Ординатура / Офтальмология / Английские материалы / Slatter's Fundemental of Vetrinary Ophthalmology 4th edition_Maggs, Miller, Ofri_2008

posterior, as follows. More detail can be found in the specific chapters relating to diseases of these individual tissues.

Eyelids

The eyelids should be examined with particular attention to the periocular skin, eyelid margin, and meibomian gland orifices. Look in particular for the following:

•Periocular discharge: serous (“epiphora”), mucoid, purulent, sanguineous, or a combination of these

•Periocular dermatitis/blepharitis: alopecia, scaling, hyperemia, crusting, swelling, ulceration, maceration, etc.

•Palpebral fissure size: narrowed or macropalpebral fissure

•Eyelid position and motion: entropion, ectropion, ptosis, blepharospasm

•Disorders of the cilia or periocular hair: ectopic cilia, distichia, trichiasis

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.

BASIC DIAGNOSTIC TECHNIQUES 87

Conjunctiva

In addition to the conjunctiva that lines both surfaces of the third eyelid, the remaining conjunctiva lining the eyelids (palpebral conjunctiva) and the anterior globe (bulbar conjunctiva) must also be examined. This requires opening and eversion of the upper and lower eyelids and examination with the globe in many positions of gaze. Look in particular for the following:

•Change in color: hyperemia, anemia, icterus, melanosis

•Chemosis: conjunctival edema

•Surface irregularities, thickening, or masses

•Inadequate or excessive surface moistness or discharge

•Subconjunctival hemorrhage or emphysema

Nasolacrimal Apparatus

The only components of the nasolacrimal apparatus visible during the external eye examination are the ventral and dorsal puncta in the palpebral conjunctiva near the medial canthus. However, pathology in any part of the nasolacrimal drainage apparatus can produce ocular and periocular signs. Look in particular for the following:

•Ocular discharge (epiphora, mucoid, purulent, sanguineous, or a combination of these)

•Tear staining at the medial canthus

•Negative fluorescein passage (Jones) test result (see later section on vital dyes)

•Occlusion or absence of one or both puncta: atresia, fibrosis/ cicatrization, canalicular foreign body (typically a grass awn)

•Abscess, swelling, or purulent dermatitis near the medial canthus (dacryocystitis)

Cornea

The normal cornea is transparent owing to a number of anatomic and physiologic features. As a result, pathology within the cornea manifests as opacity, often of a color and pattern highly suggestive of the pathologic process (see Chapter 10). Look in particular for the following:

•Loss of transparency: fibrosis, edema, melanosis, vascularization, cellular infiltrate, lipid or mineral accumulation, keratic precipitates

•Changes in contour: keratoconus, keratoglobus, globe rupture, corneal ulcer

•Surface irregularities or dullness: corneal mass/plaque, corneal ulcer/facet, keratoconjunctivitis sicca, iris prolapse/staphyloma

•Change in corneal diameter: buphthalmos, microphthalmos, phthisis

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:

•Changes in thickness: thinning with or without staphyloma or diffuse thickening with scleritis

•Surface irregularities: nodular granulomatous episcleritis, neoplasia, staphyloma, globe rupture

•Alteration in scleral “show”: increased due to exophthalmos, phthisis, microphthalmos, tetanus, macropalpebral fissure; decreased due to symblepharon, ptosis, blepharospasm

•Change in contour: due to globe rupture (often at or near the limbus)

•Change in color: episcleral injection, hemorrhage, icterus, melanosis, melanocytic tumor

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.

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,

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.

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

Iris and Pupil

The iris and pupil are assessed together because alterations in one often produce changes in the other. Both should be examined before and after pupil dilation. Obviously the iris face is best examined before dilation, but abnormalities of the posterior iris (and ciliary body) are sometimes not visible until full pupil dilation is achieved. Horses and ruminants normally have cystic excrescences of posterior iris epithelium that emerge to varying degrees through the pupil, especially dorsally and ventrally, as the corpora nigra or granula iridica. This is very highly developed in the camelids, in which it forms a series of interdigitating pleats. Look in particular for the following:

•Altered pupil shape (dyscoria) or position (corectopia): synechia, iris atrophy, iris hypoplasia, iris coloboma

•More than one aperture in the iris: iris coloboma, persistent pupillary membranes, iris atrophy, iris hypoplasia

•Iridal masses: iris cysts, neoplasia, abscess/granuloma

•Altered iris color: heterochromia iridis, rubeosis iridis, edema, melanosis, melanocytic neoplasia, iris granuloma/ abscess, chronic or acute uveitis

•Altered pupil size: uveitis, glaucoma, Horner’s syndrome, iris atrophy, retinal or optic nerve disease, central nervous system (CNS) disease, CN III paralysis, drug administration, lens dislocation

•Iridodonesis (fluttering of the iris): surgical aphakia or lens dislocation

•Altered pupil color: cataract, nuclear sclerosis, vitreous hemorrhage, retinal detachment, asteroid hyalosis

Lens

Examination of the lens, like that of other clear ocular structures, can be confusing. Here, too, the Sanson-Purkinje images are useful (see earlier). The examiner can use parallax (see Figure 5-10) or the slit beam of the direct ophthalmoscope and a source of manification to determine position of opacities within the lens. Lens pathology is relatively limited, with altered clarity (nuclear sclerosis or cataract) and dislocation (luxation or subluxation) predominating. Look in particular for the following:

•Altered size: microphakia, hypermature cataract, intumescent cataract

•Altered shape: spherophakia, lenticonus, lentiglobus, hypermature cataract, intumescent cataract, lens capsule rupture

•Altered position: luxation, subluxation, aphakic crescent

•Lens opacity: cataract, nuclear sclerosis, anterior lens capsule melanosis, intralenticular hemorrhage, persistent hyaloid artery, persistent hyperplastic primary vitreous, persistent tunica vasculosa lentis

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

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:

•The direct ophthalmoscope

•The indirect lens

•The monocular indirect ophthalmoscope

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.

To avoid interference between the examiner’s and patient’s noses, the observer should use the left eye to examine the patient’s left eye and the right eye to examine the patient’s right

BASIC DIAGNOSTIC TECHNIQUES 89

Ophthalmoscope

Light bulb

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.)

D

B

A

C

FIGURE 5-15. The direct ophthalmoscope. Controls for light intensity (A), focusing lenses (B), light aperture size and shape (C), and filters (D).

FIGURE 5-16. The direct ophthalmoscope being used to examine a horse’s fundus.

eye. This is less important with laterally placed eyes such as in horses (Figure 5-16). Ideally, the examiner’s opposite eye is left open. With the ophthalmoscope set on 0 D and so that the largest circle of light is emitted, the ophthalmoscope is rested firmly against the examiner’s brow and the patient’s eye is viewed in a darkened room from approximately 25 cm away. The tapetal reflection is located, and then the observer moves

in to within 2 to 3 cm of the patient’s eye. If necessary, adjustment may be made to the lens settings to bring the fundus into focus. The fundus is then searched in quadrants, with the optic nerve head used as a reference point. The direct ophthalmoscope is analogous to the high-power lens of a microscope and provides an upright image magnified 15 to 17µ, varying somewhat with the size of the patient’s eye. Therefore thorough examination of the fundus by direct ophthalmoscopy is at best time-consuming and often impossible because the field examined is so small and the patient’s eye is constantly moving.

Indirect Ophthalmoscopy

With indirect ophthalmoscopy, a convex lens (typically 20 to 30 D) is placed between the observer’s eye and the patient’s eye and an inverted virtual image is formed between the lens and observer (Figure 5-17). The magnification and field of view depend on the dioptric power of the lens and on the size of the patient’s eye. However, with the lenses typically used in veterinary medicine, the magnification is always less and the field of view greater than that achieved with use of a direct ophthalmoscope. Therefore indirect ophthalmoscopy allows examination of a greater percentage of the fundus with each field and is faster and more thorough than direct ophthalmoscopy. The ability to compare all regions of the fundus present in one field of view is another important advantage. With the binocular indirect ophthalmoscope (Figure 5-18), a headmounted light source between the examiner’s eyes permits both eyes to be used for the examination and creates depth perception to better interpret raised and depressed lesions within the fundus. This technique also leaves both hands of the examiner free; one hand can then be used to position the patient’s head and eyelids at arm’s length from the examiner, and the other to position the lens and further control the patient’s eyelids (Figure 5-19).

Examiner

Patient

FIGURE 5-17. Indirect ophthalmoscopy. (Modified from Vaughan D, Asbury T [1983]: General Ophthalmology, 10th ed. Lange Medical Publications, Los Altos, CA.)

FIGURE 5-19. The binocular indirect ophthalmoscope and a 2.2 panretinal lens are ideal for funduscopic examination of most domestic veterinary patients.

Monocular Indirect Ophthalmoscopy

A number of new monocular indirect ophthalmoscopes that fit onto the standard battery handset used for the direct ophthalmoscope have become available (Figure 5-20). Monocular indirect ophthalmoscopy has features intermediate between those of direct ophthalmoscopy and the indirect lens. It produces an upright image of moderate magnification and moderate field of view. The instrument can be used with one hand and is relatively easy for the beginner and infrequent user to master. However, because the observer uses only one eye, there is little depth perception. Views of a canine fundus that would be obtained using the three methods of ophthalmoscopy are shown in Figure 5-21.

NORMAL FUNDUS

The fundus of each domestic species has a characteristic but highly variable appearance, which must be learned through regular practice with a reliable ophthalmoscope. The interpretation of fundic lesions is probably one of the most difficult components of the ocular examination and is a common and justifiable reason for referral to a veterinary ophthalmologist. The following structures are found in the fundus of domestic species (Figure 5-22):

•Tapetum: A highly reflective structure in the dorsal portion of the fundus. The pig, bird, and camelids have no tapetum, and absence of tapetum in other species usually having one

Temporal Nasal

GENERALIZED FUNDUS

FIGURE 5-22. Structures in the normal fundus. For this diagram, species differences have been ignored. For individual and species differences, see Figures 5-23 through 5-29.

is considered a normal variation. When present the tapetum typically occupies the dorsal half or less of the fundus. The nonreflective ventral portion, termed the nontapetal area, may be variably melanotic with choroidal vessels evident if melanin is sufficiently lacking. Tapetal development continues postnatally. In young dogs and cats the fundus is grayish soon after the eyes open (7 to 10 days). With tapetal development the dorsal fundus progresses through lilac and successively lighter blues to the adult color and reflectivity by approximately 4 months of age.

•Optic disc, optic nerve head, or optic papilla: The region of the optic nerve where axons that form the nerve fiber layer of the inner retina turn through about 90 degrees to exit the eye and orbit as the optic nerve. It is ventrolateral to the posterior pole of the globe. The degree of myelination varies among species and even individuals of the same species (but minimally between eyes of the same individual). As for the tapetum, postnatal development

of the optic papilla occurs and myelination can continue in dogs until approximately 4 months of age. When

present, myelin produces some irregularities in the border (and therefore the shape) of the optic disc. Also, a small depression of variable size is seen in the middle of the optic disc in myelinated nerves and is known as the physiologic cup.

•Retina: The funduscopically important parts of the retina are the retinal vessels, the retinal pigment epithelium (RPE), and the neurosensory retina. The retinal vascular pattern varies among species. When present, retinal arterioles and venules can be seen emanating from the optic disc. The RPE is perhaps poorly named because it is not always pigmented (melanotic). When the tapetum (which it overlies) is present, the RPE is typically nonpigmented to permit light to pass through it and reach the tapetum. In nontapetal animals and in those nontapetal regions of tapetal fundi, the RPE may be more melanotic and may obscure the view of the choroid behind it. In some subalbinotic animals the melanin content of the RPE is limited and a clear view of the choroid is possible (Figure 5-23). The neurosensory retina itself is translucent (rather like wax paper) and, as

FIGURE 5-23. Subalbinotic fundus of a dog. The choroidal vessels can be seen easily because of sparse melanin within the choroid and retinal pigment epithelium.

such, is not seen directly. Rather, its major effect on funduscopic appearance is that it reduces the tapetal reflection dorsally and makes the ventral nontapetal area seem slightly more gray than black. This effect is usually not appreciated until retinal thickness is reduced in various forms of retinal degeneration. The effect consists of tapetal hyperreflectivity, a more mosaic pattern to the nontapetal fundus, and retinal vascular attenuation.

•Choroid: The choroid is a highly vascular, variably melanotic structure. Unlike the retinal vessels, which are relatively fine, dark red, branching vessels, choroidal vessels appear as broad, orange to pink, spokelike

vessels emanating from the optic disc. The choroid is seen when there is no overlying tapetum and when the overlying RPE is not highly melanotic. This most commonly

occurs in the ventral fundus of subalbinotic animals (see Figure 5-23).

Dog

Typically, tapetal and nontapetal areas are present in the canine fundus (Figure 5-24), although the tapetum can be normally absent in dogs. Tapetal color is usually gold, bluish green, or orange-brown. The tapetum has a fine beaded or granular appearance. The junction between tapetal and nontapetal areas is often irregular. The nontapetal fundus is deep brown to black and relatively homogeneous to slightly mottled. The choroid is normally visible in the nontapetal fundus of subalbinotic, lightly colored, or merle animals (see Figure 5-23).

The optic disc usually lies near the tapetal-nontapetal junction (depending on the size of the tapetum). When it lies in the tapetal region, a small hyperreflective ring immediately adjacent to the margin of the disc is normal and is called peripapillary conus. The disc surface tends to be white to pink, owing to admixing of myelin and numerous small capillaries. The physiologic cup is a small gray depression in the center of the optic disc. The retinal vascular pattern is holangiotic, meaning that vessels should extend from the optic nerve head to the periphery of the retina. The major retinal venules typically anastomose on the surface of the disc. The anastomoses may appear complete or incomplete, depending on the degree of myelination covering the vessels. These venules penetrate the

lamina cribrosa with the nerve fibers but soon leave the nerve to enter the orbit. Retinal arterioles (about 20) emerge from the outer portions and margin of the disc and are considerably smaller than the venules. There typically are three major retinal venules—superior, ventromedial, and ventrolateral—although additional veins and variation in orientation are common. Over the nontapetal fundus, the retinal vessels may normally exhibit a grayish silver sheen or “reflex” in the center of the vessel, which is not seen over the tapetum.

Cat

The feline fundus tends to be more uniform in appearance than the canine fundus (Figure 5-25). A large, highly reflective, relatively uniform tapetal region of gold or green is usually present. The nontapetal region tends to be highly melanotic but can be amelanotic in subalbinotic animals, especially oriental breeds. The optic disc is smaller and more circular and does not have an obvious physiologic cup because of the lack of myelin. The optic nerve head is usually present in the tapetal area. Like the dog, the cat has a holangiotic retinal vascular pattern. However, the retinal vessels emanate from the edge of the optic disc, and there is no venous circle on the optic nerve head. As with the dog, there are generally three arterioles—superior, ventromedial, and ventrolateral—which are ciliary in origin. The area centralis is a region of high cone density for enhanced visual acuity. It is superior and temporal to the optic disc, within the tapetal fundus, and is usually visible as an oval to elliptical area devoid of large blood vessels and with a slightly granular appearance.

Horse

The equine fundus retina (Figure 5-26) differs greatly in appearance from the canine or feline fundus. Most horses have a tapetum with the junction between tapetal and nontapetal areas being relatively uniform. The tapetum varies in color from bluish purple to green and yellow. As with other

FIGURE 5-24. Typical canine fundus. Note tapetum, holangiotic retinal vascular pattern, with anastomotic ring on the optic nerve head, melanotic retinal pigment epithelium and choroid, and myelinated optic nerve head.

FIGURE 5-25. Typical feline fundus. Note tapetum, holangiotic retinal vascular pattern, with vessels extending only to the periphery of the optic nerve head, melanotic retinal pigment epithelium and choroid, and nonmyelinated optic nerve head.

FIGURE 5-26. Typical equine fundus. Note tapetum, paurangiotic retinal vascular pattern, melanotic retinal pigment epithelium and choroid, and oval, salmon pink, nonmyelinated, optic nerve head. This horse has the diffuse color dilute region seen dorsal to the optic nerve head in a number of normal horses.

herbivores, the horse’s tapetum tends to be less reflective than that of the carnivores with the stars of Winslow being far more readily appreciated. These are small, uniformly scattered red or dark pink dots and lines representing end-on views of capillaries of the choriocapillaris traversing the tapetum to supply the outer retina. The nontapetal area is typically brown and relatively homogeneous. Absence of the tapetum or amelanotic nontapetal area allows the choroidal circulation to be seen but occurs relatively infrequently in horses compared with dogs and cats. However, a common normal variant is a relative reduction in tapetal coloration dorsal to the disc with variable exposure of the choroidal circulation (see Figure 5-26). The optic disc lies in the nontapetal fundus, is a horizontal oval, and has 30 to 60 short vessels supplying the surrounding retina. This is a paurangiotic vascular pattern with the arterioles and venules indistinguishable from each other. The lamina cribrosa is often visible within the optic nerve head.

Sheep, Goats, and Cattle

The fundi of sheep, goats, and cattle are similar. They are typically tapetal with a heavily myelinated optic nerve head. The optic nerve head varies in shape among these three domesticated ruminant species. In cattle it tends to be horizontally ovoid (Figure 5-27), in sheep it is kidney shaped (Figure 5-28), and in goats it forms an irregular circle (Figure 5-29). The nontapetal area is homogeneous brown/black except in albinotic and subalbinotic animals, in which it may lack melanin, revealing the subjacent choroidal vasculature. The retinal vascular pattern is holangiotic with three or four major venules and arterioles radiating from the optic disc. The retinal vessels are of large diameter and bulge (with the inner retina) into the vitreous. The dorsal retinal venule and arteriole often intertwine, and tributaries of the dorsal vessels often appear like the hanging branches of a tree. Remnants of the hyaloid vascular system may persist centrally on the optic nerve head as Bergmeister’s papilla.

FIGURE 5-27. Typical bovine fundus. Note tapetum, holangiotic retinal vascular pattern, with large retinal vessels, melanotic retinal pigment epithelium and choroid, and horizontally ovoid optic nerve head.

FIGURE 5-28. Typical ovine fundus. Note tapetum, holangiotic retinal vascular pattern, with large, intertwined retinal vessels, melanotic retinal pigment epithelium and choroid, and kidney bean–shaped optic nerve head.

EXAMINATION OF THE POSTERIOR SEGMENT

Vitreous

The anterior vitreous can be partially examined along with the anterior segment after pupil dilation and if a bright light source is used. However, examination of the vitreous is completed with ophthalmoscopy. As for the aqueous humor, the vitreous should be transparent and should effectively go unnoticed during the examination if it is normal. Therefore look in particular for the following:

•Opacities within the vitreous: persistent hyaloid artery or its remnants (perfused or nonperfused), asteroid hyalosis, synchysis scintillans, inflammatory exudates, vitreous hemorrhage, traction bands, obvious vitreous plicae

FIGURE 5-29. Typical caprine fundus. Note tapetum, holangiotic retinal vascular pattern, melanotic retinal pigment epithelium and choroid, and roughly circular optic nerve head. (Courtesy University of California, Davis, Veterinary Ophthalmology Service Collection.)

•Swirling of the vitreous with globe movement: liquefaction of the vitreous (syneresis)

•Retinal detachment/dialysis/tears

Retina

Retinal disease is usually best observed as changes in the appearance of the underlying tapetum, choroid, or RPE. Look in particular for the following:

•Changes in color: inflammatory cells, edema, melanin, hemorrhage, fibrosis (gliosis), retinal folds, neoplasia, and lipid accumulation, all of which obscure the view of the tapetum

•Changes in tapetal reflectivity: hyporeflectivity (typically active and acute processes) or hyperreflectivity (indicative of more chronic and inactive changes)

•Difficulty focusing on all of the retina at once: vitreous debris, retinal edema, retinal detachment, scleral coloboma/ectasia

•Changes in the vascular appearance: retinal vessels may appear attenuated (retinal degeneration, anemia) or enlarged/tortuous (systemic hypertension, chorioretinitis, vasculitis, collie eye anomaly)

Optic Nerve

Interpretation of optic nerve head changes requires an appreciation of the normal species-related variations as well as some individual variations. Look in particular for the following:

•Increased size or prominence: optic neuritis, papilledema, excessive myelination

•Decreased size or prominence: coloboma, optic nerve hypoplasia, micropapilla, optic nerve atrophy, glaucomatous cupping

•Vascular changes: hemorrhages, anemia, engorgement, Bergmeister’s papilla

ADDITIONAL DIAGNOSTIC TESTING

Following a through examination of the anterior and posterior segments, additional testing should be chosen on the basis of presenting complaint, history, signalment, examination findings, and disease suspicion. Discussion of such tests forms the remainder of this chapter.

Aqueous Flare

Aqueous flare (sometimes just called “flare”) is a pathognomonic sign of anterior uveitis due to breakdown of the bloodocular barrier with subsequent leakage of plasma proteins (often along with cells) into the aqueous humor within the anterior chamber. Aqueous flare is best detected with the use of magnification and a very focal, intense light source in a totally darkened room after pupil dilation. The path taken by the beam of light is viewed from an angle. In the normal eye, a focal reflection is seen where the light strikes the cornea. The beam is then invisible as it traverses the almost proteinand cell-free aqueous humor in the anterior chamber. The light beam becomes visible again as a focal reflection on the anterior lens capsule and then as a diffuse beam through the body of the normal lens because of the presence of lens proteins (Figure 5-30, A). If uveitis has allowed leakage of serum proteins from the iris or ciliary body into the aqueous humor, the light will scatter as it passes through the anterior chamber. Aqueous flare is therefore detected when a beam of light is visible traversing the anterior chamber and joining the focal reflections on the corneal surface and the anterior lens capsule (Figure 5-30, B).

A slit-lamp biomicroscope provides ideal conditions for detecting flare (Figure 5-31); however, the beam produced by the smallest circular aperture on the direct ophthalmoscope held as closely as possible to the cornea in a completely darkened room and viewed transversely with some source of magnification also has excellent results (Figure 5-32). The smallest circle of light is preferred over the slit beam on the direct ophthalmoscope because the slit beam is not as intense and does not provide as many “edges” of light where flare can be

A

B

FIGURE 5-30. Detection of aqueous flare. A, In the normal eye a focal light beam is visible only where it traverses the cornea and lens but not within the anterior chamber. B, With leakage of serum proteins from the iris or ciliary body into the aqueous humor (anterior uveitis), the beam of light is also visible traversing the anterior chamber.

FIGURE 5-31. Photograph of a dog with aqueous flare. Note that the beam of light is visible within the anterior chamber behind the cornea.

FIGURE 5-32. The direct ophthalmoscope can be used to assess aqueous flare. It is turned to the small circle of light, held very close to the patient’s cornea, and the light beam is viewed at a transverse angle in a darkened room using a source of magnification.

appreciated more easily. Assessment of flare may be easier after complete pupil dilation because the dark space created by the pupil provides an excellent “backdrop” against which the light beam can be seen traversing the anterior chamber. If flare is seen, then anterior uveitis can be diagnosed with certainty. However, it is important to note that not all eyes with uveitis have flare. The absence of this sign should not be used to eliminate the diagnosis of uveitis, and other signs of uveitis (low intraocular pressure, miosis, rubeosis iridis, keratic precipitates, etc.) should be searched for.

All red, inflamed, or painful eyes should undergo assessment for aqueous flare to facilitate the diagnosis of uveitis.

Tonometry

Tonometry is the measure of intraocular pressure (IOP) and is perhaps one of the most important but underused diagnostic tests in veterinary medicine. For too long, tonometry has been taught as simply a method of diagnosing glaucoma (in which IOP is raised). However, it is far more than this. It is also a method of diagnosing anterior uveitis (where IOP is typically reduced) and of confirming the diagnosis of all other causes of

BASIC DIAGNOSTIC TECHNIQUES 95

reddened eye, such as keratitis, conjunctivitis, scleritis, and orbital cellulitis (in which IOP should be unaffected). Following confirmation of uveitis or glaucoma, tonometry should be an essential and perhaps the most important method of monitoring response to therapy and judging the tapering or augmentation of therapy. The ready availability of the Tono- Pen—a reasonably priced, user-friendly, applanation tonometer—has made tonometry an achievable goal in all private practices and is the basis of this section. However, other methods of tonometry are also described.

Cannulation measures IOP directly; it is used experimentally but not clinically. Other methods employ measurements of corneoscleral tension to estimate IOP. Digital palpation of the globe through closed lids provides an extremely unreliable, nonreproducible measure of IOP and will fail to identify animals in which IOP is within a range for which treatment is likely to be successful. Dependence on this method leads to inaccurate diagnosis and inappropriate ocular therapy, with unacceptable consequences such as blindness or ongoing ocular pain. Therefore a method of IOP measurement is an essential part of a thorough ophthalmic evaluation.

Indentation Tonometry

The Schiøtz tonometer relies upon indentation tonometry. In this method, a standard force is applied with a metal rod to the anesthetized cornea. The distance the rod indents the cornea is measured and is inversely related to the IOP (i.e., the greater the tonometer scale reading, the lower the patient’s IOP). This method is easily understood if one regards the eye as analogous to a water-filled balloon. If the blunt end of a pencil is applied to the balloon with a given force (e.g., the weight of the pencil placed vertically), the pencil indents the surface of the balloon by a certain distance. If the pressure in the balloon is decreased (some of the water is let out), the tension in the rubber wall decreases, and the same pencil resting on the balloon indents it farther. Conversely, if the pressure in the balloon is increased, the same pencil indents it less. The Schiøtz tonometer (Figure 5-33) consists of three parts: the plunger (analogous to the pencil), the footplate assembly (a device to measure indentation), and the handle. A further refinement is added: The weight applied to the eye through the rod may be varied by adding or subtracting weights (5.5 g, 7.5 g, 10.0 g, or 15.0 g). The greater

FIGURE 5-33. The Schiøtz tonometer.

5-34. The Schiøtz tonometer in use on a cat. Note that the tonometer must be held vertically, making its use awkward or impossible in many veterinary patients.

the weight applied to the eye at a given IOP, the greater the penetration of the rod.

Before use of the Schiøtz tonometer, a drop of local anesthetic is placed on the surface of the eye. The patient is then restrained and the lids are carefully retracted without placing pressure on the globe. Since the indentation tonometer relies on gravity for an accurate reading, it must be placed vertically upon the cornea (Figure 5-34). This requires that the head be elevated so that the corneal surface is horizontal. The tonometer is placed on the eye, either with no weight in place (the plunger weighs 5.5 g) or with the 7.5-g weight in place. The scale reading is recorded. The calibration table supplied with the instrument is then used to convert the scale reading to the IOP via the appropriate column, selected according to the weight used. Human calibration tables supplied with the instrument are more accurate than veterinary versions. The data are recorded as scale reading/weight/ pressure (e.g., 5.0 units/5.5 g/21 mm Hg).

Because of difficulties in head positioning and patient cooperation, Schiøtz tonometry is difficult in many small animals and impossible for large domestic animals. Also, if, under the influence of the Schiøtz plunger, the sclera and cornea stretch because of the slight increase in IOP caused by the indentation, a higher scale reading and a lower IOP are measured, because the ocular rigidity has changed. This ocular rigidity varies considerably in dogs. A quick check to determine the influence of ocular rigidity on the accuracy of the readings may be performed by measuring the pressure with two different tonometer weights (using the lighter weight first). If the pressures obtained are similar, the influence of ocular rigidity is small. If the pressure obtained with the heavier weight is considerably less, the ocular rigidity is low and the walls of the globe are stretching. The plunger assembly of the Schiøtz tonometer must be kept clean because mucus and salts from tears can dry on the plunger rod surfaces, prevent free movement of the plunger, and cause inaccurate results. The instrument may be sterilized with ethylene oxide to prevent transfer of pathogenic microorganisms between patients.

Owing to ease of use, the Tono-Pen has largely replaced the Schiøtz tonometer in progressive practices.

Applanation Tonometry

Unlike indentation tonometry, applanation tonometry using the Tono-Pen is suitable for large animals, is unaffected by variations in ocular rigidity, requires no conversion tables, and needs no sterilization. The principle of applanation tonometry is that the force required to flatten a given area of a sphere is equal to the pressure within the sphere (the Imbert-Fick law). Therefore if the area is known (the size of the footplate) and the force is measured, the pressure can be calculated. Although there are numerous types of applanation tonometers, the most commonly used in general and specialty veterinary practice is the Tono-Pen. This instrument has changed design somewhat over the years but the basic utility of the instrument is unchanged (Figure 5-35).

The advantages of the Tono-Pen include the following:

•It is accurate and easy to use.

•The animal’s head does not need to be held vertically, although the probe must applanate the corneal surface at right angles.

•Errors induced by different sizes and curvatures of corneas in different species are less important.

•Because of the small instrument head, irregular or diseased corneal areas may be avoided, and accurate readings obtained from even small corneas of exotic species.

•The probe tip is covered with a disposable latex cap, which is changed between uses and prevents transfer of infections.

•Minimal restraint is required.

•The pressure is displayed in mm Hg without need for conversion via tables.

Before use, a drop of topical anesthetic is applied to the cornea. The patient is minimally restrained by an assistant so as not to artificially raise IOP via direct pressure on the jugular veins, and the operator carefully retracts the eyelids using the nondominant hand. Care must be taken not to place pressure on the globe. The dominant hand holding the Tono-Pen should then be stabilized against the hand holding the eyelids, and the central cornea gently touched with the instrument’s tip using multiple, very light “blotting” movements. Particular attention

5-35. A series of Tono-Pens showing the evolution of the instrument through the years. Despite these changes, the utility of this instrument is unchanged.