample are simple: Assess the cells native to the sampling site, the cellular response to the condition, and the noncellular elements that are present.

Only a limited number of cell types will be found in corneal samples, and the list of “other elements” commonly present in cytology samples from ulcers is short. With a little practice, typical findings can be easily discerned by equine practitioners and veterinary technicians (Fig. 2-13).

Figure 2-13 Drawing of common cellular elements.

Slides should be examined first under low-power magnification. Areas of interest should be scanned under high dry power, then viewed under the oil-immersion lens. A cytology report should be drafted that describes numbers, types, and staining characteristics of native corneal cells, infiltrating nonresident cells, and noncellular elements.

Scrapings of healthy cornea should contain nothing but sheets of epithelial cells (Fig. 2-14). Cells from the superficial layers are flattened with large amounts of blue cytoplasm and central basophilic nuclei. Cells from intermediate layers are more polyhedral, while cells from basal layers are more cylindrical and round and stain more darkly, showing less cytoplasm. Normal corneal cells do not contain bacteria and are exfoliated in sheets. Individual cells that are flat in contour may roll up into tight scrolls on the slide and must not be mistaken for hyphae or foreign bodies.

Figure 2-14 Photomicrograph of healthy corneal epithelium. Normal corneal epithelial cells do not contain bacteria and are exfoliated in sheets (Diff-Quik stain ×100).

Scrapings from corneas with a cellular inflammatory response will show an infiltrate. The most common finding is a suppurative infiltrate, where neutrophils with or without toxic changes dominate the response (Figs. 2-15 and 2-16). A small number of lymphocytes, monocytes, or plasma cells may also be present. Although a suppurative infiltrate is not pathognomonic for infection, it is highly suggestive of it. Occasionally a corneal sample will show an eosinophilic infiltrate. Presence of eosinophils or basophils is abnormal and suggests either acute allergic hypersensitivity or eosinophilic keratitis (Fig. 2-17). Some fungal infections incite a granulomatous infiltrate consisting of epithelioid macrophages and giant cells. If hemorrhage has occurred due to the disease process or sampling trauma, red blood cells will be present in large numbers.

Figure 2-15 Photomicrograph of corneal cytology showing a suppurative cellular response (numerous neutrophils) and intracellular cocci indicating bacterial infection.

Figure 2-16 Photomicrograph of corneal cytology showing a suppurative cellular response (neutrophils) and intracellular rods indicating bacterial infection.

Figure 2-17 Photomicrograph of corneal cytology showing eosinophils interspersed with a few epithelial cells indicating eosinophilic keratitis.

The most common noncellular elements found in corneal scrapings are infectious agents. Bacteria commonly seen include cocci, large rods, small rods, and bipolar rods. Most cocci are gram positive in their staining patterns, and most rods are gram negative, but some small rods may have gram-positive stain patterns. Intracellular bacteria are highly correlated with infection.

The presence of even a few fungal hyphae in a cytology sample is indicative of mycotic keratitis. Hyphae are slender, septate, branching, or linear structures. Most fungal hyphae stain well with Romanovsky stains and can be seen as a tangle of spaghetti-like densities interspersed with native epithelial cells (Fig. 2-18). Care must be taken to differentiate “scrolled” epithelial cells and long, thin strands of necrotic cellular debris from fungal hyphae (Fig. 2-19). The presence of fungal elements is always significant in ulcer samples, but the absence of hyphae does not rule out infection; hyphae may be present in corneal layers deep to the sampling site.

Figure 2-18 Photomicrograph of corneal cytology showing fungal hyphae. Fungal hyphae can be positively identified by their branching morphology and parallel walls (Diff-Quik stain ×100).

Figure 2-19 Photomicrograph of corneal cytology of corneal epithelial cells (upper left) showing necrotic cellular debris (lower right) that can easily be confused with fungal hyphae. Necrotic cellular debris does not present in a branching morphology, and the walls of the strands of degenerative cytoplasm are of variable thickness (Diff-Quik stain).

Occasionally, filamentous rod-shaped organisms may be present in cytology samples. These organisms (as well as some fungal species) do not stain with Diff-Quik stains, and their elements may be seen as “negative images”—transparent structures interspersed among stained structures. Special stains available through reference labs may be needed for analysis of these samples.

Other noncellular elements that may be seen on corneal cytology include intracytoplasmic melanin granules (dark green to black granules found in cells scraped from the limbal region or from pigmented areas [Fig. 2-20]), vegetative foreign bodies (Fig. 2-21), and mineralized crystals. The latter may stain light blue with Diff-Quik and may correlate with deposits of calcium in the subepithelial layers of horses with stromal keratopathies.³

Figure 2-20 Photomicrograph of corneal cytology showing epithelial cells removed from paralimbal region of the cornea. The cells contain pigmented melanin granules. These normal cytoplastic elements should not be confused with intracellular bacteria.

Figure 2-21 Photomicrograph of corneal cytology from a horse showing vegetative foreign material (V). This material should not be confused with fungal hyphae or other forms of infectious keratitis (Diff-Quik Stain, ×40).

Treatment Choices Based on Corneal Cytology

Rational therapeutic choices will follow as the cytology results are correlated with the clinical conditions described in Chapter 5 and the dominant infectious agents are identified. Most of the time, cytology will aid in choosing antiinfective, antiinflammatory, and antiprotease therapy, but in cases of suspected indolent ulcers, verification of the absence of cellular reaction or infectious elements is a critical step that must precede deeper débridement of a chronic lesion.

Other Ocular Cytology Indications

Cytology may also be indicated in tissues other than cornea. Samples can be taken from bulbar, palpebral, or nictitans conjunctiva that is inflamed, infected, or abnormal in appearance and analyzed as described earlier. Conjunctival samples may contain goblet cells in addition to the cellular and noncellular findings described in cornea. These cells have eccentric nuclei and pale blue cytoplasm (Fig. 2-22). A lymphocytic or eosinophilic infiltrate may be apparent in cases with allergic inflammation. Material can be collected from the tarsal margin to analyze cases of chalazion or meibomianitis.

Figure 2-22 Photomicrograph of normal conjunctival cytology (Diff-Quik Stain, ×100).

Lesions that are suspect for adnexal or corneal neoplasia can be analyzed by cytology samples of cells débrided from the surface, cells from impression smears, and cells from fine-needle aspirates. It is beyond the scope of the practice laboratory to analyze these specimens. They should be submitted to a reference laboratory. Most lesions suspicious for neoplasia should also be biopsied and fixed tissue samples submitted for histopathology (Fig. 2-23). Reports from clinical pathologists focus on several criteria when assessing dysplastic, anaplastic, or metaplastic cells for malignant potential³:

Figure 2-23 Photomicrograph of a histologic section of a horse with a corneal squamous cell carcinoma. There is a proliferation of neoplastic epithelial cells and formation of a keratin pearl, suggesting that this tumor is well differentiated. Diagnosis of ocular surface neoplasia is beyond the scope of the practice laboratory—these specimens should be sent off to a reference laboratory for evaluation.

The report will list the cell type(s) found and give a presumptive or definitive diagnosis. Treatment or referral choices can then be made using information on the specific condition as outlined within this text.

Digital Photography

Inexpensive, high-quality digital cameras have made imaging of ophthalmic problems simple and accessible to all practitioners. Just as radiography and ultrasound are widely used to image the axial skeleton, body cavities, and reproductive tract, digital photography is appropriate for imaging the anterior segment of the eye. A digital image provides accurate documentation of the observable problems, permits further study of the lesions as they relate to surrounding anatomy, and provides a baseline for monitoring subsequent progression or resolution of said lesions.

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