Ординатура / Офтальмология / Английские материалы / Veterinary Ocular Pathology A Comparative Review_Dubielzig, Ketring, McLellan_2010

•Species with eyes adapted for vision in dim light generally have a relatively rod-rich retina and, therefore, a relatively thick outer nuclear layer, e.g. consisting of about 20 rows of nuclei in the cat, whereas species adapted for vision in bright light and those adapted for maximum visual acuity, typically have a thinner outer nuclear layer

•This layer is thinner in the periphery of the retina, where photoreceptor density is lowest

•Cone nuclei are slightly larger and lighter staining than rod nuclei and, usually, are positioned adjacent to the outer limiting membrane.

The outer limiting membrane

•The outer limiting membrane is not a physical layer but consists of a continuum of tight junctions connecting the cell membranes of photoreceptor cells and Müller cells

•The tight junctions of the outer limiting membrane along with tight junctions at the apices of the RPE cells are the anatomic features that delineate the sub-retinal space

•The subretinal space is the potential space between the photoreceptor outer segments and the retinal pigment epithelial cells (RPE) and, since it is enclosed by tight junctions, it normally only contains secreted substances, within the inter-photoreceptor matrix. This is an important factor in maintaining organization of the outer retina and preserving the intimate contact between the neural retina and the RPE.

The photoreceptor layer

The photoreceptor layer is composed of:

•Photoreceptor inner segments

■Cytoplasmic component of the photoreceptor cell packed with mitochondria and rough endoplasmic reticulum

■Photoreceptor cells are among the most metabolically active cells in the body, both in terms of oxygen consumption and protein production

■Rod and cone photoreceptors can be distinguished by the shape of their inner segments

■The central retina, superior and temporal to the optic nerve, is a relatively cone-rich zone in many species. Furthermore, many avian, reptile and primate species have foveae, regions that are largely devoid of rods

•Photoreceptor outer segments

■The site of phototransduction

■Both rod and cone outer segments are made up of parallel stacked membranes containing visual pigments that consist of the appropriate opsin proteins charged with photosensitive vitamin A-derivative, retinal

■The photoreceptor outer segments are complex modifications of cilia and maintain some of the structural features of cilia at their junction with the inner segments.

The Retina Chapter 11

•Together, the basement membranes of the RPE and adjacent choriocapillaris form the structure known as ‘Bruch’s membrane’

•The RPE has a number of important roles in maintaining the functional integrity of the overlying neurosensory retina:

■Oxygen and nutrients for the outer retina are transported through the RPE from the choriocapillaris of the choroid

■The RPE serves to phagocytose the outermost portion of the photoreceptor outer segment that is cyclically shed, in a continuous process of outer segment renewal

■‘Spent’ retinol from visual pigment cycles from the outer segment to the RPE and back. The RPE cell essentially regenerates the retinal which is a component of visual pigment, so that it is ready to function in the process of phototransduction again

■The melanin pigment of the RPE absorbs light and therefore reduces intraocular light scatter and glare

■Tight junctions between cells of the RPE help maintain the blood-retinal barrier, and delineate the subretinal space

■The RPE limits photo-oxidative damage that is associated with the high levels of oxygen, light and free-radicals in the outer retina, by a number of antioxidant mechanisms.

Retinal blood vessels (Fig. 11.2i,ii)

•Retinal blood vessels have tight junctions and function as part of the blood-retinal barrier

•Where present, retinal blood vessels are found within the nerve fiber, ganglion cell and inner plexiform layers of the inner retina; capillaries are also present in the inner nuclear layer

•The pattern of retinal blood vessels is highly variable between different species:

■Holangiotic, with blood vessels distributed throughout most of the neurosensory retina, supplying the inner retina only.

–Carnivores, primates, even-toed ungulates, most rodents

■Paurangiotic, with blood vessels limited to the retina immediately adjacent to the optic nerve head and the rest of the retina being avascular

–Horses and other odd-toed ungulates

–Guinea pigs and some rodents

■Merangiotic, with retinal blood vessels confined to a linear horizontal streak on both sides of the optic disc, accompanying streaks of myelinated nerve fibers

–Lagomorphs

■Anangiotic

–Other vertebrate groups have a variety of vascular structures within the vitreous adjacent to the retina, but only mammals have true intra-retinal vasculature

–Birds have no retinal vessels but they have a complex vascular structure, the pecten, which extends into the vitreous from the optic nerve head.

The retinal pigment epithelium (RPE)

•The RPE is a monolayer of cells that is derived from the outer layer of neuroepithelium of the optic cup, and is continuous with the outer pigmented epithelium of the ciliary body at the ora ciliaris retinae

•Apical microvillous processes of each RPE cell invest the outer segments of numerous photoreceptors

•Although typically densely pigmented, RPE cells are nonpigmented in the area that overlies the tapetum (see Ch. 9), losing their melanosomes by a process of autophagocytosis concurrent with tapetal development within the choroid

Comparative Comments

•The normal anatomy of the human retina resembles in most respects that of other vertebrate species

•The human macula, a feature shared with most other primates, represents a refinement of the area centralis of other mammals. A xanthophyll pigment gives the macula a yellow appearance clinically and on gross examination, but the xanthophyll dissolves during tissue processing and is not present in histologic sections. The macula is the portion of the retina that is responsible for the high degree of visual acuity in primates. In its histologic and

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Comparative Comments (continued)

pathologic usage, the term ‘macula’ refers to that portion of the retina having more than a single layer of ganglion cells. At the center of the macula are the fovea and the foveola, which are distinctive features of the primate retina within the Mammalia, but are present in birds and reptiles

•As noted, the pattern of retinal blood vessels is highly variable among different mammalian species. In the human retina, the intrinsic retinal blood vessels nourish the nerve fiber layer, ganglion cell layer, inner plexiform layer, and inner two-thirds of the inner nuclear layer. The choroidal vasculature in turn provides the blood supply to the outer one third of the inner nuclear layer, outer plexiform layer, outer nuclear layer, photoreceptors, and retinal pigment epithelium (RPE). Consequently, ischemia produced by intrinsic retinal vascular lesions produces atrophy of the inner retina, whereas choroidal ischemia produces outer retinal atrophy.

Significance of retinal disease to a mail-in pathology service

Primary retinal diseases, primarily congenital or hereditary retinal diseases, although fairly common, are seldom submitted for evaluation by a pathology service and, for that reason, are under-represented in the COPLOW collection.

•When the eyes of animals with retinal disease are submitted to an ocular pathologist, it is often not until there are secondary complications such as retinal detachment, cataract or intraocular hemorrhage that may subsequently lead to painful ocular disease, by which time the background retinal disease is chronic or otherwise changed by superimposed pathology

•When globes are submitted for evaluation of a disease primarily affecting the retina the likely reasons include the following:

■A request for an opinion regarding a diagnosis of hereditary progressive retinal atrophy in an animal dying of other causes

■A request to evaluate the retina for suspected toxic retinopathy in an animal dying of other causes

■A request for an opinion from a researcher to describe the pathology in an animal with a novel funduscopic appearance.

CONGENITAL AND HEREDITARY DISEASES

Congenital disorders of the retina (see also Ch. 3)

Retinal folds and retinal dysplasia (Figs 11.3–11.6)

•Retinal dysplasia represents the disorganized development of retinal tissue. Lesions may be focal or multifocal curvilinear retinal folds; larger geographic lesions that may be accompanied by localized retinal degeneration and pigmentary changes, or, in its most severe form, total retinal non-attachment with vitreoretinal dysplasia

•There is seldom a clinical indication for enucleation of eyes with retinal folds or dysplasia in isolation. Often, eyes are removed for secondary or other unrelated disease and, after a significant amount of time has elapsed

The Retina Chapter 11

■The challenge is to be able to recognize the features of retinal disease that are due primarily to congenital or hereditary dysplasia and to differentiate these from secondary or superimposed abnormalities, related to retinal detachment or degeneration, glaucoma, trauma, or inflammation, for example

•Acquired retinal folds may be recognized in areas of retinal re-attachment subsequent to detachment, for example associated with sub-retinal effusion or exudates

•Before interpreting pathologic lesions as representative of an underlying breed-related retinal condition, there should be evidence in the form of tissue changes or cellular changes allowing the observer to distinguish a pathologic fold from a purely artifactual wrinkling of the retina. These might include gliosis, vascular changes, or changes in the density of intact retinal elements, such as photoreceptors or Müller cells.

Collie eye anomaly

•This recessively inherited abnormality of the canine fundus has its primary, ophthalmoscopically visible, effects on the RPE and choroid, with affected dogs demonstrating variable degrees of choroidal hypoplasia. A proportion of affected dogs also have colobomatous defects involving the optic nerve head, or peripapillary choroid and sclera

•Retinal detachment is not uncommon in severely affected eyes, and often relates to the presence of large colobomas, through which liquefied vitreous gains access to the subretinal space (see also Chs 3 and 12)

•The underlying genetic mutation, a deletion in the NHEJ1 has been identified and segregates with disease in all affected breeds, and a commercially available, mutation-based genetic test is available.

Congenital stationary night blindness in horses

•There is a breed-predisposition for the Appaloosa, in which the disease appears to be linked to the Leopard complex coat pattern

•Although pronounced behavioral and electrophysiological abnormalities are apparent, no ophthalmoscopic or histopathological abnormalities are detectable in affected animals.

Comparative Comments

•The most serious congenital disease of the retina is retinopathy of prematurity (ROP)

–This disease can be induced in neonatal animals and animal models have been the source of extensive study of this abnormality, but ROP rarely occurs spontaneously in non-human species. However, a condition described previously in cats (see Ch. 3) shares many of the pathologic features of the human disease

–ROP is classically defined as an oxygen-induced vitreoretinal disease of premature infants. ROP occurs bilaterally, almost exclusively in infants with an immature, incomplete retinal vascular system and particularly in premature infants who weigh less that 1.5 kg at birth and who receive oxygen therapy

–The classic theories for pathogenesis were based on kitten, mouse, and puppy models, in which it was found that hyperoxia induces a vaso-obliterative phase with functional

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Comparative Comments (continued)

constriction of the immature retinal blood vessels, followed by structural obliteration. If the hyperoxia is sufficiently prolonged, on cessation of oxygen exposure the peripheral retina remains ischemic. A proliferative phase ensues, with intense fibrovascular proliferation beginning at the junction of the vascular and avascular zones and continuing through the internal limiting membrane into the vitreous

•Other important congenital abnormalities of the retina in humans include albinism, medullated nerve fibers, vascular anomalies, and congenital hypertrophy of the RPE.

Inherited retinal degenerations

Generalized progressive retinal atrophy (Figs 11.7–11.9)

•The collective term, progressive retinal atrophy (PRA) represents a diverse group of inherited, or presumed inherited, photoreceptor disorders that share a number of key features:

■Bilateral, symmetrical ocular involvement

■Photoreceptor degeneration that is progressive and leads to blindness

■Common clinical features include:

– Reduced pupillary light reflexes

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–Diffuse hyper-reflectivity of the tapetum, where present

–Attenuation of the retinal blood vessels

–Pigment clumping visible in the non-tapetal fundus

•Many of these disorders, particularly in dogs and cats, are considered to represent important models for human inherited retinopathies, and as such have been extensively characterized. Diagnostic pathologists are unlikely to be involved in these studies, but may be asked to voice an opinion regarding the presence or absence of PRA in a globe removed from a domestic pet, often years after onset of blindness. Such globes are typically enucleated because of a secondary or unrelated condition, e.g. glaucoma or ocular neoplasia

•Dogs are much more commonly affected by PRA than other species, as breeding practices have increased the incidence of inherited disorders in this domestic species. Most, but not all, forms of PRA are recessively inherited. X-linked PRA and autosomal dominant PRA have also been identified

•The disease features presented in this chapter clearly represent a simplistic generalization. Comprehensive discussion of the full extent of current knowledge of the incidence, early structural and functional abnormalities and the genetic basis of this spectrum of retinal disorders is outwith the scope of this text. Hence, a general synopsis is provided for pathologists attempting to interpret ocular specimens submitted for

diagnostic purposes and for the clinicians who will be making use of those interpretations

•This diverse group of disorders may be broadly subdivided into:

■Early onset: photoreceptor dysplasias and dystrophies

–Morphological and electrophysiological abnormalities are detectable prior to maturation of the retina (i.e. prior to about 8 weeks in dogs) and significant visual deficits are evident early in life

■Late onset: photoreceptor degenerations

–Although the photoreceptors appear morphologically normal in young animals, rod photoreceptor outer segments are often preferentially affected, becoming shortened, disorganized, then degenerating. Ultimately, cone photoreceptors are also affected. Blindness ensues in middle-aged adults

■These disorders may be further classified according to the principal photoreceptor type involved

–Many of the well-characterized forms of PRA affect the rod photoreceptors initially and more severely

–Cone dystrophies and degenerations are much less common than disorders involving the rod photoreceptors, and many forms do not cause night blindness, or do not demonstrate rod photoreceptor abnormalities until late in the course of disease

•Early morphologic features of PRA include:

■Shortening, disorganization and distortion of photoreceptor outer segments, progressing to loss of photoreceptor outer and inner segments. The age of onset and relative extent of involvement of rods and cones depends on the underlying disease process

■Loss of photoreceptor nuclei, with thinning of the outer nuclear layer

■Although retinal disease is diffuse, often photoreceptor loss is not uniform across the fundus. This topographic and temporal gradation of lesion severity differs between the various disorders

•Morphologic features of longstanding PRA, in globes removed for other reasons, include:

■Retinal atrophy, most profoundly affecting the outer retina

■Presence of phagocytic cells within the neurosensory retina

■Photoreceptor atrophy progresses to end-stage retinal atrophy with gliosis

■Common intraocular complications of long-standing PRA in dogs include:

–Retinal detachment

–Cataract, which may, in turn, lead to lens-induced uveitis or lens luxation, and to glaucoma

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A B

C D

A

B

C D

Figure 11.6  Retinal folds and retinal dysplasia, pathology. (A) Low magnification photomicrograph of

a canine retina with retinal folds.

(B,C) Subgross and higher magnification photomicrographs showing retinal dysplasia. (D) Low magnification photomicrograph of a dog retina showing widespread retinal dysplasia.

■It is seldom possible to definitively diagnose PRA on the basis of morphological findings alone. It is generally only possible to say that the changes seen are consistent with that diagnosis

–Knowledge of commonly affected breeds may raise an index of suspicion for inherited PRA

–Very commonly affected breeds include the miniature and toy Poodle, Labrador retriever, American and English Cocker spaniel (prcd) and Miniature Schnauzer, although primary photoreceptor dystrophies and degenerations have been reported and characterized, or are suspected to be inherited, in a large number of breeds. The reader is referred to the most recent edition of Ocular Disorders Presumed to be Inherited in Dogs, published by ACVO for regularly updated information on affected breeds

•Differential diagnoses for retinal degeneration include:

■Sudden acquired retinal degeneration syndrome (SARDS) in dogs

■Toxic retinopathies

■Choroidal perfusion problems/vascular disease

■Nutritional retinopathies

■Diseases of the RPE such as lipofuscinosis

•A number of genetic mutations have been identified in affected breeds and genetic tests are available for an ever increasing

number of different forms of PRA. The reader is referred to online molecular genetic and citation databases, such as OMIM, OMIA and Medline, as well as other online resources such as the website of the commercial genetic testing company, Optigen (www.optigen.com) for up-to-date information on available tests

•Inherited photoreceptor degeneration in cats:

■Aside from retinal degeneration secondary to vascular disease, nutritional or toxic causes (discussed later in this chapter), diffuse photoreceptor degeneration is uncommon in cats

■Clinical and pathologic features of inherited retinal degeneration are broadly similar to canine PRA, as described in the previous section

■Two distinct forms of inherited retinal degeneration have been extensively studied in the Abyssinian cat:

–An early-onset form with dominant inheritance

–A later-onset, more slowly progressive form with recessive inheritance

■An early-onset, recessively inherited retinal

degeneration has also been described in a colony of Persian cats

■Older Siamese cats have also been over-represented in some studies of feline retinal degeneration.

Retinal dystrophy in Briard dogs, congenital stationary night blindness (CSNB), lipid retinopathy

•Clinical features of this disease include:

■Affected dogs are night-blind, with variable degrees of visual impairment in daylight conditions

■Fundus appearance is normal for the first few years of life but slowly progressive, subtle color changes may be noted in the tapetal fundus, with white, yellowish or brown spots visible in some animals and associated with tapetal hyper-reflectivity

•Morphological features of this disease include:

■Vacuolation of the RPE, with accumulation of large, lipidlike, electron-lucent inclusions

■Disorganization and degeneration of rod outer segments

•This disease is caused by a mutation in the gene which codes for RPE65, an RPE protein which is essential to the regeneration of visual pigment that, in turn, is vital to photoreceptor function

•This disease is not common but is considered an extremely important animal model for a human disease (Leber congenital amaurosis) that can result from the same genetic mutation

■Reports of restoration of vision in affected dogs marked the first successful demonstration, in a species with comparable eye size to that of humans, of gene therapy to preserve or restore vision in a primary, inherited retinal disease.

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Canine multifocal retinopathy (Fig. 11.10)

•This unusual retinopathy is characterized by multifocal gray or tan lesions that appear throughout the fundus of young dogs

•Morphologically, these lesions correspond with serous detachments of the retina and RPE, and associated multi-focal areas of RPE hypertrophy, lipofuscin accumulation (see below) and degeneration. These RPE lesions are accompanied by variable degrees of neurosensory retinal degeneration

•Canine multifocal retinopathy demonstrates autosomal recessive inheritance in the breeds characterized to date

•Mutations in the bestrophin gene, which codes for a protein that is expressed in the RPE, have been identified in the Great Pyrenees, English Mastiff and Bull Mastiff (cmr1) and Coton du Tulear (cmr2) and in humans with Best macular dystrophy.

Lysosomal storage disorders and the neuronal ceroid lipofuscinoses

•This diverse group of neurodegenerative disorders are characterized by progressive neurological abnormalities and gradual loss of vision. Storage products accumulate within lysosomes in various tissues throughout the body, impairing tissue function

•Several of these disorders have been extensively characterized in domestic species, including sheep, dogs and cats, and serve as valuable animal models for human diseases

•The range of lysosomal storage disorders that may be associated with retinal pathology includes, but is not limited to:

■GM1 gangliosidosis

■MPS VII

■Fucosidosis

•The neuronal ceroid lipofuscinoses (NCL) are a heterogeneous group of diseases characterized by intracellular accumulation of ceroid lipofuscin (see below)

■In contrast to disorders associated with photo-oxidation, in which lysosomal storage bodies primarily accumulate in the RPE, in NCL this autofluorescent ceroid lipopigment generally accumulates within the neurosensory retina, particularly retinal ganglion cells, as well as other tissues

■Characteristic ‘fingerprint profiles’ may be seen within these granules on ultrastructural studies.

Comparative Comments

A number of generalized chorioretinal degenerations are seen in humans, many of which have striking similarities to those occurring in dogs and other species. Although clinically and histologically there is little evidence of inflammation, this family of diseases is referred to as retinitis pigmentosa and has a complex and varied genetic basis. Although eyes with these conditions are rarely submitted to the pathology laboratory, the disease is characterized primarily by the loss of rod photoreceptor cells by apoptosis, with cone photoreceptors generally degenerating secondarily to the rods.

OTHER DEGENERATIVE CONDITIONS

OF THE RETINA

Sudden acquired retinal degeneration syndrome (SARDS) in dogs

Although SARDS is a relatively common cause of sudden-onset blindness, there is little indication, beyond research interest, to remove the eyes from SARD-affected animals for pathologic evaluation.

•Typically, globes from dogs with SARDS are only submitted to a pathologist after the animal dies for other reasons, or if they subsequently develop complicating ocular conditions that necessitate enucleation, or if they adjust so poorly to their loss of vision that euthanasia is elected

•There are only 20 cases in the COPLOW collection.

Clinical features typical of SARDS in dogs

•Sudden onset of blindness noticed by the owner

•Dilated resting pupil size, with pupillary light reflexes that can still be elicited in response to intense stimuli that are a source of blue light

•Normal, or near normal, fundus appearance in early disease

•A ‘flat’ electroretinogram, indicating absence of photoreceptor function/phototransduction.

•Over many months, ophthalmoscopic findings are consistent with gradually progressive diffuse retinal degeneration

•The clinical history may suggest ill-defined metabolic problems including:

■Polyuria and polydipsia

■A high-normal ACTH response test or abnormalities in the results of other tests of adrenal cortical function

■Obesity, or recent weight gain.

Morphological features of the retina in SARDS (Fig. 11.11)

•Early in the course of disease, the retina is nearly normal except a narrowing of the outer plexiform layer

•Apoptotic photoreceptor cells may be detected within the outer nuclear layer

•From several weeks or months into the course of disease, there is an atrophy of photoreceptors, resembling PRA. However, unlike early stages of PRA, involvement is diffuse and relatively uniform throughout the retina

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•Lymphoplasmacytic inflammatory cells are seen but are present in very small numbers

•The retinal degeneration is slowly progressive and ultimately, can involve all the layers of the retina.

Feline central retinal degeneration, taurine deficiency (Figs 11.12, 11.13)

•Taurine deficiency in cats is associated with both retinal disease and dilated cardiomyopathy

•Cats need a dietary source of taurine as they are unable to synthesize adequate amounts of this amino acid from cysteine

•The classical retinal disease is a focal outer retinal degeneration, initially seen in the area centralis and slowly progressing to involve both the temporal and nasal fundus superior to the optic nerve head

•In the earliest stages of this disease, cone disorganization and dysfunction is greater than involvement of rod photoreceptors

•In chronically deficient cats, more generalized photoreceptor degeneration is seen and these cats may be irreversibly blind (Fig. 11.13)

•Taurine deficiency should be considered in the differential diagnosis of photoreceptor degeneration in cats.

Lipofuscin in the RPE and effects on the retina (Fig. 11.14)

Lipofuscin is a general term for a variety of autofluorescent compounds containing cross-linked proteins and lipids. The precise constituents of lipofuscin vary according to the tissue in which it is formed as a lysosomal storage product.

•The main constituent fluorophore of RPE lipofuscin has been identified as A2E, a product of vitamin A metabolism and photoreceptor membrane lipids.

•Accumulation of small amounts of lipofuscin in the RPE is a common finding in eyes submitted to COPLOW

•Lipofuscin may be recognized as a feature of normal aging, and is also known as ‘age pigment’

•Lipofuscin in the RPE is probably the most common ocular abnormality seen in exotic or wild species in the COPLOW collection

•Lipofuscin accumulates in conditions associated with enhanced oxidative stress, or a reduction in the activity of intrinsic defense mechanisms against oxidative stress.

•Too much lipofuscin in the RPE impedes further phagolysosomal degradation, contributes to photo-oxidative damage and can impact photoreceptor function, leading to outer retinal degeneration.

Clinical features of retinal diseases associated with increased lipofuscin in the RPE cells

•Accumulations of RPE lipofuscin may be visible within the tapetal fundus (where the RPE is normally non-pigmented) as multiple foci and patches of light-brown pigment

•Subsequent retinal degeneration may lead to other changes such as attenuation of retinal blood vessels, pigmentary changes in the non-tapetal fundus, and focal regions of tapetal hyper-reflectivity

•Depending on the underlying cause of lipofuscin accumulation, other systemic signs of disease such as neurologic abnormalities or muscle weakness may be evident.