Chapter 5 Basic Diagnostic Techniques
MEDICAL HISTORY
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).
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.
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.
ASSESSMENT OF PUPIL SIZE, SHAPE, SYMMETRY, AND MOBILITY
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).
EXAMINATION OF THE ANTERIOR SEGMENT
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.
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).
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.
Eyelids
Third Eyelid
Nasolacrimal Apparatus
Cornea
Sclera
Only the anterior portion of the scleral coat may be seen on direct external examination of the eye. Even then, it is seen through the almost transparent bulbar conjunctiva. The posterior sclera also is usually not directly visualized except in dogs with a subalbinotic fundus. In these patients, the inner aspect of the posterior sclera (the lamina fusca) is visible between and through choroidal tissue. This tends to have a more tan appearance than the external anterior sclera and is discussed more fully later as part of the ocular fundus. Pathology of the posterior aspects of the sclera can sometimes create notable changes in the adjacent choroid or retina, which can be seen during the fundic exam or using ultrasound. However, scleral changes can go unnoticed unless special care is taken to include this tissue on the mental checklist for the eye examination. When examining the anterior portion of the sclera, one should in particular look for the following:
Anterior Chamber
When assessing the anterior chamber, one should look in particular for the following:
Iris and Pupil
Lens
Examination of the lens completes examination of the anterior segment, which will identify the vast majority of ocular lesions encountered in general practice. However, a complete examination also necessitates examination of those structures in the posterior segment: the vitreous and various structures of the ocular fundus. This requires some equipment and techniques additional to those used for examination of the anterior segment, which are described in the following sections.
OPHTHALMOSCOPY
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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