MEDICAL HISTORY

A thorough, relevant history is an important part of the diagnostic process. To use the problem-oriented approach, the clinician first determines the major problems that have caused the owner to present the animal for examination. These problems form a temporary problem list and not only will direct the examination but also may suggest further avenues of questioning while the clinician is obtaining the complete history from the owner. Delaying taking the complete history until after the initial problems have been determined saves time and avoids the collection of irrelevant historical data.

Questions that are often helpful during the collection of a history for an ophthalmic patient include the following:

EXAMINATION PROCEDURE

An ophthalmic examination requires a minimum of equipment (Box 5-1). Ideally the examination is conducted in dim ambient light, preferably in a darkened room or stall, to minimize interfering reflections. Once sedated, a horse can be examined with the examiner’s and the horse’s heads under a blanket or dark cloth. Although the order in which the examination is conducted is not critical for all components, certain tests either would compromise later parts of the ocular examination or should not be performed until certain conditions have been ruled out because they could exacerbate or complicate those conditions or render them impossible to further examine. The major components of the ocular examination and the order in which each should be completed are described in Figure 5-1. Sometimes this order can be best recalled and examination findings best recorded on a form designed specifically for the ophthalmic examination (Figure 5-2).

Figure 5-1 Suggested order for the complete ophthalmic examination in all species. IOP, Intraocular pressure; PLRs, pupillary light reflexes; STT, Schirmer tear test.

Figure 5-2 Sample ophthalmic examination sheet.

(Courtesy University of Missouri, Columbia, Veterinary Ophthalmology Service Collection.)

SCHIRMER TEAR TEST

The Schirmer tear test (STT) is a semiquantitative method of measuring production of the aqueous portion of the precorneal tear film. It must be performed before application of any topical solutions, because these would artificially but temporarily raise the STT value. In addition, some topical solutions exert a more protracted inhibitory effect. For example, topically applied anesthetics or parasympatholytic drugs used to induce mydriasis, and local anesthetics, both will reduce STT values. Finally, manipulative procedures such as corneal or conjunctival scrapings, flushing of the lacrimal apparatus, and potentially even application of bright lights to an inflamed eye will result in artificially elevated STT values. For these reasons, if the STT is to be performed, it should be done as the first component of the ophthalmic examination.

The test is performed with sterile, individually packaged strips of absorbent paper with a notch 5 mm from one end. Each strip is folded at the notch and hooked over the middle to lateral third of the lower lid for 60 seconds (Figure 5-3). The distance from the notch to the end of the moist part of the paper is measured immediately on removal of the strip from the eye. This is the STT 1, which measures basal and reflex tearing, including that due to corneal stimulation provided by the test strip itself. That is why the STT strip should be placed in the middle to lateral region of the lower eyelid, where it can gently contact the corneal surface. If it is placed more medially, the third eyelid can protect the cornea and reduce STT 1 results. In normal dogs, the STT 1 result should exceed 15 mm in 1 minute. Readings of less than 10 mm in 1 minute are considered diagnostic for keratoconjunctivitis sicca. Values between 10 and 15 mm in 1 minute are considered highly suggestive of keratoconjunctivitis sicca, particularly if appropriate clinical signs are present.

Figure 5-3 Schirmer tear test being performed on a cat. Note that the strip is placed into the lower lateral conjunctival fornix so that it lightly contacts the lateral cornea.

The reported range for STT results in normal cats is 3 to 32 mm in 1 minute with a mean of 17 mm in 1 minute. However, experience suggests that lower readings than the reported mean can be expected in a clinical setting. This is probably due to autonomic control of secretion and short-term alterations in tear flow due to stress in the examination room. Values should still be recorded in cats but should be interpreted with caution and always in conjunction with clinical signs. Commercial strips are often unsuitable for horses if left in place for 60 seconds because of greater tear production in this species, which quickly saturates the entire strip. Some recommend broader strips, but these must be prepared in a very uniform manner. Rather, it is probably better to leave a standard strip in place for only 30 seconds in this species. STT results are also published for a number of exotic species (see Chapter 20).

For measurement of the STT 2, corneal sensation is abolished with topical anesthetic, and lower test values result because the afferent limb of the reflex path is blocked and reflex secretion by the lacrimal and nictitans glands is reduced. The STT 2 has not received widespread clinical application in animals but is sometimes referred to in texts and research studies.

MICROBIOLOGIC SAMPLING

Ocular surface samples (typically a swab or scraping) may be assessed for presence of a microbial pathogen by cytologic assessment, culture, polymerase chain reaction, or immunofluorescent antibody labeling. Some of these tests, especially microbial culture, may be affected by many of the drugs applied topically to the eye and by the preservatives that accompany them. Although topical anesthetic agents do contain preservatives, application of a topical anesthetic is essential for the safe and humane collection of samples from the ocular surface. Therefore, if indicated, samples for microbiologic analysis should be collected early in the examination process, immediately after the STT if it is done. The indications for collection of samples for microbiologic assessment include notable purulent inflammation; chronic, unresponsive, or severe corneal or conjunctival lesions; deep corneal ulcers with stromal loss or malacia (“melting”); and severe blepharitis or periocular dermatitis.

Traditionally, microbiologic specimens have been collected with a moist swab. However, some ophthalmologists prefer to collect more cellular specimens from corneal or conjunctival lesions by scraping or with a cytology brush. These samples can be submitted for culture and sensitivity testing, cytologic assessment, and Gram staining. A sterile instrument such as a Kimura spatula, a cytology brush, or the handle-end of a scalpel blade should be used (Figure 5-4). Samples should be placed into or onto media suitable for the organisms being sought, as advised by the laboratory that will receive and interpret the samples, and shipped appropriately and as soon as possible.

Figure 5-4 Instruments suitable for collecting ocular surface samples for cytologic and microbiologic examination. Left to right, Kimura platinum spatula, handle-end of Bard-Parker scalpel blade, cytology brush.

ASSESSMENT OF PUPIL SIZE, SHAPE, SYMMETRY, AND MOBILITY

Pupil dilation is essential if a thorough examination of the half of the eye behind the iris (the ciliary body, lens, vitreous, retina, tapetum, optic nerve head, choroid, and posterior sclera) is to be conducted. However, pupil dilation will completely prohibit assessment of pupil size, symmetry, shape, position, and reaction to light. It will also limit examination of the iris itself. Therefore anisocoria, dyscoria, corectopia, or altered pupillary light reflexes (PLRs) will be missed if a patient’s pupils are dilated before two straightforward tests are completed. Before application of a topical mydriatic agent, patients should be examined by retroillumination and their direct and consensual PLRs should be assessed. Pupil shape and the speed and magnitude of the PLR differ among species. The feline pupil is a vertical ellipse, the dog’s and bird’s are circular, and most large herbivores are horizontal slits. Despite these differences in pupil shape, pupils in these species dilate to approximately circular.

Retroillumination

Retroillumination is a simple but extremely useful technique for assessment of pupil size, shape, and symmetry. A focal light source (Finoff transilluminator or direct ophthalmoscope) is held up to the examiner’s eye and directed over the bridge of the patient’s nose from at least arm’s length from the patient so as to equally illuminate the two pupils and elicit the fundic reflection (Figure 5-5). This reflection is usually gold or green in tapetal animals and red in atapetal individuals. With each eye equally illuminated, the fundic reflex is used to assess and compare pupil size, shape, and equality (Figure 5-6). Retroillumination can also be used to judge the clarity of all of the transparent ocular media (tear film, cornea, aqueous humor, lens, and vitreous). Opacities in the ocular media will obstruct the fundic reflection and can be noted for more detailed subsequent examination. Retroillumination is particularly useful for differentiating nuclear sclerosis from cataract (see Chapter 13).

Figure 5-5 Retroillumination is performed with a focal light source held close to the examiner’s eye or with the direct ophthalmoscope held up to the examiner’s eye. The examiner stands at arm’s length from the patient and directs the light over the bridge of the patient’s nose so as to illuminate both pupils equally.

Figure 5-6 Retroilluminated view of a dog with bilateral nuclear sclerosis (visible as a translucent ring inside the pupils of both eyes). The tapetal reflection is used to “backlight” (or retroilluminate) all the clear ocular media (vitreous, lens, anterior chamber, cornea, and tear film).

EXAMINATION OF THE ANTERIOR SEGMENT

Every attempt should be made to avoid sedation or anesthesia before or during the ophthalmic examination in small animals, because its use complicates the examination in many ways. Reflexes and responses, vision, pupil size, globe movement and position, STT value, moistness of the corneoconjunctival surface, palpebral fissure size, and deviations of the visual axes cannot be assessed accurately if anesthesia or sedation has begun. Further, the globe becomes enophthalmic and rolls ventromedially, and the third eyelid covers much of the globe, making a complete eye examination impossible.

The exception to this rule is in the horse, in which sedation and an auriculopalpebral nerve block are essential for a good eye examination and should be performed after the assessment of pupil size, vision, STT (if necessary), and all reflexes and responses. The auriculopalpebral nerve block provides paresis of the orbicularis oculi muscle and limits eyelid closure, which is very forceful in the horse. Without an auriculopalpebral block, it is generally not possible to examine the whole globe of the horse and peripheral lesions will be missed. The auriculopalpebral nerve is found by running a finger up and down over the zygomatic process of the facial bone. The nerve can be palpated as it runs transversely in the subcutaneous space across the lower third of this process (Figure 5-7, A). Approximately 2 mL of lidocaine is then injected over the nerve (Figure 5-7, B). Sometimes sedation can be avoided in horses if a twitch is used. For food animals, a suitable crush or race/chute and (especially in cows) nose tongs are useful.

Figure 5-7 Performing an auriculopalpebral block. A, The auriculopalpebral nerve (yellow line) is palpated by running a finger up and down over the zygomatic process of the facial bone (white arrows). B, Approximately 2 mL of lidocaine is then injected over the nerve.

In all species and before sedation (if used), the eyes and periocular region should first be examined from a distance and in normal ambient light for gross abnormalities, including asymmetry, palpebral fissure size, ocular or nasal discharge or dryness, periocular alopecia, deviations of the visual axes, redness or other color change, and corneal clarity and moistness (reflectivity). The animal’s response to the new environment of the examination room may also be used to judge visual function. Neuroophthalmic and visual testing may then be conducted. Vision testing could include maze testing, tracking of objects that do not emit sound or odor (thrown cotton balls or a laser pointer are excellent). The menace response, if conducted in such a way that air currents and direct contact with the vibrissae are avoided, can also be used. Normal palpebral reflex (a complete blink in reaction to digital stimulation of the eyelid skin) should then be verified for both eyes as a test of CN V and CN VII function. These tests are more thoroughly described later. Finally, globe movements and position are examined. As the head is elevated, depressed, and moved laterally, the eye should return to or remain in the center of the palpebral fissure, producing a physiologic nystagmus. Both globes should retropulse normally and symmetrically.

Examination in dim ambient light is then commenced with a magnifying loupe and a focal light source (Figure 5-8). A Finoff transilluminator, which fits to the direct ophthalmoscope or otoscope handset, produces a brighter and more focused beam of light than a penlight and is therefore preferred. A source of magnification is absolutely essential to conduct a complete ocular examination. The most useful instrument for general practice is a simple magnifying loupe with a power of 2 to 4 μ and a focal length of 15 to 25 cm (6 to 10 inches). Using this combination of a focal light source and magnification, the clinician should examine the eye from many angles while the light is directed from many contrasting angles. Particular attention should be paid to the Sanson-Purkinje images, which result from reflections from ocular structures. When a focal light beam is examined from an oblique angle as it passes through the eye, three reflections can usually be seen: the cornea, the anterior lens capsule, and, sometimes, the posterior lens capsule (Figure 5-9). The combination of variable viewing and illumination angles permits the examiner to gain intraocular depth perception by virtue of parallax, shadows, perspective, and reflected light (Figure 5-10).

Figure 5-8 Finoff transilluminator and Optivisor magnifying loupe.

Figure 5-9 The Sanson-Purkinje reflections off the cornea and anterior and posterior lens capsules provide perspective during the examination and should be used to judge the depth of lesions within the eye.

Figure 5-10 The use of parallax to localize intraocular opacities. 1, Corneal opacity; 2, cataract on anterior lens capsule; 3, cataract on posterior lens capsule. The pupil is dilated for this examination.

(From Komar G, Szutter L [1968]: Tierarztliche Augenheilkunde. Paul Parey, Berlin.)

The slit-lamp biomicroscope (Figure 5-11) is a more sophisticated optical instrument that combines up to 40 μ magnification and illumination and can be used to examine many different microscopic and optical features of the patient’s eye such as individual layers of the cornea that normally are invisible to the naked eye of the examiner. As such, it allows pathologic processes to be described more accurately so as to better guide diagnosis, prognosis, and treatment. Because of the requirement for training and skill in their use, slit-lamp biomicroscopes are usually found only in specialty practices and teaching institutions.

Figure 5-11 A slit-lamp biomicroscope in use. Slit-lamp biomicroscopes provide binocular magnification (with stereopsis) and a focal light source whose angle, shape, and intensity can all be altered. These greatly facilitate ophthalmic examinations in all animals.

Using a source of focal light and magnification, the clinician examines ocular structures sequentially according to a mental checklist (see Figure 5-2). A logical order appears to be to work simultaneously from peripheral to axial and from anterior to posterior, as follows. More detail can be found in the specific chapters relating to diseases of these individual tissues.

Third Eyelid

The position of the third eyelid should be examined at rest, and then its anterior face further examined through gentle digital retropulsion of the globe through the upper lid. This later step should be omitted if a deep or penetrating corneal or scleral lesion renders the globe unstable. The posterior or bulbar face of the third eyelid may be examined by means of protrusion and eversion of the third eyelid with a pair of fixation forceps or mosquito hemostats after application of topical anesthetic (Figure 5-12). Look in particular for the following:

• Increased prominence at rest: orbital mass, enophthalmos, phthisis, microphthalmos, Horner’s syndrome, Haw’s syndrome

• Scrolled third eyelid cartilage

• Masses: prolapse of the gland of the third eyelid (“cherry eye”), neoplasia

• Irregularities of the margin or surfaces: chronic conjunctivitis (“pannus” or conjunctivitis), trauma

• Foreign bodies

• Changed color: melanosis, hyperemia, anemia

• Surface moistness and discharge: dacryocystitis, keratoconjunctivitis sicca

Figure 5-12 Use of two hemostats to exteriorize the third eyelid for examination following application of a topical anesthetic. This horse has a squamous cell carcinoma on the leading edge of the third eyelid.

Anterior Chamber

The anterior chamber is the aqueous humor–filled space between the iris and posterior cornea. As such, it can be difficult to examine. The following three techniques assist with this problem:

1. Assess the globe from the lateral side, looking “across” the anterior chamber (Figure 5-13).

2. Assess the clarity of the view of the rest of the intraocular structures, especially the iris face. If this is decreased, then corneal disease and/or anterior chamber debris is likely.

3. Give attention to the Sanson-Purkinje images as described previously.

Figure 5-13 Transverse view of the left eye of a cat. The view from this angle is very useful for examination of the anterior chamber in animals. Magnification and a focal light source are also essential.

When assessing the anterior chamber, one should look in particular for the following:

• Alterations in depth: increased with posterior lens dislocation, microphakia, buphthalmos, hypermature cataract or after surgical lens removal. Decreased with anterior lens dislocation, iris or ciliary body tumors/cysts, iris bombé, many acute glaucomas, intumescent cataracts, and aqueous misdirection in cats.

• Abnormal contents: anterior lens luxation, foreign body, hyphema, fibrin, hypopyon, aqueous flare, iris cysts, tumors, persistent pupillary membranes, vitreous, and anterior synechia

OPHTHALMOSCOPY

The clinician can examine the fundus of larger eyes (notably those of the horse, cow, and many raptors) directly through a widely dilated pupil by aligning the light beam with his/her visual axis and standing a short distance from the patient, as for retroillumination (see Figure 5-5). However, this maneuver is not possible for dogs and cats, and even for horses and cows, accurate assessment requires examination using one of three methods of ophthalmoscopy:

Although various writers describe one or others of these ophthalmoscopes as easier to use, the old adage that practice makes perfect applies, and persistence and practice are essential to master any ophthalmoscopic technique. Therefore rather than concentrating on the technique that is alleged to be easier to learn, budding ophthalmoscopists should perhaps focus on the technique most likely to be useful to them throughout their careers with the aim being to become competent at that technique. Most ophthalmologists prefer to scan the whole fundus with an indirect lens and then to examine more closely any regions of interest using the direct ophthalmoscope. This approach takes advantage of the greater field of view associated with indirect ophthalmoscopy and the higher magnification permitted by direct ophthalmoscopy. Regardless of the ophthalmoscope used, thorough examination of the fundus requires complete pupil dilation, which is achieved approximately 15 to 20 minutes after application of 1 drop of tropicamide. The extent and speed of dilation can be enhanced in some patients by application of a second drop about 5 minutes after the first.

Direct Ophthalmoscopy

The direct ophthalmoscope directs a beam of light into the patient’s eye and places the observer’s eye in the correct position to view the reflected beam and details of the interior of the eye (Figure 5-14). It is called a direct ophthalmoscope because it provides a direct and upright image of the fundus rather than a virtual and inverted image as provided by the indirect ophthalmoscope. The direct ophthalmoscope (Figure 5-15) has a rheostat to control the light intensity, colored filters, a slit beam for viewing elevations and depressions within the fundus, an illuminated grid to project onto the fundus to measure lesions, and a series of lenses on a rotating wheel that adjusts the depth of focus within the eye. These lenses may be used to examine structures other than the fundus or to measure the height of lesions by changing the focus from the tip of the lesion to the surrounding retina and determining the dioptric difference. However, many of these features are of limited use in noncompliant veterinary patients.

Figure 5-14 Direct ophthalmoscopy. Arrows show orientation of images in examiner’s and patient’s eyes.

(Modified from Vaughan D, Asbury T [1983]: General Ophthalmology, 10th ed. Lange Medical, Los Altos, CA.)

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