BACK TO BASICS: FOUNDATIONAL CLINICAL SKILLS FOR EQUINE PRACTICE


peripheral to axial.9 Retroillumination is used to visu- alize the tapetal reflection through the pupil; a focal light source is held close to the observer’seye and directed toward the horse’s eye at arm’slength.6,9 This allows assessment of pupil size and symmetry and highlights any opacities impeding the reflection, including corneal scar, corneal blood vessels, cataract, pigment, and so on. Transillumination (focal illumina- tion) is used to sequentially examine all structures in the anterior segment.9 A focal light source should be directed at the eye from many angles while the exam- iner also varies the viewing angle, permitting assess- ment of the eye from all 3 dimensions.6,9 Deeper structures are then examined using focal illumination, the slit-beam setting on the direct ophthalmoscope, and fundic examinationwith indirect ophthalmoscopy.


Adnexa, Eyelids, and Conjunctiva


The examiner may use a bright light source to provide direct illumination to inspect the adnexa, eyelids, and conjunctiva.Magnification loupesa are inexpensive and very helpful in identifying lesions on intricate struc- tures. Varying degrees of magnification and focal dis- tance are available. Any ocular discharge should be noted. The eyelid borders should be smooth and free of scars or regions of ulceration. Eyelid closure should be complete.1 The globe should be retropulsed to extrude the third eyelid for examination. The conjunctiva should be smooth and pink with no irregularities. Common defects to look for include tumors (squamous cell carcinoma, sarcoid, melanoma, etc.), eyelid irregu- larities from prior trauma, entropion, and conjunctival hyperemia6 follicles consistentwith conjunctivitis. **The remainder of the ophthalmic examination is


best performed in a dark or dimly lit area.Magnification is helpful in examining the cornea, anterior chamber, iris, and lens.2


Cornea


The cornea should be smooth and clear with a lus- trous tear film. Start with diffuse focal direct illumi- nation and identify any abnormalities. Then change to the slit beam on a direct ophthalmoscope head to further evaluate corneal topography, thickness, depth of any previously identified lesions, and any areas of stromal loss. As with adnexa, magnification is very beneficial. Light directed perpendicular or diagonal to the cornea should help reveal opacities against the dark background of the iris/pupil interface.2


Anterior Chamber


The anterior chamber should be optically clear.1 Again start with diffuse focal direct illumination to look for any abnormalities or opacitieswithin the anterior cham- ber.Then change to the slit beamon a direct ophthalmo- scope head to further evaluate clarity of the aqueous humor and the iris/lens interface. It is very important to look for aqueous flare, which represents protein 6 cells in the anterior chamber and is an important indicator of anterior uveitis.2,5,6 Aqueous flare can be detected by


shining the focal light source directly on the cornea so it is a focused beam; the examiner looks perpendicular to the beam of light as it crosses through the anterior chamber. The anterior chamber should be clear and not cloudy or murky. Use the slit beam or the smallest focal circular beam of light to focus the light directly on the cornea.6 Typically, the light source is held close to the cornea (;5–10mm) in order to focus the light. The examiner should view the light from the side (45–90°) as the light courses from the cornea through the ante- rior chamber.Normal eyeswill showthe light beam hit-


ting the corneal surface ! a clear void in the anterior chamber ! light hitting the anterior lens capsule and iris/lens interface ! light coursing through the lens to the end at the posterior lens capsule.6 A light beam visi-


ble in the anterior chamber between the cornea and iris/ lens interface (like a car head light beamin the fog) indi- cates the presence of aqueous flare and thus uveitis.6 Dark exam settings andmagnification are helpful.


Iris Including Corpora Nigra


The iris should be examined for tissue architecture, smooth topography, variations in pigmentation, or mass effect. The iris may vary in color; combinations of brown and blue irides (heterochromia iridis) are a variation of normal. These should be differentiated from regions of hyper- or hypopigmentation of the iris from past uveitis. The corpora nigra or granula iridica are normal


round structures that arise from the posterior pig- mented epithelium of the iris and are found at the dorsal and ventral pupillary margins.7 Corpora nigra mayhave a wide variationofnormal interms of shape and size. It is important to compare the cor- pora nigra between both eyes to look for symmetry. Atrophy, shrinkage, or coalescence of corpora nigra may be indicative of past uveitis. Corpora nigra may also become cystic and enlarged, which can result in visual impairment frompartial blockage of the pupil or impingement on the corneal endothelium causing cor- neal lesions.7 Iridal cysts may be present separate fromthe corpora nigra along the margin of the pupil or rarely free-floating in the anterior chamber.10 Iris cysts arise from the posterior pigmented epithelium of the iris and may peek over the pupillary margin into the anterior chamber. It is impossible to predict which cysts are likely to enlarge. The examiner should ensure the pupil is mobile and


look for any synechia (adhesions). Adhesions of the iris to the cornea are known as “anterior synechia.”7 Synechia are usually indicative of past corneal ulcera- tion/rupture or uveitis. Anterior synechia should be differentiated from persistent pupillary membranes (iris to cornea),which are congenital. Persistent pupil- lary membranes (PPMs) typically originate at the iris collarette, and anterior synechia typically originate from the pupillary margin.11 Adhesions of the iris to the lens are known as “posterior synechia” and gener- ally indicate previous uveitis.1 Differentiating a single episode of uveitis vs equine recurrent uveitis is gener- ally not possiblewith a single examination.


AAEP PROCEEDINGS / Vol. 68 / 2022 183


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