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

15-3. Function of the tapetum. Three incoming photons are shown. Two are absorbed by the photoreceptors and contribute to a visual sensation, and the third passes through the retina without being absorbed. In the nontapetal fundus (right) this photon’s energy dissipates in the pigment epithelium and is therefore wasted. In the tapetal fundus (left) the photon is reflected back onto the photoreceptors. In this case it is absorbed and contributes to vision, thus increasing sensitivity to low levels of light. Because the photon is eventually absorbed by a photoreceptor that is not in its original trajectory, the resulting image is blurred. This blurring affects the acuity of daytime vision but has less impact at night, when the cones are not active. Note that the overlying retinal pigment epithelium is pigmented where there is no tapetum and nonpigmented over the tapetum.

FIGURE 15-4. Retinal pigment epithelium (RPE, blue) phagocytosis of photoreceptor outer segments (brown). The phagosome, containing the ingested material consisting of bleached photopigment, enters the RPE cytoplasm, where it merges with lysosomes to facilitate digestion of the outdated membranes. (Adapted from Steinberg RH, et al. [1977]: Pigment epithelial ensheathment and phagocytosis of extrafoveal cones in human retina. Philos Trans R Soc Lond 277:459.)

in the inner plexiform layer between bipolar and ganglion cells, and laterally between horizontal and amacrine cells and bipolar and ganglion cells. These lateral connections between cells coordinate and integrate retinal function.

The ganglion cell layer (layer 8) consists of cell bodies of the ganglion cells. Except in the central retina, the ganglion cell layer is usually one cell thick. Axons of ganglion cells form the nerve fiber layer (layer 9). They run parallel to the retinal surface and converge onto the optic disc. Here they form bundles of nerve fibers that constitute the optic nerve, which exits the eye through the lamina cribrosa (a sievelike opening in the sclera). These ganglion cell axons will reach their first synapse in the lateral geniculate body (although those axons contributing to the pupillary light response (PLR) will synapse in the pretectal nucleus).

The innermost layer, facing the vitreous, is the internal limiting membrane (layer 10). It is a basement membrane to which the inner ends of Müller’s cells are closely attached (see Figure 15-1).

Nuclei of axons in the optic nerve lie in the ganglion cell layer of the retina.

Intuitively, it may seem that the orientation of the retina, with the photoreceptors being the outermost layer facing the choroid and the ganglion cells the innermost layer facing the vitreous, is illogical. Indeed, this anatomic arrangement is called an inverted retina—incoming light must transverse the entire retina to be absorbed by the outer photoreceptors, and the generated signal must again transverse the entire retina to exit the eye through the axons of the inner ganglion cells. The reason for this anatomic arrangement is the high metabolic requirements of the photoreceptors, which necessitate placing these cells next to their “private” blood supply, that is, the choroid (see next section).

Blood Supply

The retina is the most metabolically active tissue in the body, as indicated by its high oxygen consumption. Therefore in most species it has a dual blood supply. The outer retina (i.e., the photoreceptors) is supplied by the choroid, and the inner retina and midretina are supplied by inner retinal vessels, which are usually visible ophthalmoscopically on the inner retinal surface. Arterioles, capillaries, and venules originating in these inner vessels penetrate the retina to supply the midretina. Interruption of either choroidal or retinal vasculature quickly results in ischemia and severe, irreversible loss of function, despite reserves of glycogen within Müller’s cells. (The clinical implication of this feature is that retinal detachment must be treated early to avoid irreversible loss of function.) The blood-retina barrier therefore has two components. The first is the RPE, which separates the retina from the choroid. The second component is formed by the endothelial cells of the inner retinal capillaries and their basement membrane. Both of these barriers limit the passage of substances into the retina. There is little extracellular space in the retina, and transport of solutes from capillaries occurs via Müller’s cells and astrocytes.

In animals, the inner retina is supplied by vessels arising from the short posterior ciliary arteries (which are therefore called cilioretinal arteries) that penetrate the sclera in a circle around the optic disc. A notable exception is in primates, whose retinas are supplied by a single central retinal artery, making them susceptible to ischemia due to occlusion. Retinas of domestic animals are classified according to the pattern of their inner retinal vasculature (Table 15-2). The most common pattern is holangiotic, whereby most of the inner retinal surface is transversed by blood vessels (Figure 15-7). In merangiotic retinas, the vessels extend from the optic disc laterally and medially but other regions of the retina are uncovered (Figure 15-8), whereas in paurangiotic retinas only the area around the optic disc is supplied by short, peripapillary inner retinal vessels (Figure 15-9). In species with paurangiotic supply, such as the horse, the retina therefore depends more on choroidal supply; thus the consequences of interruption to choroidal supply by trauma or anemia are more serious for the equine retina. The avian fundus is characterized by the presence of a pecten—a pigmented vascular structure protruding into the vitreous from the retina (Figure 15-10). The avian retina is usually avascular, and the pecten may have a nutritional role. A similar structure, a conus papillaris, is found in many reptilian and amphibian species (Figure 15-11).

Most mammals, including the dog, cat, cow, sheep, rat, mouse, and primates

Rabbit

Horse, rhinoceros, elephant, marsupials

Most nonmammalians, including birds, reptiles, and amphibians; some mammals, including beaver, chinchilla, porcupine, armadillo, sloth, guinea pig, and bats

FIGURE 15-6. Structure of Müller’s cells. Note how the (dark colored) cell transverses 9 of the 10 retinal layers, thus leading structural support to the entire retina and forming the outer and inner limiting membranes. a, Radial processes; b, honeycomb meshwork; c, horizontal fibers; d, fiber baskets; l., layer; mem., membrane. (From Hogan MJ, et al. [1971]: Histology of the Human Eye. Saunders, Philadelphia.)

PHYSIOLOGY AND BIOCHEMISTRY

Rods and Cones

As mentioned earlier, the outer segments of the rods and cones contain light-sensitive photopigments that absorb the energy of incoming light particles (photons). Because rods and cones have differing functions (Table 15-3), the pigments in each are different, and they also vary with species. Rods are much more sensitive than cones to low levels of light and to small changes in illumination. Therefore they function in dim environments and at night (scotopic vision). Rods are also responsible for processing motion. Cones are less sensitive to small fluctuations in light levels, functioning predominately at high levels of illumination (photopic vision). On the other hand, cones are capable of greater visual discrimination than rods, thus providing for high-resolution vision; in many species, cones also contain pigments for color vision.

Some of the difference in sensitivity between rods and cones is accounted for by retinal summation. For instance, there are approximately 130 million photoreceptors in the human retina but only 1.2 million axons in the optic nerve. This means that

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Table 15-3 Characteristics of Rods and Cones

inevitably some axons have more than one photoreceptor (typically rods) associated with them. By converging large numbers of rods in a particular area onto a single bipolar cell, and by converging several bipolar cells onto a ganglion cell, the rod pathway can amplify the response to low levels of light (Figure 15-12). This is because just a single photon falling anywhere in a large area can activate the regional ganglion cell (see Figure 15-12, left panel). However, this amplification occurs at the expense of fine discrimination—if this regional ganglion cell fires, there is no way of knowing which of its associated rods was hyperpolarized by a photon. Maximum visual discrimination occurs when one photoreceptor is connected to one bipolar cell and to one ganglion cell, which is the typical synaptic cone pathway (see Figure 15-12, right panel). This is because the firing of any given ganglion cell can be triggered only by hyperpolarization of a single, specific cone that is associated with this ganglion cell. However, more photons are required for the activation of this system, making it active only at high intensities of light. The effect of retinal anatomy and function on vision is described in detail in Chapter 1.

The retinas of birds and primates possess a specialized area called the fovea. This region is populated only by cones (and their associated bipolars and ganglion cells) and provides these species with their high resolution and rich color vision. In most (nonprimate) mammalian species, this function is served by a region called the area centralis, which has a relatively high cone concentration. In these species, however, rods outnumber cones even in the area centralis, accounting for the lower visual resolution and greater light sensitivity of most domestic species. In all animals, including those that possess a fovea, the rod-to-cone ratio rises towards the peripheral retina, which is typically characterized by low-resolution, light-sensitive vision (Figure 15-13). Some nocturnal animals have a pure rod retina with no cones, whereas some raptor species have two foveas, giving these birds very high visual resolution.

Visual Photopigments

Molecules that absorb light are termed photopigments. Visual photopigments in the photoreceptors can absorb a range of wavelengths, with each class of photopigment having peak absorption at a particular wavelength. The molecule is also capable of absorbing other wavelengths with decreasing efficiency, thus forming a bell-shaped curve of absorption

versus wavelength, with its peak is known as the absorption

maximum, or Lmax (Figure 15-14).

Visual photopigment molecules consist of two parts, a chromophore (which is a derivative of vitamin A) and a protein, or opsin. The chromophore is the part of the molecule that transduces the energy of the light photon into a chemical reaction,

which generates a neuronal signal, as detailed in the next section (Photochemistry). The opsin is the part of the molecule that determines the wavelength the photopigment will absorb, thus allowing the eye to perceive color in that spectrum. Therefore species possessing trichromatic vision, such as primates, have three cone populations as defined by their respective

FIGURE 15-8. A merangiotic blood supply of the inner retina in an albino rabbit. Only the nasal and temporal retinas are supplied by vessels that can be seen at the optic disc at the 3 o’clock and 9 o’clock positions. The thick red bands elsewhere in the fundus are choroidal blood vessels. (Courtesy University of California, Davis, Veterinary Ophthalmology Service Collection.)

opsins. These three opsin populations enable the cones to absorb light in three primary colors (typically red, green, and blue). The richness of the human color vision, and the number of shades we can see, is made possible by the overlapping absorption curves of these three primary colors. Species with dichromatic vision have two classes of cones (possessing two types of opsins). Contrary to popular belief that animals see in black-and-white, most domestic species, including dogs, horses, and ruminants, are dichromatic, enabling these animals limited color vision consisting of two primary colors and their intermediate shades; cats may even possess trichromatic vision. Tetrachromatic species, such as birds, have a fourth class of cones, with an extra opsin absorbing ultraviolet light and allowing for color vision that is richer than that of humans.

The photopigment that has been studied most extensively is rhodopsin. This photopigment, found in rods, also consists of a chromophore and an opsin. The rod chromophore is a vitamin A1 derivative (11-cis retinal aldehyde). The rod opsin has an absorption maximum of approximately 495 nm (see Figure 1514). Much of what we know of visual photochemistry comes from the study of rhodopsin, although it is assumed that cone photopigments function similarly.

Retina

Choroid

Sclera

Efferent vein

Pecten

Optic nerve

FIGURE 15-9. A paurangiotic blood supply of the inner retina in an 8-year- old Welsh pony. Note that vessels are restricted to the area around the optic disc. (From Rubin LF [1974]: Atlas of Veterinary Ophthalmology. Lea & Febiger, Philadelphia.)

Photochemistry

When a photon of light is absorbed by a rhodopsin molecule, it initiates a chemical process that results in phototransduction of its energy into a neuronal signal: The opsin breaks off the chromophore (i.e., 11-cis retinal aldehyde) and the chromophore is isomerized into the more stable all-trans retinal aldehyde. The isomerization triggers a complex chain reaction involving numerous enzymes. The final step in this cascade is hydrolysis of cyclic guanosine monophosphate (cGMP) into GMP by phosphodiesterase. The resulting decrease in cGMP levels closes sodium channels in the outer segments, leading to hyperpolarization of the photoreceptor; that is, a neuronal signal. (Photoreceptors are exceptional neurons in that they are depolarized in their resting state [at darkness] and are hyperpolarized following excitation [by light].) Mutations in the genes encoding for any of the enzymes involved in this cascade cause inherited retinal degeneration in a number of species, notably dogs and humans (see Inherited Retinopathies).

One of the by-products of the phototransduction process is the isomerized chromophore, all-trans retinal aldehyde. This

FIGURE 15-10. A, Structure of the pecten, showing its relationship to the entrance of the optic nerve and its vascular connections. The supplying artery sends a branch to each fold. The efferent vein receives a branch from each angle of the fold. B, Ophthalmoscopic view of the fundus of a barn owl (Strix flammea). Note that no vessels can be seen on the surface of the retina. (A modified from Duke-Elder S [1958]: System of Ophthalmology, Vol I. Mosby, St. Louis; B courtesy University of California, Davis, Veterinary Ophthalmology Service Collection.)

FIGURE 15-11. A gecko fundus, typical of the anangiotic blood supply of the reptilian retina. No vessels can be seen, and the optic disc is obscured by a conus papillaris. (Courtesy University of California, Davis, Veterinary Ophthalmology Service Collection.)

molecule, representing bleached photopigment, is shed by the photoreceptor and phagocytized by the RPE for “recycling” (see Figure 15-4). In the RPE, the all-trans retinal aldehyde can be isomerized back to the 11-cis retinal aldehyde and re-form rhodopsin (Figure 15-15), or it can be reduced to all-trans retinol and esterified. The esters are stored in the pigment epithelium until required for dark adaptation (see later). After all-trans ester has been deesterified, oxidized, and isomerized, it is available for spontaneous regeneration of rhodopsin in the dark.

Vitamin A in the eye turns over very slowly with other body stores of vitamin A, and only a small proportion of ingested vitamin A reaches the eye to form the chromophore of the visual photopigment. Vitamin A deficiency does not affect the eye until other body stores are depleted. Hypovitaminosis A causes loss of rod function (owing to depletion of the rhodopsin) and, when chronic, leads to complete retinal degeneration and

FIGURE 15-12. Retinal summation. The rod pathway is converging, as large numbers of rods are connected to a bipolar cell (although only two to three converging rods are shown here) and several bipolar cells are connected to a ganglion cell. In the cone pathway there is little or no summation, because one cone synapses to one bipolar cell that synapses with one ganglion cell. This arrangement allows cones faster conduction and high visual resolution at the price of low sensitivity to light levels.

blindness. In young animals it may also cause bone remodeling, leading to stenosis of the optic foramen and blindness due to the resulting optic nerve atrophy.

Vitamin A deficiency causes night blindness (nyctalopia), retinal and optic nerve atrophy, and convulsions in cattle, and microphthalmia and nyctalopia in the offspring of deficient sows.

Dark Adaptation

Dark adaptation is the transition of the retina from the lightadapted (photopic) to the dark-adapted (scotopic) state. Visual

FIGURE 15-13. Comparative anatomy of the central and peripheral retina. The different ratios of rods and cones in these regions result in different relative numbers of bipolar and ganglion cells, leading to varying degrees of summation in the optic nerve fibers and producing characteristic differences in the relative thickness of the retinal layers. (Modified from Duke-Elder S [1958]: System of Ophthalmology, Vol I. Mosby, St. Louis.)

Receptors

(many cones) summated but little in:

Bipolar cells

finally summated but little in:

Ganglion cells

Receptors

(mostly rods) summated extensively in:

Bipolar cells

finally summated extensively in:

Ganglion cells

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80

60

40

20

0

Wavelength in Mm

FIGURE 15-14. Absorption spectrum of rhodopsin (at different light intensities) as a function of wavelength. As can be seen, wavelength sensitivity is not all-or-nothing. Rhodopsin has absorption maximum, or

Lmax, values of 495 and 525 nm (depending on light intensity) but can absorb shorter and longer wavelengths with decreased effectiveness.

(From Moses RA [1970]: Adler’s Physiology of the Eye, 5th ed. Saunders, Philadelphia. Modified from Hecht.)

Rhodopsin

FIGURE 15-15. Rhodopsin (containing II-cis, retinal) is isomerized by light into all-trans retinal. All-trans retinal may be either isomerized to re-form rhodopsin or reduced to all-trans retinol. After esterification it can be stored in the retinal pigment epithelium until needed for dark adaptation. DPN, Diphosphopyridine nucleotide; TPN, triphosphopyridine nucleotide. (From Wald G [1968]: Molecular basis of visual excitation. Science 162:230.)

acuity is greatest in the photopic state, whereas light sensitivity is maximal in the scotopic state. The three physiologic processes contributing to the increased light sensitivity of the retina in darkness are dilatation of the pupil, synaptic adaptation of retinal neurons, and increase in the concentration of rhodopsin available in the outer segments. Together, these three processes may increase the sensitivity of the eye by 5 to 8 log units (i.e., by up to 100 million). Maximal sensitivity is reached after 30 minutes or more in the darkness, depending on the species and light level before adaptation began. In making the transition from light to dark, the brighter the preexisting light level, the longer the eye takes to reach maximal

S

FIGURE 15-16. Increasing amplitude of the b wave (arrow) in response to flashes of light during dark adaptation in a normal dog during electroretinography. Note that as the animal spends more time in the dark (indicated in minutes in the left column), the amplitude of the response increases. S, The timing of the light flash stimulus that elicits the retinal response. (Modified from Aguirre GD, Rubin L [1971]: The early diagnosis of rod dysplasia in the Norwegian elkhound. Am Vet Med Assoc 159:429.)

b

a

s

FIGURE 15-17. Electroretinogram of a normal dark-adapted canine in response to a white light flash, 0.02 second in duration (denoted as s). The downward deflection, the a wave, is composed mostly of photoreceptor activity. It is followed by an upward (positive) deflection, the b wave, representing bipolar and Müller’s cell activity. (From Rubin LF [1974]: Atlas of Veterinary Ophthalmoscopy. Lea & Febiger, Philadelphia.)

sensitivity, presumably because rhodopsin stores are lower after exposure to bright light and have to be reconstituted from stores in the pigment epithelium.

Dark adaptation in domestic animals is measured by (1) increase in amplitude of the electroretinogram (ERG) with time spent in the dark (Figure 15-16) and (2) the ability to detect dimmer lights with time spent in the dark. The latter may be measured electrophysiologically as a decrease in the stimulus intensity required to produce a given ERG amplitude in the dark.

Electroretinography

The ERG is the electrical response recorded when the retina is stimulated by flashes of light (Figure 15-17). Although it is possible to relate different parts of the ERG wave to different structures within the retina (e.g., the a wave to the rods and cones, the b wave to the bipolar and Müller’s cells, and the

15-18. Electroretinography (ERG) performed in 6-month-old American Bulldog using the handheld multispecies ERG device (HMsERG). For the purpose of the recording, the dog is anesthetized, its pupil is dilated, and the lids are retracted with an eyelid speculum. The flash stimulus is delivered by the HMsERG, and the electrical response of the retina to the stimulus is recorded by means of a contact lens electrode placed on the cornea (visible as a red wire). (Courtesy Kristina N. Narfström.)

c wave to the pigment epithelium), such attempts are an oversimplification of a complex process that is incompletely understood. For clinical purposes the ERG is best considered a mass response of the entire outer retina to flashes of light. Therefore the ERG is usually used to assess outer retinal function in animals affected with disorders of the rods and cones. Although electroretinography requires sophisticated equipment (Figure 15-18) and specialized training in its operation and interpretation of results, it is an extremely valuable diagnostic tool for the veterinary ophthalmologist.

Electroretinography is useful in the following circumstances:

•Routine preoperative evaluation of retinal function before cataract surgery: Unfortunately, many dogs may be simultaneously affected with both retinal dystrophy and cataract. Regardless of whether these two diseases are related or independent, it is obvious that cataract surgery will not restore vision if the retina is not functioning. Because the cataract prevents a thorough ophthalmoscopic evaluation of the retina, an ERG is required to determine the prognosis of the surgery. It is important to note that

even in the presence of cataracts (or a corneal opacity) that affect vision, sufficient light reaches the retina to elicit an electrophysiologic response, provided that the retina is functional. This is also the reason why PLR can be elicited in cataractous patients.

•Diagnosis of retinal disorders in which no ophthalmoscopic abnormalities are evident: These include early stages of retinal dysplasia, day blindness (hemeralopia) in Alaskan malamutes and German short-haired pointers, congenital stationary night blindness (CSNB) in dogs and horses, and sudden acquired retinal degeneration (SARD). In all of these diseases, ERG abnormalities may be recorded even though the fundus may seem normal.

•Differentiating between retinal and postretinal causes of blindness: For example, cases of SARD and retrobulbar optic neuritis may manifest similarly as acute loss of vision, a normal-looking fundus, and fixed, dilated pupils. An ERG may be used to differentiate between the two, because the response will be extinguished in SARD (which is a retinal disease) but normal in optic neuritis (which is a postretinal disease).

•Early diagnosis of inherited photoreceptor atrophies: In many dog breeds and in some cat breeds, the ERG may detect changes in retinal function long before ophthalmoscopic or behavioral signs are observed. This early detection is invaluable to breeders wishing to screen their animals for inherited retinal diseases (Figure 15-19).

Flash electroretinography is a summed response of the outer retina. Focal retinal lesions (e.g., scars), or inner retinal disease (e.g., glaucoma) may not affect the flash ERG.

Based on the clinical indication for the ERG, two recording protocols have evolved for performing the test in dogs. The first is the rapid, “yes-no” protocol used to demonstrate retinal function. It is conducted to rule out SARD or to determine whether the patient is a suitable candidate for cataract surgery. For early detection and evaluation of inherited photoreceptor diseases, a more exhaustive recording protocol is required. This protocol involves extensive testing of rod and cone function, based on their different physiologic properties (see Table 15-3). It includes testing the process of dark and light adaptation (see Figure 15-16), responses to dim and bright light (see Figure 15-19), responses to red, blue, and white

500

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1 2

50

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FIGURE 15-20. Flicker fusion responses recorded in the same cats as in Figure 15-19. The signals are recorded in response to light flashes presented at a high frequency. In the normal cat (1), there is a normal signal in response to each flash, whereas in the affected cat (2), the responses are diminished and delayed, because the diseased photoreceptors do not fully recover before subsequent flashes (note that the two recordings appear similar but their scales differ). If the frequency of the flashes is increased further, the normal photoreceptors will not be able to recover fully either. At a threshold frequency, the flashes are so rapid that the photoreceptors do not recover at all, and their responses “fuse.” Rods and cones are characterized by different fusion threshold frequencies. (Courtesy Kristina N. Narfström.)

lights, and flicker fusion frequency (FFF) recorded in response to rapid flashes of light (Figure 15-20). FFF is the frequency of stimulation beyond which individual ERG responses are not recorded, and it depends on whether the rods and cones are functioning under the prevailing levels of illumination. ERG results are typically reported as a- and b-wave amplitudes and implicit times in response to the various stimuli used (Figure 15-21). A summary of electroretinographic alterations in various ocular disorders is given in Table 15-4.

The ERG is a test of retinal function, not of vision. Therefore it may be normal in some cases of blindness. For example, the ERG is normal in cases of postretinal blindness such as optic neuritis or cortical disease, even though the patient is blind.

APPLIED ANATOMY (OPHTHALMOSCOPIC VARIATIONS)

Before pathologic processes can be recognized, common variations in fundus appearance must be appreciated. Detailed fundus diagnosis is the province of the veterinary ophthalmologist, but familiarity with common, normal fundus variations is essential so that the general practitioner can recognize and distinguish them from pathologic processes. Students and clinicians are encouraged to examine the fundus of every patient (including

ERG CALCULATIONS

FIGURE 15-21. Electroretinography parameters. (From Howard DR, et al. [1973]: Clinical electroretinography: a protocol for testing the retina. J Am Anim Hosp Assoc 9:219.)

those presented for nonophthalmic reasons) as part of a comprehensive examination and in order to familiarize themselves with its normal appearance.

Tapetum

The tapetum is a reflective layer located in the choroid. It can be found in many mammalian species (with the notable exceptions of primates, pigs, and rodents) as well as in nocturnal nonmammalian species. Although the structure of the tapetum differs among different species (i.e., it may be fibrous or cellular), its role is similar. The tapetum acts as a mirror that reflects the light back toward the photoreceptor layer, thus increasing the probability that the light will be absorbed by the photopigment and contribute to visual sensation in dim light (see Figure 15-3). Because melanin absorbs light and would prevent it from reaching the tapetum, the RPE overlying the tapetum is nonpigmented, thereby allowing it to fulfill its physiologic role.

Color variations in the tapetum occur in all species. They are most frequent in dogs, where various shades of yellow-orange and green-blue are commonly observed, although other colors may also be seen. In newborn pups the fundus is dark at birth; the tapetal area gradually changes shades into gray and blue before adult colors appear (Figure 15-22).

FIGURE 15-22. Fundus of a 13-week-old Alaskan malamute. The area of the future tapetum is blue at this age. (From Rubin LF [1974]: Atlas of Veterinary Ophthalmoscopy. Lea & Febiger, Philadelphia.)

Ultrasound may be of greater diagnostic value

ERG useful in differentiating retinal and cortical blindness

ERG useful in cases of retrobulbar neuritis in which no ophthalmoscopic lesions can be observed

In early cases of glaucoma there is ganglion cell loss with no change in flash ERG response. In advanced stages the damage spreads to the outer retina, affecting the ERG response. The flash ERG is not diagnostic for glaucoma.

ERG essential for definitive diagnosis of hemeralopia, as fundus looks normal

ERG may be abnormal 10 wks before onset of ophthalmoscopic signs

ERG essential for definitive diagnosis of CSNB, as fundus looks normal

In some breeds (Irish setter, miniature schnauzer) the ERG can be diagnostic before 2 months of age

The ERG can be diagnostic months or years before the onset of behavioral and ophthalmoscopic signs

Pigment is occasionally observed in the normally unpigmented pigment epithelium in the tapetal area and should be differentiated from pathologic pigmentation. The pigmented areas are more common at the tapetal-nontapetal junction (Figure 15-23). The transition between tapetum and nontapetum may be gradual or sharply demarcated.

Absence of the tapetum occurs in all species, although it is most prevalent in subalbinotic and color-dilute eyes. Absence of the tapetum may be total (Figure 15-24) or focal (Figure 15-25). The RPE is often unpigmented in association with tapetal agenesis. In these regions, where there is no tapetum and the RPE is nonpigmented, the underlying wide choroidal vessels are visible through the retina as numerous thick, parallel red stripes, the so-called tigroid fundus (see Figures 15-24, B, and 15-25). Note that the much finer retinal vessels are visible overlying the choroidal vasculature. This type of fundus causes a red funduscopic reflection (through the pupil or ophthalmoscope), and although it is a normal variation, it is sometimes mistaken for hemorrhage.

FIGURE 15-23. Multifocal epithelium overlying the (From Rubin LF [1974]: Febiger, Philadelphia.)

areas of pigmentation of the retinal pigment tapetum in a dog. This is a normal variation. Atlas of Veterinary Ophthalmoscopy. Lea &