Ophthalmology Made Simple
Dr. Natalie Stilwell provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting. In addition to her DVM obtained from Auburn University, she holds a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida.
Ocular examination and diagnostic testing don’t have to be intimidating. Here’s what you need to know to hone your skills.
Ophthalmic examination offers unique insight into a patient’s health status, according to Elizabeth Giuliano, DVM, MS, DACVO. At the 2018 Western Veterinary Conference in Las Vegas, Nevada, Dr. Giuliano provided a general ophthalmology overview for veterinary practi- tioners that included valuable information on important diagnostic tests and useful tips to improve ophthalmic exam skills.
Dr. Giuliano, a professor and the ophthalmology section chief at the University of Missouri Veterinary Health Center and former president of the American College of Veterinary Ophthalmologists, began by highlighting the organ’s important anatomic features, noting that “when something goes wrong in one part of the eye, it can rapidly affect and cause disease in many other parts of the eye.”
The 3 basic tunics of all mammalian eyes are the outer fibrous tunic, consisting of the cornea and sclera; the middle vascular tunic, or uveal tract, including the iris, ciliary body, and choroid; and the inner neural tunic, or retina. Internal optical media include the aqueous humor, lens, and vitreous humor. Although these anatomic features exist across species, Dr. Giuliano also emphasized a great amount of interspecies variation. She recommended IVALA Learn veterinary anatomy and clinical learning content (ivalalearn.com) as a useful resource for anatomy of the eye.
Practitioners can gain valuable information by obtaining a thorough patient history. Questions for the owner should address the presence and duration of clinical signs; whether vision problems are progressive, static, or improving; and whether problems are noted in certain light levels or environments. Also, note any genetic ophthalmic issues in the pet’s relatives. Small animals, especially cats, may even hide blindness from the owner for years, Dr. Giuliano stated, especially if their living environment has not been altered.
In addition to vision, it is equally important to gauge the patient’s comfort level by asking the own- er to describe clinical signs seen at home, such as redness, swelling, tearing, or squinting. Always ask the owner to bring any topical and systemic medica- tions to the exam and determine dosing schedules, as well as when the owner last gave medications, to interpret signs appropriately. For example, in cases of chronic keratoconjunctivitis sicca (KCS),
Dr. Giuliano performs a Schirmer tear test within 3 to 4 hours after the most recent cyclosporine dose to accurately interpret the medication’s effect on tear production.
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TIPS FOR A SUCCESSFUL OPHTHALMIC EXAMINATION
Ophthalmology offers unique insight during a veterinary exam, Dr. Giuliano explained, as the eye is the only organ with directly visible vessels and nerves, and the anterior chamber allows examination of an interstitial space. In many cases, the eye can also provide evidence of systemic disease. Ocular pathology aids the diagnostic process and can help the practitioner prioritize diagnostic testing, which is why she stressed that all enucleated eyes should undergo diagnostic testing.
The minimum ophthalmic database should include a complete ophthalmic exam, neurophthalmic exam, Schirmer tear test, tonometry, fluorescein stain, and additional diagnostics based on exam findings. Every clinic should also have a dedicated ophthalmic exam sheet to keep with patient records.
A successful ophthalmic exam depends on having the proper equipment and using the correct technique. A bright focal light source, such as a transilluminator, head loupe, or slit lamp, in an otherwise dark environment provides optimal visualization of the eye. Dr. Giuliano also regularly uses a camera to monitor lesions over time.
Practitioners should examine the patient at eye level, with the light source illuminating the eye in 3 distinct ways:
- Direct focal illumination
- Tangential illumination, which is when the light source is held out to the side of the patient’s globe and the eye is illuminated from the side
- Retroillumination, which reflects light off the tapetum and iris to highlight important lesions such as anisocoria or opacities in optical media
- (eg, cataracts)
OPHTHALMIC TESTSPupillary Light Reflex
Dr. Giuliano reminded the audience that the pupillary light reflex (PLR), which measures the function of cranial nerves (CNs) II and III, “does not equate to vision.” The reflex may be intact in cases of cortical blindness or significant retinal dysfunction. Also remember to observe for direct and consensual responses due to crossover of CN II and III. The PLR may be naturally decreased in an excited patient with high sympathetic tone, and iris atrophy can also cause an absent or decreased PLR.
While the neuronal pathway for the dazzle reflex is not clearly defined, this test evaluates subcortical reflexes (CN II and VII function) rather than vision. The dazzle test requires a very bright light, such as a smartphone flashlight, to induce an involuntary blink. Dr. Giuliano generally chooses this test to evaluate whether an eye has any residual function remaining (eg, when the globe is proptosed) or if it is in the patient’s best interest to have the eye enucleated. She noted that dazzle may still be present even with significant retinal dysfunction.
RELIABLE VISION TESTSMenace Response
The menace response, which has a complex pathway including CN II and VII, is absent in very young animals and learned at different rates in precocial and altricial species. An intact menace response should be present by approximately 12 weeks of age in small animal species, while large animal species develop the response by 2 to 3 weeks. A proper menace response test should be performed in a still environment with no air current. Dr. Giuliano also reminded the audience to test each eye separately by closing 1 eye and using a finger to menace both sides of the visual field near the lateral and nasal canthus. The menace response is typically subdued in feline patients.
Tracking behavior is a simple vision test to evaluate whether the eyes follow a dropped object, such as a cotton ball. As with the menace response, some feline patients may not respond to this test and will not track an object even if vision is normal.
“A grossly underutilized test that greatly helps to distinguish vision problems,” is how Dr. Giuliano described the maze test. Practitioners can perform this simple test in a dark exam room with either rheo- stat lighting or a flashlight placed in the corner. Put a trashcan or stool in front of the animal, and position the owner on the other side of the room. Gently disorient the patient by spinning it a few times, then observe the level at which the patient can avoid objects and the speed at which it navigates. This test also evaluates for progressive retinal atrophy by differentiating night blindness from day blindness, as affected animals lose night vision first.
The corneal reflex tests CN V for corneal sensation and CN VII for a blink response. Make sure no topical anesthetic agents, such as proparacaine, have been applied to the eye before performing this test. Dr. Giuliano also emphasized the importance of performing this test gently, with a cotton-tipped applicator rather than a finger.
Similar to a menace test, the palpebral reflex is induced by touching the periocular area to look for a blink reflex induced by CN V and VII function. Stimulating the lateral canthus, medial canthus, and base of the ear will test function of the maxillary, ophthalmic, and mandibular branches of CN V, respectively. Although this is not a test of vision, it does test the facial nerve response in animals and thus tells the veterinarian whether the patient is able to blink.
IMPORTANT DIAGNOSTIC TESTSSchirmer Tear Test
The Schirmer tear test measures both basal and stimulated tear production over a 1-minute period. Although most references state that more than 15 mm of absorbency on a test strip in 60 seconds is acceptable, Dr. Giuliano said that most normal dogs exhibit upwards of 20 mm of absorbency. She strongly suspects KCS when results range from 10 to 15 mm in 60 seconds; less than 10 mm in 60 seconds is diag- nostic for KCS. Also, she emphasized that tear produc- tion can fluctuate throughout the day, so low tear production should be taken seriously.
Dr. Giuliano hinted that KCS typically presents in dogs about 8 years or older, around the same time as the onset of nuclear sclerosis. When Schirmer tear test results are hovering around 14 to 15 mm in 60 seconds, she recommended beginning administration of preservative-free tears twice daily, not only to increase eye lubrication but also to encourage owners to inspect the eye regularly and acclimate patients to eye drops. She then performs serial Schirmer tear tests over the following few months to monitor tear production. If the cornea is already exhibiting signs consistent with KCS such as a lackluster appearance, even if the strip measures 15 mm of wetting in 60 seconds, she will frequently initiate therapy with a lacrimostimulant drug such as topical cyclosporine or tacrolimus.
Proper test strip placement is in the middle lower fornix of the lower lid. If the strip is placed too lateral, it will contact sclera instead of cornea, whereas a strip that is too medial will encounter the third eyelid. If the strip falls out of the eye, the same strip may be used to finish the test, as it has already absorbed the lacrimal lake, or pool of tears in the lower conjunctival cul-de-sac.
Fluorescein stain adheres to hydrophilic areas of the cornea, such as stroma, thus making epithelial defects easier to visualize. Practitioners should perform the stain in dim light with a cobalt blue filter rather than other light sources such as a Wood lamp. To perform the test correctly, moisten the strip with eyewash and touch sclera (not cornea), then rinse generously. Stain can be diluted in a 3- to 5-mL syringe for large animal patients, but she does not advocate this technique in small animal patients. Always use a fresh strip for each patient.
Fluorescein stain is also used to perform the Jones test, during which the appearance of stain in the nares indicates an intact nasolacrimal outflow tract. With the Seidel test, the presence of pale rivulets of stain in a focal area on the cornea indicates aqueous humor leaking through a perforation site.
Dr. Giuliano stressed that “intraocular pressure [IOP] should be measured in every patient with a red eye, as long as the cornea and sclera are still intact.”
Avoid performing tonometry if the eye is suspected or confirmed to be perforated, with a descemetocele or deep stromal or melting ulcer, for example. IOP should also be measured in patients with a family history or breed risk for glaucoma as part of the minimum ophthalmic database or to monitor uveitis and ophthalmic surgery cases.
The 3 main types of tonometry are Schiötz (or indentation tonometry), applanation, and rebound. Although it is inexpensive and accurate with practice, Schiötz tonometry is inconvenient for some species because it requires the cornea position to be horizontal. This type of tonometer is also relatively difficult to clean.
Applanation and rebound tonometry are becoming increasingly popular in veterinary ophthalmol- ogy for several reasons. Although more expensive than Schiötz, both methods are digital and simple to use and clean. They are also convenient for patients with small corneas, and vertical cornea positioning is acceptable.
Veterinary practitioners should perform applanation tonometry with topical anesthesia, such as proparacaine, as a certain amount of force is necessary to flatten the cornea. Although applanation provides accurate measurements in patients with corneal abnormalities, it is also easy to elevate IOP artificially by applying neck pressure or digital pres- sure directly on the globe, particularly in brachyce- phalic breeds.
The tonometer should be calibrated prior to use or daily, and calibration instructions should be followed strictly to ensure accurate results. Use readings only with a coefficient of variation of less than 5% among 3 to 6 instrumental readings. Also, Dr. Giuliano advised standing slightly to 1 side of the patient rather than directly in front to better visualize corneal contact with the tonometer.
Unlike applanation tonometry, rebound tonometry does not require topical anesthesia or calibration. The instrument uses an electromagnetically propelled probe to bounce gently off the cornea.
Regardless of tonometry method, always make sure the animal is calm, and avoid applying pressure to the globe, eyelids, head, or neck. Appropriate placement of the hands is under the chin and behind the base of the skull. A general rule of thumb is that the lowest final reading is the most accurate. A pressure difference of more than 5 mm Hg between eyes should be considered abnormal. Normal reference ranges for IOP are 12 to 25, 12 to 27, and 17 to 28 mm Hg for dogs, cats, and horses, respectively.
Ophthalmology is a valuable component of the veterinary diagnostic workup. Many of the ophthalmic tests Dr. Giuliano described are simple to perform and require minimal equipment. Furthermore, proper handling and restraint are essential to ensure accurate results and optimize patient comfort.
Dr. Stilwell is a medical writer and aquatic animal veterinarian in Athens, Georgia. After receiving her DVM from Auburn University, she completed an MS degree in Fisheries and Aquatic Sciences, followed by a PhD degree in Veterinary Medical Sciences, at the University of Florida.