Chapter 11 Retina, Choroid, Sclera
The retina is a multicellular layer 100 to 250μm thick that lines the posterior segment of the eye. It is sandwiched between the vitreous and retinal pigment epithelium (RPE) and choroid, from which its outer layers receive nutrition. The normal retina is almost transparent; the cellular arrangement is such that light can pass through the retina with limited impediment to reach the photoreceptors (rods and cones) that absorb light and convert it into electrochemical energy. The photoreceptors are adjacent to the RPE. Some light is absorbed by the retinal layers, however, which becomes evident when the retina undergoes degeneration and becomes thinner. In dogs and cats some of the light incident on the retina is absorbed as it passes through the photoreceptor layer. The light then hits the tapetum cellulosum in the choroid, which acts as a mirror and reflects the light back via the retina, where more is absorbed to maximize the light-gathering power of the eye. For this to be possible, little light must be absorbed in the intervening RPE cell layer, and it is for this reason that the RPE is largely nonpigmented over the area of the tapetal fundus. The tapetal fundus occupies the upper hemisphere of the fundus. The nontapetal fundus occupies the lower hemisphere of the posterior segment and also surrounds the periphery of the tapetal fundus superiorly, nasally, and temporally.
Figure 11-97 Rod-cone dysplasia type 1 in a 14-month-old Irish setter—the same dog as shown in Figures 11-98 and 11-99. This photograph shows the peripheral tapetum and marked tapetal hyperreflectivity and vessel attenuation.
Figure 11-98 A-B A, Rod-cone dysplasia type 1 in an Irish setter at 4 months of age. Note the juvenile fundus, which ophthalmoscopically appears normal. B, Rod-cone dysplasia type 1 in the same dog at 10 months of age. This photograph is of the same dog as shown in Figures 11-97 and 11-99. Tapetal hyperreflectivity, vessel thinning, and optic atrophy can be seen.
Figure 11-99 Rod-cone dysplasia type 1 in a 14-month-old Irish setter—the same dog as shown in Figures 11-97 and 11-98. Tapetal hyperreflectivity, vessel thinning, optic atrophy, and glial proliferations at edge of the nerve (gliosis) can be seen.
Figure 11-184 Mastiff (2 years old) with retinal dysplasia. Clinically this resembles multifocal retinal detachments with hemorrhage, although the red material settling out in the detached areas is slightly more orange or brown than blood would appear. Similar-looking dysplasias have been described in other giant breeds.
The layered structure of the retina, RPE, choroid, and sclera can be reduced to a schematic to illustrate how in various areas of the normal fundus the ophthalmoscopic view appears as it does. The appearance of and variations in the fundus owe much to the pigmentation in the eye as a whole, which can vary considerably among species and individual animals or even between the two eyes of an animal.
When a dog’s tapetal fundus is viewed ophthalmoscopically, retinal vessels are superimposed on the highly colored tapetum. The only visible components of the retina are the blood vessels. The RPE in this region is nonpigmented to allow light to reach and be reflected by the tapetum. Therefore, the RPE layer is not visible either. The tapetum is visible as the aggregate of numerous tapetal cells that overlap one another to make a layer of variable thickness, depending on the location in the posterior segment, and varies between eyes according to the amount of pigment in the eye because the tapetum consists of modified pigment cells. The tapetum may be gray, violet, blue, green, yellow, or reddish orange. The surface area of the posterior segment covered by the tapetum also may vary, tending to occupy a larger area in larger dog breeds (and including the optic nerve in the tapetal area) and a smaller area (usually more on the temporal area of the fundus) in small breeds. Some dogs have no tapetum at all. The border of the tapetal and nontapetal fundus may be sharply defined (usually more likely in short-coated breeds) or irregular with interlocking of tapetal and nontapetal regions (in longer-coated animals). Dogs of same breeds often have the same tapetal appearance and often color.
Because the tapetum acts as a mirror to reflect light for a second pass through the retina, the underlying choroid and sclera in the tapetal area of the fundus are not usually visible (at least if the tapetum is well developed).
Figure 11-1 This schematic shows a cross-section through a normally pigmented tapetal area of the fundus. The vitreous is not shown but would be above the retina in this figure. RPE overlying the tapetum is not pigmented. Black dots in the choroid represent melanin, because the choroid in these animals is pigmented, although this cannot be seen ophthalmoscopically. The viewer looks down on these layers from above.
Figure 11-11 Normal retina with odd myelination of the optic disc. The dark areas around the disc are areas where the tapetum ends before the optic disc begins; choroidal pigment (conus) is visible here.
Figure 11-12 Normal retina. Tapetal and nontapetal junction is feathered (scalloped); the disc is circular, because little extension of myelination is present along the axons in the retina around the disc.
Figure 11-14 Normal retina. A physiologic cup is visible in the center of the optic nerve. The circular outline of the optic nerve can be seen inside the myelinated area of the nerve head. The neurons at the edge of the nerve are losing myelin (hence the gray tint to the nerve).
Figure 11-19 Normal canine retina with poor myelination in the optic nerve. A pigmented conus is present around the nerve. The arrangement of both arterioles and veins arising toward the edge of the nerve is more typical of the feline fundus. In dogs the veins usually arise more centrally on the optic nerve.
Figure 11-22 Peripheral tapetum in a normal dog. The radiating dark lines appear to correspond to underlying choroidal blood vessels. This should be differentiated from thinning of the peripheral retina in dogs with degenerative retinopathies (e.g., progressive retinal atrophy [PRA]).
Figure 11-26 Normal retina. The tapetum is white with dark spots, which are the end-on view of capillaries passing through the tapetum from the deeper choroidal vessels to reach the choriocapillaris layer beneath the RPE. The nontapetum is subalbinotic.
Figure 11-30 Tapetal hypoplasia. If the pigment is absent in the RPE, as it is in the diagram, an area of thin or absent tapetum reveals choroidal vessels and pigment as islands within the tapetal fundus. Areas of absent tapetum may also be present with pigment in the RPE, which is visible as pigmented spots in the superior fundus or as a greatly enlarged nontapetum and reduced-to-absent tapetum (this is not illustrated).
(Courtesy Dr. E. Dan Wolf.)
Figure 11-37 Normal retina. The tapetum is poorly developed; speckled tapetal cells of green and yellow predominate. Superiorly the tapetal color is somewhat diluted and is highlighted by reflection from the inner limiting membrane of the retina.
Figure 11-47 Normal retina. No tapetum is present. The RPE is pigmented superiorly, so this part of the fundus looks dark brown. The RPE is sparsely pigmented inferiorly where the red glow from the choroidal vessels is visible.
Figure 11-49 Schematic diagram of the nontapetum of a dog. The RPE is pigmented (brown ovals in the cells represent pigment granules). When the fundus is viewed, the choroid and sclera cannot be seen because of the overlying RPE pigment.
(Courtesy Dr. Robert Playter.)
Figure 11-57 In dogs with a light (color dilute) coat color (e.g., chocolate Labrador retriever) the iris is a light brown, as in this German short-haired pointer, and fundus pigmentation can be expected to be correspondingly reduced.
Figure 11-59 Normal fundus in a liver-colored German shorthaired pointer. The RPE in the nontapetum is less pigmented than that of non–color-dilute dogs and looks reddish brown because of the underlying choroidal vessels and the bright reflection from the flash.
Figure 11-62 Red fundus. This phenomenon appears in some breeds with a poorly developed or absent tapetum but with some pigment in the RPE. The red appearance is assumed to be an optical effect of viewing red blood in the choroidal vessels through a thin layer of pigment.
Figure 11-69 Tigroid nontapetal fundus, so called because of the tiger-striped appearance of the choroidal vessels and choroidal pigment. The pigment is reduced or absent in the RPE, which allows the viewer to see retinal vessels superimposed on a background of choroidal vessels and pigment. The sclera is not visible because the choroidal pigment absorbs any light incident on it.
(Courtesy Dr. E. Dan Wolf.)
Figure 11-74 Subalbinotic fundus. Pigment is reduced or absent in the RPE and choroid. The resulting funduscopic appearance is of retinal vessels superimposed on choroidal vessels, all superimposed on the white background of the sclera.
As in the dog, the ophthalmoscopic view of the tapetal fundus of the cat shows retinal vessels superimposed on the highly colored tapetum. The only visible components of the retina are the blood vessels. The RPE in this region is nonpigmented to allow light to reach and be reflected by the tapetum. Therefore the RPE layer is not visible either. Compared with dogs, cats have fewer variations in the tapetum. Usually the tapetal color is yellowish, and the optic nerve is usually in the tapetum. The nontapetum in cats is usually dark brown, with retinal vessels superimposed. Blue-eyed white cats usually have no tapetum and have a subalbinotic fundus, similar to blue-eyed dogs. Blue-eyed cats with Siamese point coloring (e.g., Siamese, Himalayan) usually have a normal tapetum, but the nontapetum is subalbinotic.