Examination of the Patient From a Distance
The first part, and probably the most overlooked portion of the ophthalmic examination, is to scrutinize the patient from distance. Many important abnormalities are most easily seen with a careful distant exam. The questions to ask are: 1) What is the mental status and overall posture of the patient? The blind animal typically holds the eyes widely open in an attempt to increase light stimulation to the retina. 2) Is there symmetry to the eyes and orbits? Changes that affect position and size of the globe are most easily detected in this manner; differentiating globe enlargement (buphthalmos) from globe protrusion (exophthalmos) can be difficult with close inspection, but should be easy with careful distant examination. 3) Is there symmetry to the face and eyelids? Blepharospasm, ptosis, and facial “droop” should be evaluated. 4) Is there normal conjugate eye movement as the animal surveys its environment? Abnormalities of ocular innervation and motility are only easily detected by distant examination. Also, consider if the animal blinks completely and with regular frequency. This is the only way to detect lagophthalmos, which accounts for much of the corneal disease in brachycephalic breeds.
Through careful scrutiny of the patient from a distance, you will have inadvertently evaluated nearly all cranial nerves of ophthalmic importance (cranial nerves II, III, IV, V, VI, and VII). Checklist for distant exam: vision, mental status, globe and orbital symmetry, symmetry to adnexa and facial structures, blink characteristics, and ocular motility.
Examination of the Adnexa
Adnexal examination begins with Schirmer's tear test. The Schirmer's tear test should be done to any eye in which there is discharge, signs of inflammation, or corneal disease (there is no contraindication for this procedure). The tear test is especially important in cases of corneal ulceration. The location of placement of the tear strip in the palpebral fissure has little affect on the result. In uncooperative patients, the eyelids may be manually held closed. The average dog should produce 20 mm wetting in 60 seconds, and the average cat slightly less. Values over 10 mm are considered normal. Values between 5 and 10 mm indicate the possibility of sicca, while values < 5 mm are diagnostic. The password for good adnexal examination is magnification. Although any magnifying lens may be used, the method I prefer is to use the lens of an otoscope; this lens is powerful enough to detect nearly all common abnormalities and is easy to manipulate. The only thing better is a slit lamp biomicroscope. Checklist for the adnexal exam: distichiasis, trichiasis, ectopic cilia, foreign bodies under third eyelid, follicle formation, conjunctival vascular changes (inflammation and congestion), character of ocular discharge, eyelid neoplasms, chalazions, and blepharitis.
Examination of the Cornea
Fluorescein dye remains the agent of choice for detecting corneal ulcers. It is important that excess fluorescein be thoroughly irrigated from the cul de sac to avoid confusing healed ulcers with active ones; fluorescein will pool in epithelialized stromal defects. If fluorescein absorption occurs, distinguish complete versus partial epithelial cell loss by intensity of the staining; in cases of superficial keratitis the epithelium is likely to faintly stain, although true ulceration is absent. Ultraviolet or cobalt blue-filtered light is helpful but rarely necessary. The cornea is then examined for other opacities such as edema, pigment, and vascularization.
Vessels should be categorized based on depth (superficial vessels branch in tree fashion and can be seen crossing the limbus, deeper vessels possess a brush type border and emerge from the sclera). Proliferative-vascular lesions are sometimes encountered in the corneas of small animals. In most cases, they are secondary to chronic irritation, or are associated with healing ulcers. If close to the limbus, primary inflammatory lesions should be considered (especially in dogs, e.g., fibrous histiocytoma, chronic superficial keratitis, pannus). Finally, the cornea is examined for white opacities that usually are scars, dystrophies, or metabolic infiltrates. If a dense white opacity is present, the cornea is examined for adherent iris tissue (synechia). Such a lesion is referred to as an adherent leukoma. Checklist for corneal exam: ulceration, edema, vascularization, pigment, inflammatory masses, and white opacities (scars, dystrophies, and metabolic infiltrates).
Examination of the Anterior Chamber and Anterior Uvea
The most commonly overlooked anterior chamber abnormality is aqueous flare. Aqueous flare (plasmoid aqueous) occurs when the blood-aqueous barrier has been compromised. Although most commonly due to inflammation, breakdown of the blood-aqueous barrier also occurs with intraocular neoplasms. Detection of aqueous flare is important because it is frequently the only definitive sign of intraocular disease. When a disease process disrupts iris blood vessels, high molecular weight proteins are allowed access to the anterior chamber. The normally transparent aqueous humor becomes transparent; however, aqueous protein can only be detected with the correct application of optical principles.
To see aqueous flare an intense and focal light source must be held close (1 cm) to the corneal surface. A direct ophthalmoscope set at its smallest circular aperture of white light, and to its highest light intensity, works best (a pen light is inadequate for this procedure). Aqueous flare is present if the light beam is visible in the anterior chamber. Other abnormal anterior chamber contents include purulent exudate (hypopyon), blood (hyphema), and neoplastic masses. The detection of these abnormalities is facilitated by examining the anterior chamber with magnification from the temporal limbus, perpendicular to the visual axis. It is only from this perspective that subtle changes in anterior chamber depth, indicative of lens displacement, can be detected.
When examining the iris, abnormalities to look for are dyscoria, anisocoria, iris swelling, changes in iris color (heterochromia), iris cysts, persistent pupillary membranes, synechia, and atrophy. Examination of the ciliary body can only be accomplished with the aid of complete mydriasis. Check list anterior chamber and anterior uveal exam: aqueous flare, anterior chamber depth, hypopyon, hyphema, synechia, iris cysts, iris atrophy, heterochromia, persistent pupillary membranes, iris swelling.
Mydriasis and the Fundus Reflex
Tropicamide (Mydriacyl®) is the agent of choice; two drops instilled at five minute intervals produces mydriasis in 20 minutes which lasts for 3–5 hours. Glaucoma is the only contraindication to dilating the pupils. Examination of the fundus reflex is the most simple and yet useful methods for estimating the clarity of the ocular media. Using a direct ophthalmoscope set on 0 diopters, the light reflected from the ocular fundus is viewed from of distance of 12–18 inches.
The quality of reflected light seen varies with the breed and coat color type of the animal; animals with a tapetum will have a bright fundus reflection of yellow, orange, or green, while those without a tapetum usually have a red fundus reflection (produced by the choroidal vessels). Regardless of the color, the fundus reflex should be uniform throughout the pupil, and be free of aberrations. By examining the fundus reflex, the clinician can accurately estimate the amount of light reaching the retina, and thereby estimate the quality of sight. For example, in an animal presented with the complaint of blindness suspected to be due to cataracts, which has a reasonably normal fundus reflex, blindness cannot be attributed to lens opacification. The same principle applies to opacification anywhere within the eye, including the cornea, anterior chamber, lens, and vitreous. Once an opacity has been identified, it is examined in more detail using other techniques.
Examination of the Lens
The slit lamp biomicroscope is designed for examining the lens; however, for almost all practical purposes, the lens can be examined adequately with a direct ophthalmoscope and otoscope. The first part of the lens exam occurs when evaluating the fundus reflex. If an opacity obscuring the fundus reflex is suspected to be in the lens, its location is most easily verified by ruling out opacification of corneal and anterior chamber structures as described above. The fundus reflex is then used to localize the opacity within the lens. Because the lens nucleus is close to the rotational axis of the globe, the directional movement of lens opacities (cataracts) indicates their location; opacities that move in the direction of globe movement are in the anterior portion of the lens, those that appear to remain stationary are centrally located (nuclear), and those that move opposite are in the posterior portion. If cataracts are found, the anterior capsule of the lens is examined for irregularities (viewing perpendicular to the animals visual axis) with the otoscope. Changes in position of the lens should have been detected when examining the anterior chamber. However, subtle lens displacement may only be evidenced by a small crescent shaped area within the pupil that is devoid of lens material (aphakic crescent); aphakic crescents are also most easily detected by viewing the fundus reflex. Finally, the lens capsule is examined for the presence of adhered uveal tissue (posterior synechia, congenital pigment remnants), persistent pupillary membranes, and hyaloid artery remnants. Checklist for lens exam: cataracts, lens displacement, persistent pupillary membranes, hyaloid artery remnants, and synechia.
Examination of the Ocular Fundus
Mydriasis is absolutely imperative when evaluating the ocular fundus. Ophthalmoscopy may be done with either of two techniques, direct or indirect. With the direct method (direct ophthalmoscope), you view a real image of the fundus at a distance of 1–2 cm from the eye. With the indirect method (indirect ophthalmoscope) an inverted image of the fundus is seen from a distance of one to two feet from the eye. What is the practical difference between these two techniques? The most important difference is that indirect ophthalmoscopy provides a wide view of the ocular fundus while with the direct method only a very small area can be viewed. In most ophthalmology textbooks, the direct method is described as easier.
However, I disagree with this assertion. Because the patient’s eye is constantly moving, it takes considerable experience to see all parts of the fundus when viewing such a small surface area. With the indirect method, such a large portion of the fundus can be seen that identifying abnormal areas is far easier. Indirect ophthalmoscopy also produces an image with considerable depth of field, making it easy to tell if a lesion is raised or depressed. The disadvantage of indirect ophthalmoscopy is that the image is inverted. The fundus reflex is first viewed from a distance of 12–18 inches (lens diopter setting of 0). Move to a distance of 1–2 cm from the patient’s cornea and begin viewing the fundus, starting first with the optic papilla. Confusion exists regarding the use of different strength lenses of the direct ophthalmoscope. For most of us the instrument will remain set at 0 diopters.
In some small animals, because of their myopia, a lens setting of -1 to -3 may provide a clearer image. The only time other lenses are used, is to estimate the height or depth of fundic lesions; if a lesion is raised it will come into focus with a more positive lens setting, and if depressed will come into focus with a more negative setting. A difference of 3 diopters indicates a depth change of approximately 1 mm. The more positive (+8 to +20 diopter) settings can be used to examine more superficial structures in the eye; however, because these lenses are small and provide a limited view, the large otoscope lens is easier to use for these structures. When doing indirect ophthalmoscopy the fundus reflex is first viewed with either a direct ophthalmoscope set on 0 diopters, a bright penlight, or transilluminator (Finhoff) from a distance of 1–2 feet from the eye. One a clear reflex is observed, hold an indirect lens approximately 1 cm from the patient’s eye. The fundus will now be in view. Increase the magnification by moving the lens slowly away from the eye; the distance that produces the maximum magnification depends on the dioptric strength of the lens (those with the smaller number produce the greatest magnification).