How to Perform an Ophthalmic Examination: It Should Not Be Complicated, It Should Not Be Expensive
Koret School of Veterinary Medicine, Hebrew University of Jerusalem, Rehovot, Israel
An ophthalmic examination should not be a scary experience! Though admittedly interpretation of the findings may sometimes be challenging, the examination itself follows a logical, anatomical order. Furthermore, it does not require expensive equipment. In fact, the most important items required are non-ophthalmic in nature: A room that can be darkened, a good source of focal light and a magnifying loupe. A hand held lens, a direct ophthalmoscope, a tonometer and some disposable items (stains, solutions, etc.) complete the list of basic equipment.
The patient should be observed as it walks into the room, since this is an unfamiliar environment which may highlight visual deficits; these will be further evaluated later on. Following the anamnesis and physical examination, the ocular assessment begins by careful observation of the patient from a distance, without touching the patient (as this may cause distortion of palpebral fissure).
While observing, ask yourself:
Are both eyes open normally? Is there evidence of pain or photophobia? Is the animal blinking normally?
Are the eyes of normal size and position? Is there evidence of exophthalmos or buphthalmos? Are the pupils of equal size?
Is the eyelid conformation normal? Is there evidence of entropion or ectropion (usually of the lower lid)? Is the 3rd eyelid elevated?
Is there ocular discharge? What is its nature?
This involves making a sudden threatening gesture which is supposed to elicit a blink response. It is important to note that the menace response involves cerebral cortical integration and interpretation and therefore is not a reflex. Rather, it is a cortical response that requires the entire peripheral and central visual pathways, as well as the visual cortex and the facial nucleus and cranial nerve, to be intact. The menace response should be evaluated in one eye, while the other eye is being covered. Be careful not to touch the eyelashes/hair of the patient, or to cause wind movement, as this may lead to a "false positive" response; consider making the menace gesture behind a glass partition. Likewise, "false negative" results (lack of a menace response in a visual animal) are also possible. One possible reason is facial nerve paralysis, which is ruled out using the blinking reflex. The menace response is absent in very young (< 10–12 weeks) animals, and may also be affected by the patient's mental state.
Examination in the Dark
After the light has been dimmed, the dilation of the pupils should be evaluated. Use a dim light (to prevent constriction), and stand at a distance so you can visualize both pupils simultaneously, using the tapetal reflection. The tapetal reflection also serves to highlight (by means of retroillumination) any ocular opacities, particularly in the lens or vitreous.
Next, use a bright light to evaluate the pupillary light reflex (PLR). Unlike the menace response, the PLR is a subcortical reflex. Therefore, it does not test vision, and a normal PLR may be found in a cortically blind animal. Furthermore, the PLR is usually present (though it may be diminished or slow) in animals suffering from outer retinal degeneration (PRA), cataracts, and other causes of subcortical blindness. Nevertheless, the PLR is a very important test, which helps localize the lesion which causes loss of vision.
If one of the pupils does not react to light, or if it cannot be visualized (e.g., in cases of severe corneal edema or hyphema), the consensual PLR should be checked. Alternatively, you can check the dazzle reflex. This is also a subcortical reflex, which is manifested as a bilateral, partial blink in response to a bright light.
Next, using magnification and a focal light source, the anterior structures of the eye are examined in an anatomical order.
Eyelids & Eyelashes
Evaluate the size of the palpebral fissure, looking for a narrowed or enlarged fissure. Carefully examine the skin, looking for discharge and for signs of dermatological disorders, such as dermatitis, alopecia, scaling, swelling, crusting, ulceration, etc. Pay particular attention to the eyelid margin. In a normal animal, you should see the entire margin in close contact with the globe. Lack of contact may be due to ectropion (drooping lid). On the other hand, if you cannot see the margin, or parts of it, the lid may be everted (entropion). Eyelash abnormalities may be better visualized if the lid is slightly retracted. Dark lashes can then be highlighted against the background of the white conjunctiva.
Third Eyelid & Conjunctiva
At rest, the 3rd eyelid should be mostly retracted, and hardly visible. Look for increased prominence at rest, scrolled margin, or "cherry eye" and other masses. The inner aspect of the 3rd eyelid margin may be examined after application of topical anesthetic and eversion of the lid with fine forceps. Look for foreign bodies or hyperplasia of lymphatic follicles.
Similarly, examine the conjunctiva lining the inner aspect of the eyelids and globe for change in color, congestion, edema, prominent vessels, masses, thickening, discharge, moistness or subconjunctival hemorrhage.
Cornea & Sclera
The normal cornea should be smooth and transparent. Any deviation from these characteristics represents pathological changes. Look for loss of transparency due to edema, pigmentation, vascularization, cellular infiltration, lipid or mineral deposition, or fibrosis. Look for surface irregularities which may be due to ulceration, perforation and iris prolapse, granulation tissue or keratoconus. Evaluate the corneal diameter. An enlarged diameter may indicate glaucoma, while a reduced diameter will indicate a phthisical or microphthalmic eye.
Assess the depth of the anterior chamber (best visualized from the side), as it may be increased or decreased in various intraocular diseases. In normal animals, the aqueous filling the anterior chamber should be clear. Look for any opacities or masses such as blood, fibrin, hypopyon, aqueous flare, luxated lens, persistent papillary membranes, iris cysts or vitreous strands.
Iris & Pupil
Look for alterations in pupil shape, which may be due to adhesions, or iris atrophy, hypoplasia or coloboma. Changes in the color of the pupil may indicate cataract, hemorrhage or retinal detachment. The size of the pupil may be altered in uveitis, glaucoma and various diseases of the retina or the nervous system.
Examine the surface of the iris for any masses or changes in color. These may be due to inflammation, hemorrhage or neoplasia. Fluttering of the iris may indicate lens luxation.
The lens may be examined with direct visualization, or by retroillumination, using tapetal reflection. The two main pathologies are luxation or opacities, which would indicate cataract. A comprehensive lens examination requires dilation of the pupil.
This part of the examination is the one which clinicians usually dread the most. Part of this undoubtedly stems from the large range of normal variations in the appearance of the canine (and, to a lesser extent, the feline) fundus. Admittedly, if you are not in the habit of examining fundi, you will find it difficult to diagnose abnormalities. You should therefore make a habit of examining, however briefly, the fundus of every patient that you see. Your clients will appreciate the extra touch, and you will gain the required proficiency.
Ophthalmoscopy should be conducted in a dark room, following dilation of the pupil. First evaluate the tapetal reflection from a distance, to detect any lenticular or vitreal opacities. As you approach the patient, focus on successively more posterior structures - cornea, iris, lens and vitreous - till you are focused on the fundus. Carefully inspect the entire fundus, evaluating changes in the tapetum, non-tapetum, blood vessels and optic disc. It is best to stay in stationary position and let the patient's eye movements bring the structures to you, instead of trying to "chase" them.
Schirmer tear test is used to evaluate tear production and diagnose keratoconjunctivitis sicca. It should be conducted at an early stage of the examination, as any ocular manipulation may induce reflex tearing.
Fluorescein staining is used to diagnose corneal ulcers. Superficial ulcers may be stained with Rose Bengal.
Samples for bacteriology, mycology and cytology may be taken as indicated. The first two should be taken before any drops are put in the eye, as solutions frequently contain preservatives.
Nasolacrimal patency is evaluated by passage of fluorescein from the eye to the nose, by cannulating the nasolacrimal system and by dacryocystorhinography.
Ultrasound is frequently used in ophthalmology. The main indications are imaging of the retrobulbar area, and imaging of the posterior segment when it cannot be visualized (e.g., due to hyphema or cataract). CT and MRI techniques may be used in certain cases.
Additional tests, including gonioscopy (evaluation of the iridocorneal angle as part of the diagnosis of glaucoma) and electroretinography (recording electrical responses of the retina to flashes of light, to determine retinal function) may be available in referral centers.