Chapter 12
Retina, Optic Nerve, Vitreous Body

Embryology, Anatomy and Physiology

Development

The walls of the optic vesicle are the retinal anlage. When the optic vesicle invaginates to form the optic cup there is apposition of the walls to form two layers: the outer is the future retinal epithelium and the inner is the future sensory retina.

The outer layer becomes a single layer of cuboidal cells which is the future retinal epithelium. The inner layer thickens into a multilayered neuroblastic zone from which all the cells of the sensory retina are derived.

In the future sensory retina, an inner-most layer of cells forms and begins growing axons; these cells are the ganglion cells. The ganglion cell axons converge and leave the eye as the optic nerve through the fetal fissure.

Mesodermal (vascular) tissue enters the eye through the same fetal fissure and becomes hyaloid vasculature. Fibrils from lens, retina and the hyaloid artery form the primary vitreous. Secondary, or definitive, vitreous is produced mostly by the retina. The hyaloid vasculature normally resorbs completely prior to or shortly after birth. It often is prominent for some months after birth in ruminants and rats.

Structure and Function

Retina

The retina ranges in thickness from about 100-500 µm. It is a composite of numerous cellular and synaptic layers which can be grossly split into an outer epithelial layer (referred to as the retinal epithelium or retinal pigment epithelium) and an inner sensory layer (referred to as the sensory retina or neuroretina). The retina is one of the most metabolically active tissues in the body (Young). Its major function is to convert light energy into chemical and electrical energy so that vision can occur (if a functional brain is present).

The order of retinal layers starting from outer to inner layers (that is, from choroid to vitreous) is as follows:

Retinal epithelium - single layer of cuboidal cells whose basement membrane merges with that of the choriocapillaris. The epithelial cells may or may not contain melanin granules; in particular, those overlying the tapetum generally are amelanotic; they have apical processes which interdigitate with the photoreceptor cell processes. Nourishment of the outer retinal layers is from the choriocapillaris by way of the retinal epithelium. The retinal epithelium also recycles photoreceptor proteins by phagocytizing shed outer segments and metabolizing them.

Photoreceptor outer segments - it is in these structures that light causes chemical changes that initiate the vision process.

Photoreceptor inner segments - these provide for basic function of the photoreceptor cells and the continued production of outer segments.

Outer or external limiting membrane - this is a misnomer because it is not a membrane, rather it is a line of junctions between photoreceptor cells and Müller cells (major glial cells of the retina). By light microscopy, it appears as a membrane and because of tradition, it will continue to be called a membrane.

Outer or external nuclear layer - nuclei of the photoreceptor cells, the rods and cones. Rods are so-called night vision cells because they are functional in very dim illumination (called scotopic conditions). Cones are day vision cells because they respond to higher intensity light (called photopic conditions). See elsewhere for information on electroretinography which evaluates the status of these cells.

Outer or external plexiform layer - layer of synapses between photoreceptor cells and the next order neurons, the bipolar cells. Modifying cells known as horizontal cells and interplexiform cells also synapse here. The line of synapses in this layer sometimes is referred to as the middle limiting membrane due to its light microscopic appearance.

Inner nuclear layer - nuclei of bipolar cells, horizontal cells, amacrine cells and Müller cells. The horizontal cells are located externally and modify signals between photoreceptor cells and bipolar cells. Bipolar cells are the second order neurons which carry the vision impulse to the next order neurons (ganglion cells). Amacrine cells are located on the inner aspect of the inner nuclear layer and modify the signals from the bipolar to the ganglion cells. Interplexiform cell perikarya are located among amacrine cells, but their processes extend to the inner and outer plexiform layers; interplexiform cell function is unclear. Müller cells are the major glial cells for the retina; their processes extend from the external to the internal limiting membranes; they have a high glycogen content and are important in retinal nourishment.

Inner plexiform layer - layer of synapses between bipolar, amacrine, ganglion and interplexiform cells.

Ganglion cell layer - these cells comprise the third order neurons and their axons leave the eye together as the optic nerve. Also in this layer are other retinal glial cells.

Nerve fiber layer - axons of the ganglion cells which converge to meet at the optic disk before leaving the eye as the optic nerve. The vision impulse traverses the optic nerve to reach the brain.

Internal limiting membrane - a true membrane secreted by the terminal footplates of the Müller cells. This membrane is adherent to the vitreous body.

Retinal vasculature: There is a great deal of variation between species. The ocular fundus often is described according to the type of vasculature found.

Holangiotic - retinal blood supply (inner layers) is from central or cilioretinal arteries. This is seen in dogs, cats, cattle, goats, sheep, pigs, rats, mice, primates and others.

Merangiotic - only part of the inner retina is supplied by retinal vessels. This is the type present in the rabbit.

Paurangiotic - the retinal vessels are small and extend only a short distance from the optic disk so that only the inner retina immediately surrounding the disk is supplied; the rest of the retina is dependent on the choriocapillaris. This is seen in the horse. The guinea pig has vessels in the optic disk (not easily visible clinically), and because the optic disk is composed of retinal ganglion cell axons, this could technically be considered retinal vasculature. Functionally, however, the vessels probably have little to do with the retina proper and, therefore, the guinea pig should be considered as being anangiotic.

Anangiotic - no part of the retina contains vessels and therefore the entire retina is dependent upon the choriocapillaris. This is present in some rodents such as the degu. Birds also have anangiotic ocular fundi, but have a structure called the pecten which arises in the region of the optic disk to protrude into the vitreous and may be important in providing inner retinal nourishment.

Optic nerve

Responsible for conveying the visual impulse generated in the retina to the brain. It also is the afferent pathway for the pupillary light response.

The optic nerve may be divided into three parts:

Intraocular or optic disk - about 1-2 mm in diameter.

Intraorbital.

Intracranial (optic foramen to chiasm).

The optic nerve consists of retinal ganglion cell axons and supporting cells (astrocytes, oligodendrocytes) and is surrounded by extensions of the central nervous system meninges (pia mater, arachnoid and dura mater). In most animals (the dog, rabbit and bird being exceptions), myelination of the nerve fibers does not begin until after the fibers pass from the ganglion cells through the lamina cribrosa (area cribrosa). The dog and rabbit normally have myelination of the intraocular portion of the optic nerves and this is easily seen ophthalmoscopically. In birds, myelination is present at the optic disk level, but largely is obscured due to the overlying pecten. Myelin in the optic nerves is derived from oligodendrocytes.

Vitreous body

Also known as vitreous humor. It is about 99% water, but usually is present in a gel state due to its collagen content; also contains hyaluronic acid, chondroitin sulfate and a few cells.

Most of the volume of a globe is due to the vitreous which provides support and assists in maintaining intraocular pressure. Besides being optically clear to allow light to reach the retina, the vitreous body is important for the metabolism of intraocular tissues.

The vitreous cavity is bounded by the posterior surface of the lens, the ciliary body, the retina and the optic disk. Anteriorly the vitreous body has a depression called the patellar fossa in which rests the posterior aspect of the lens. Where the primary or embryonic vitreous humor used to be is called Cloquet's canal which extends from the optic disk to the posterior aspect of the lens.

The vitreous humor is not a stagnant structure and is active metabolically.

Interpretation of normal appearance

Because the sensory retina is optically clear, ocular fundus appearance depends on retinal vasculature, degree of melanosis of the retinal epithelium and choroid, and presence or absence of a tapetum. Retinal epithelial cells overlying a tapetum usually are amelanotic so that this region of the ocular fundus appears as the color of the tapetum. In nontapetal regions or eyes having no tapeta, heavily melanotic retinal epithelial cells and choroid impart a dark brown hue to the ocular fundus. Less melanosis leads to lighter shades of brown. As the ocular fundus becomes lighter in color, the deeper structures, such as choroidal vessels or sclera, become more visible. In some individuals there may be very little retinal epithelial melanin and moderate choroidal melanin so that choroidal vessels, which are somewhat orange and red compared with the red of retinal vessels, are visible in a background of brown. In amelanotic or mildly melanotic eyes, the choroidal vessels will be clearly visible with a white background which is, of course, the sclera.

There is a region of increased ganglion cell and cone density lateral to the optic disk (and slightly dorsal in most species), called the area centralis and which is relatively devoid of vessels. Within the area centralis many species (some primates and birds, and a few reptiles and fish), possess a fovea which is an indentation of the retinal surface and is the source of peak visual acuity. In primates the region immediately surrounding the fovea is called the macula. Note that none of the domestic animals possess a macula or fovea.

Normal fundus appearance for some species

Dog

Generally 3-4 major veins, 15 or so arterioles. Optic disk usually is myelinated, but to different degrees so that the disk assumes various shapes (from roughly triangular to circular); its position is constant, but it may appear variable depending on how much tapetal development there is (the tapetum may end short of the disk or may surround it); it is well vascularized and venous pulsation is visible in some individuals. May or may not have tapetum (Bellhorn, et al.); tapeta in small breeds often are much smaller (disproportionately) than in large breeds; stars of Winslow are present (end-on views of choriocapillaries as they penetrate the tapetum).

Cat

Generally three major retinal veins and 3-5 arterioles. Well developed tapetum. Optic disk is small, non-myelinated, circular, slightly depressed, generally surrounded by tapetum and may be slightly melanotic. Stars of Winslow are present.

Horse

60-70 small vessels (cannot differentiate arterioles from venules) arise from border of the optic disk and extend about one disk diameter from the disk (Gelatt and Finocchio). Tapetal region shows some diffuse, but light melanosis. The optic disk is situated ventral to center and is non-myelinated, ellipsoid (long axis horizontal) and slightly depressed; the tapetum usually ends a short distance above it. Stars of Winslow are prominent.

Cow

Three major veins (very prominent) and three or more arterioles; the dorsal vein and arteriole often twist around each other. Well developed tapetum. Optic disk is non-myelinated and horizontally oval or sometimes round, often with indistinct borders; the tapetum usually ends at or around the dorsal border. Stars of Winslow are prominent.

Sheep

Generally similar to cattle. Optic disk has a typical 'kidney bean' shape to it, with one half often appearing 'incomplete.'

Goat

Several major veins with several arterioles. Tapetum similar to cattle. Optic disk is non-myelinated and round; the tapetum often develops to surround it. Stars of Winslow are prominent.

Pig

Four major veins and several arterioles. Optic disk is non-myelinated and horizontally ellipsoid. No tapetum.

Bird

No retinal vessels; if there is little melanin in the retinal epithelium and choroid, the choroidal vessels and sclera will show through. Optic disk is covered by pecten which protrudes into vitreous cavity and usually is heavily melanotic; optic disk is myelinated, but most of it is obscured by pecten. There is a fovea dorsal and lateral to pecten; some birds, such as falcons and eagles, may have two fovea. Most birds have no tapetum.

Rabbit

Retinal vessels lie on top of retina and send capillaries down into it; extend primarily lateral and medial to optic disk along with the medullated nerve fibers, and a short distance dorsally and ventrally from edge of optic disk. The optic disk is deeply cupped and is situated far dorsally within the fundus. No tapetum.

Guinea pig

No retinal vessels per se; few vessels on surface of optic disk, extend very short distance into retina. Optic disk small, round and indistinct. Nerve fibers prominent as radiating fine lines converging on optic disk. No tapetum.

Congenital Abnormalities

Because the retina, optic nerve and vitreous are closely related, abnormalities in one often are associated with abnormalities in the others.

Persistent hyaloid artery and remnants

Various degrees of persistence of hyaloid system may occur, usually without affecting vision. If large regions of posterior lens capsule are opacified, however, this may reduce vision particularly in daylight conditions when the pupil is small.

Clinical appearance

Mittendorf's dot - small posterior polar capsular opacity where artery attached to the lens; easiest seen with biomicroscopy. No measurable effect on vision.

There may be a short remnant attached to the lens or the optic disk. The remnant attached to the lens may hang down into the vitreous and have a coiled appearance; the remnant on the optic disk appears as a small out-of-focus gray dot when viewed straight-on and is common in cattle.

Persistent artery - may extend from the optic disk into the vitreous as far as the lens where it may terminate on the posterior capsule with a tree branch appearance. If extensive, it may cause blindness.

Persistent hyperplastic primary vitreous - extensive mass of fibrovascular tissue adherent to the posterior aspect of the lens. These eyes often have other problems such as microphthalmia, and usually are blind (Grimes and Mullaney; Lewis, et al.).

Significance

Most of these problems are not common and treatment is neither necessary nor practical.

Optic nerve aplasia

This is extremely rare in all species, but has been reported in cats (Barnett and Grimes). If it is unilateral, it may not be clinically recognized; the pupillomotor pathway is likely to be normal and this will result in the abnormal eye having an almost normal sized pupil; the eye will not, however, have a direct pupillary response itself and stimulation of it will not result in a response in the opposite eye. If bilateral, the individual will be blind with dilated, unresponsive pupils.

Ophthalmoscopy shows absence of optic disk and retinal blood vessels.

There is no treatment.

Optic nerve hypoplasia

This is uncommon, but has been reported in cats, dogs, cattle, horses, mice and others (Barr, et al.). Affected eyes may be otherwise normal or may be malformed.

Signs

Depends on whether bilateral or unilateral. Affected eyes usually are blind or have very poor vision. It is not unusual to be unilateral in which case it is clinically silent; diagnosis in these cases usually is incidental (the patient fails the swinging flashlight test).

Pupillary responses to light:

Unilateral condition - poor direct with good or poor response in opposite eye when affected eye is stimulated.

Bilateral condition - poor responses.

Ophthalmoscopy: Reduction in size of optic disk, but retinal vasculature usually normal in amount and size.

Breeds affected

Poodles especially, in which it appears to be heritable (Kern and Riis); collies; others.

Pathogenesis

Usually unknown. Histologically there usually is a reduction in number of ganglion cells.

Treatment

No treatment is available. Genetic counseling for people with affected dogs.

Optic nerve coloboma

Also could be called optic disk coloboma, optic disk ectasia or optic disk pit. These may all be the same pathologically, but called a different name depending on the observer.

Seen especially in dogs. It is more common than aplasia or hypoplasia. Occurs as part of the collie eye anomaly (about 30% of affected collies). In the collie, however, the nerve (disk) appears colobomatous, but often is near normal further into the orbit; the colobomatous appearance is due to scleral ectasia in the region of the optic disk causing distortion of the nerve as it leaves the eye. Appears to be heritable in the basenji. Is a heritable trait in Charolais cattle (Falco and Barnett; Wijeratne and Curnow).

Clinical signs depend on the extent of the coloboma, but usually none because most are not severe.

Ophthalmoscopy may reveal changes ranging from a tiny pit in the optic disk , notching of the optic disk , or an enlarged, excavated disk which may be several times normal size and several millimeters deep .

No treatment is available.

Myelinated nerve fibers

The dog, rabbit and bird normally have myelination of the nerve fibers comprising the optic disk (in the bird, the pecten obscures this). Amongst other species which normally do not have myelination, you may occasionally see an individual who has aberrant myelination of the optic nerve fibers comprising the optic disk or those adjacent the disk. The aberrant myelination appears as discrete, white, sometimes fluffy zones bordering or near the optic disk. This is a benign anomaly, but you must differentiate it from inflammatory or other lesions. In contrast to inflammatory or neoplastic lesions, these zones of myelination have well defined borders and are largely in focus with the surrounding tissue. Furthermore, there are no abnormalities of optic nerve function; i.e., vision and pupillary responses to light are normal.

Lack of or poor development of tapetum (tapetal hypoplasia)

The tapetum is part of the choroid. Lack of tapetum is not an abnormality per se, but it is mentioned here because it alters the appearance of the ocular fundus.

Occurs especially in individuals having white or color dilute hair - Dalmatian dogs, white cats, horses with hypomelanotic periocular skin, some merle collies, individuals with heterochromia iridis, albinos (Gelatt, et al.), and some 'toy' breed dogs (e.g., Chihuahuas). The latter in particular have very small tapetal areas. Beagles have an abnormality of development which appears to be heritable (Bellhorn, et al.).

If the retinal epithelium is melanotic, ocular fundus appears uniformly brown in these individuals. If the retinal epithelium is amelanotic, there will be various degrees of choroidal and scleral visibility.

There is no apparent clinical significance to this condition - individuals not having tapeta appear to have normal vision.

Collie eye anomaly

Also known as the collie ectasia syndrome, this is the most significant ocular disease in collie dogs (Roberts, et al.). About 85% of the collies in the USA demonstrate phenotypic evidence of being affected by this disease; the prevalence of genotypic involvement may be much higher. The disease is somewhat less prevalent in other countries (Bedford).

This disease is genetically determined. The ocular defects occur during formation of the eye and are present to their fullest extent (usually) at birth; i.e., there is no progression of the disease as the individual gets older, except for the possible development of retinal separation, as described below. The abnormalities not only involve the retina, optic nerve and vitreous, but also the sclera and choroid. The same syndrome is seen in smooth and rough coated collies, merle collies, and Shetland sheep dogs. A similar syndrome occasionally is seen in German shepherd dogs.

Clinical signs and ophthalmoscopic findings

Most lesions cause no clinically apparent visual disturbance. The disease always is bilateral, but not always symmetrical. Affected individuals may have one or more of the defects listed.

The characteristic lesion is chorioretinal hypoplasia located lateral to the optic disk . This appears as a zone of hypomelanosis in its mildest form; in its most serious form, there will be a large region in which there will be very few choroidal vessels, and ectasia of the sclera. The choroidal vessels which are present may be large and anomalous. In the severe form it is called staphyloma (ectasia lined by uveal tissue).

Tortuous retinal blood vessels - a common finding, but can occur alone in normal dogs.

Ectasia or coloboma of the optic disk - this may range from a small pit to a large excavation with enlargement of the disk region and ectasia of the sclera . May not be a true coloboma of the optic nerve in all cases (see previous discussion of optic nerve colobomas).

'Vermiform streaks' - this term is used to describe the retinal folds which often are part of the syndrome and appear as irregular, white lines.

Microphthalmia - this is difficult to relate just to this syndrome because collies normally are bred to produce small eyes.

Retinal separation - about 5-10% will have retinal separation (at the photoreceptor-epithelial junction) and will be blind; this generally occurs in eyes with the most severe maldevelopment and ranges in extent from a small circumscribed zone (usually in the region of chorioretinal hypoplasia) , to total separation with dialysis (tearing of the retina from its strong attachments which are around the optic disk and at the ora serrata); some individuals are born with this condition whereas others develop it over time; if retinal separation is going to occur, it usually does so before two years of age, if not already present by the time the eyes open.

Neovascularization from the retina into the vitreous body - uncommon.

White, granular deposits either on the retinal surface or in the vitreous body - uncommon, but often associated with neovascularization into the vitreous.

Intraocular hemorrhage - following retinal separation or neovascularization. May result in the entire vitreous cavity becoming filled with blood.

Corneal opacities - deposits in anterior stroma. Relationship to collie eye anomaly not known, but frequently disappear with maturity.

Diagnosis

Ophthalmoscopic appearance is diagnostic. Because lesions are present at birth, early examination is possible and should be encouraged at as early as 6 weeks.

There are two instances in which diagnosis is a challenge:

'Go-normals' - these are individuals who have mild chorioretinal hypoplasia at birth, but whose retinal epithelium becomes melanotic in the affected zones over time so that the choroidal changes are obscured with age (usually by six months to a year). These individuals nevertheless are affected and stress the necessity for examining collies at an early age.

Merle collies - these dogs are color-dilute and therefore you must be careful in diagnosing hypoplasia on the basis of lack of melanin alone; there must be an actual lack of structural development or the presence of other structural defects mentioned previously.

Control

There is no treatment available and, because the disease is heritable, selective breeding is the only control. The actual mode of inheritance is unclear, but it is believed to be autosomal recessive. The degree of ocular changes differs greatly between affected dogs. There appears to be one genotype which can give rise to any variety of phenotypes. Although it has been thought that dogs with minor abnormalities may produce offspring with only minor abnormalities, there is no proof to this claim. Collies having any degree of clinical manifestation of this disease should not be used for breeding, nor should those who appear normal but have relatives showing signs of disease. In the United Kingdom the prevalence has been reduced to about 15% through strict control over breeding and registry.

Grading of the various manifestations

Many people use a grading system to describe the extent of involvement of an individual. There are two major systems being used today; both have shortcomings, one much more than the other. The genetic behavior of this disease indicates that an individual is of one status only: affected or not affected. A grading system often leads to confusion among the collie breeders and has the potential of lulling people into thinking collies with 'better' grades could be used for breeding.

Australian shepherd syndrome

This disease is similar in many respects to the collie eye anomaly. It is heritable and is congenital. It is characterized by microphthalmia and multiple colobomas, and is seen in the Australian shepherd dog (Gelatt and McGill).

Clinical signs and ophthalmic findings

Only those individuals having severe microphthalmia and colobomas show signs of being blind or having deficient vision. Affected individuals may have one or more of the defects listed.

Microphthalmia - various degrees from slight to a tiny eye with cysts.

Persistent pupillary membrane - not an important finding, but does indicate the presence of disease.

Iridal coloboma and dyscoria - there usually is heterochromia iridis although this in itself is not abnormal .

Cataract - often posterior or nuclear.

Staphyloma involving equatorial region of globe .

Retinal folds and separation.

Retinal dysplasia - retina may be present extrasclerally.

Choroidal colobomas.

Posterior staphyloma, similar to the collie.

Optic nerve hypoplasia.

There is an association between coat color and the disease - most affected individuals are blue merle or have a high percentage of white.

Cardiac anomalies may be present.

Diagnosis

Because the disease is congenital, diagnosis can be done at early age. The ocular findings mentioned above are diagnostic although not all necessarily will occur in the same individual.

Control

No treatment; must control by selective breeding. Appears to be a recessive trait. Individuals having any degree of clinical manifestation of this disease should not be used for breeding, nor should those who appear normal but have relatives showing signs of disease.

Retinal dysplasia

This is generalized abnormal development of the retina and can be manifest as benign appearing retinal folds or more serious disorganization of the entire retina with total separation of the sensory retina from the retinal epithelium. The serious form also is known as vitreoretinal dysplasia because the vitreous often is involved.

Most forms of retinal dysplasia are heritable.

Breeds affected

The serious form is seen as a heritable condition in the Bedlington terrier (simple autosomal recessive) (Rubin), Labrador retriever (possibly dominant with variable penetrance) (Carrig, et al., a, b; Nelson and MacMillan), Sealyham terrier (recessive) (Ashton, et al.), and English springer spaniel (recessive) (Lavach, et al.; Schmidt, et al.).

In other canine breeds such as the American cocker spaniel, the dysplasia is manifested only as retinal folds (multifocal retinal dysplasia). More severe lesions never have been reported. Thus, the condition is considered a minor anomaly.

Various degrees of retinal dysplasia also are seen in Australian shepherds and collies, as part of the general ocular maldevelopment syndrome heritable in these breeds.

In cattle, retinal dysplasia is seen as part of multiple ocular anomaly syndrome especially in shorthorns.

Retinal dysplasia as an isolated defect can occur sporadically in any species .

Clinical signs and diagnosis

Most of the changes occur prenatally (or very soon after birth in species, such as the dog, whose retinas are immature at birth). Therefore, you can often diagnose the disease as soon as ocular fundus examination is feasible. It has been shown, however, that in some cases the disease may not manifest itself for many months after birth (Holle, et al.). It is recommended that dogs of breeds known to develop retinal dysplasia be re-evaluated when at least a year old even if found to be ‘normal’ at an early age.

The clinical signs will depend on the severity of the condition.

A common manifestation in breeds such as Bedlington terriers is bilateral retinal separation usually in the shape of a funnel with dysplastic retina in contact with the posterior aspect of the lens. Individuals with this condition will be blind in the affected eye. Such severe dysplasia also is part of the syndrome in Labrador retrievers and English springer spaniels.

Less severely affected individuals may not show visual disturbance. Ophthalmoscopic examination of these will reveal various degrees of retinal folding, sometimes with strands of retina extending into the vitreous, especially from the optic disk region. The folds involve the outer retinal layers, and their clinical appearance depends on their location. The folding of the tissue results in a focal change in optical quality to the sensory retina. Folds overlying the tapetum usually are dull gray and may have a hyperreflective border as well, whereas those in the nontapetal region are white due to contrast with the melanin. They may be circular, linear, curvilinear, or branched in shape. These individuals are just as affected, genetically speaking, as those showing more severe signs.

Other abnormalities sometimes present include nystagmus, microphthalmia, corneal opacities, persistent pupillary membrane and cataract (usually posterior in location).

Control

There is no treatment. Reduction of prevalence must be by selective breeding. Individuals having any degree of clinical manifestation of this disease should not be used for breeding, nor should those who appear normal but have relatives showing signs of disease. The only exception may be the American cocker spaniel in whom, as stated, the dysplasia is expressed only as retinal folds.

Acquired Abnormalities

The heritable retinopathies presented in this section are, of course, congenitally predetermined and some are manifest congenitally, but are covered here because most do not cause clinical disturbance for some time after birth.

Optic neuritis

Primary optic neuritis

Signs - usually have visual impairment or blindness of sudden onset (if bilateral); usually bilateral; pupils dilated and unresponsive to light.

Ophthalmoscopic appearance - optic disk is swollen (edema) and may have blood on it or adjacent; retinal veins may be congested .

Cause - unknown.

Diagnosis - based on signs and appearance.

Treatment - must be vigorous - systemic steroids (oral) at high anti-inflammatory concentrations (prednisolone, prednisone or dexamethasone); should be continued for two weeks, but re-evaluate weekly; if no resolution in two weeks, treat one more week before considering it permanent; antibiotics should not be used unless specifically indicated; vitamin B complex has been suggested as supportive therapy. You may want to refer to a specialist for treatment with retrobulbar steroids.

Prognosis - fair if diagnosed early; poor if axons have been damaged beyond repair. Long term prognosis is not good because it tends to be recurrent and degeneration is a common sequela.

Retrobulbar optic neuritis

Reported in dogs and often is associated with proptosis of the globe or orbital cellulitis. In most cases, cause is unknown.

Signs - sudden onset of blindness (if bilateral) with fixed, dilated pupils. If associated with proptosis or orbital cellulitis, signs relevant to those conditions will be present.

Ophthalmoscopic signs - optic disks are normal.

Diagnosis - if only blindness is present (i.e., no evidence of trauma, etc.), and central nervous system disease or sudden acquired retinal degeneration is ruled out, then the diagnosis may be by exclusion. The electroretinogram is very helpful in these cases. If the electroretinogram is normal or nearly so, this rules out primary retinal disease and strengthens the suggestion of optic nerve disease.

Treatment and prognosis - similar to primary optic neuritis. Relapses are frequent in idiopathic cases.

Secondary optic neuritis

More common than primary. May accompany severe, diffuse retinitis, choroiditis, panophthalmitis, toxicities (e.g., alcohol, arsenic, thallium), trauma or infectious diseases (especially canine distemper).

Treatment is aimed at the primary problem as well as supportive for the optic neuritis unless contraindicated as in distemper.

Optic nerve degeneration

Rarely primary; usually associated with other diseases such as retinal degeneration, optic neuritis, retrobulbar pressure from neoplasm, glaucoma , more central disease (e.g., chiasmal or optic tracts), etc.

May have higher prevalence in older, subalbinotic horses.

Signs and appearance - optic disk is pale gray to white with loss of vasculature; optic disk may be sunken in appearance (as with glaucoma) and usually is reduced in size with irregular margins. Patient is blind or has visual impairment. No pupillary responses on affected side.

Diagnosis - made by signs and appearance. Must differentiate from optic nerve hypoplasia which has different prognosis.

Treatment - none; prognosis depends on cause.

Optic nerve neoplasia

Neoplasms arising primarily from the optic nerve are uncommon, but include such entities as gliomas and meningiomas.

Clinical signs depend on the location and size of the neoplasm, but usually are characteristic of a space-occupying lesion of the orbit: exophthalmos, immobility of the globe, strabismus, etc. Blindness on the affected side may occur along with mydriasis and loss of pupillary responses. If the lesion invades the globe, it can be visualized ophthalmoscopically .

Treatment is by exenteration of the orbit after determining that the patient is free of clinical signs of metastatic disease. Neoplasms such as meningioma often arise from within the cranial cavity and grow down the optic nerve to the eye so that exenteration is of no benefit.

The prognosis generally is poor for survival.

Progressive retinal degeneration

Also known as progressive retinal atrophy. The term atrophy is not descriptive because there appears to be no ability of the affected tissue to return to normal (cf. muscle atrophy). Degeneration is a more appropriate term, but atrophy has been accepted by virtue of common usage.

Progressive retinal degeneration describes a broad group of diseases characterized by a progressive loss of outer retinal function and eventual degeneration and disappearance of the photoreceptors. Most of these conditions are considered to be heritable (Aguirre and Acland; Priester), but may be of different pathogenesis (see below) (Bellhorn, et al.; Buyukmihci).

Pathogenesis

Bear in mind that the final outcome for all the different types of progressive retinal degenerations is the same: loss of photoreceptors. In order to understand how to diagnose and differentiate the various types, it is convenient to classify this group of diseases by the nature of the first pathologic changes. If one keeps this in mind, then the clinical signs easily are explained, especially with respect to the age of onset.

Rod-cone dysplasia: The rods and cones never develop normally; hence, the term dysplasia (do not confuse this with the generalized retinal dysplasia described earlier). Affected individuals have early visual loss as a result. Eventually the abnormal cells degenerate and disappear. Rod-cone dysplasia is recognized as the primary form of progressive retinal degeneration in Irish setters (Aguirre and Rubin; Aguirre, et al.) and collies (Wolf, et al.), but other canine breeds or other species may also be affected.

Rod dysplasia: The rods never develop normally, but there is almost normal cone development. Because of this, there is early loss of night vision (nyctalopia), but day vision is unaffected for many months or years. Eventually, the cones degenerate, causing the affected individual to become completely blind. Rod dysplasia is recognized as the primary form of progressive retinal degeneration in the Norwegian elkhound (Aguirre and Rubin), but can be seen in other canine breeds and species.

Rod-cone degeneration: The rods and cones appear to develop normally, but then degenerate beginning from weeks to many months after cellular maturity. As a result, vision loss does not become apparent until the affected individual is at least a year old, often several years old. This may be the most common form of progressive retinal degeneration in all species. Amongst dogs, it is common in 'miniature' and 'toy' poodles (Aguirre and Rubin), American and English cocker spaniels and Siberian huskies, to name just a few.

Degeneration of unclear type: Miniature schnauzers affected with progressive retinal degeneration have electroretinographic evidence of photoreceptor problems before 3 months of age. These dogs show histologically severe rod degeneration and moderate to severe cone degeneration at least as early as 3 months of age. The findings are suggestive of generalized photoreceptor dysplasia. The vision in these individuals, however, seems unaffected until relatively late. Some individuals with advanced degeneration of rods and cones appear to have adequate day vision and good night vision. Also, their ocular fundi often appear normal ophthalmoscopically until late. The conclusion is that progressive retinal degeneration in the miniature schnauzer is complex and does not fit into the general pattern described previously. It is clear that clinical assessment alone is not sufficient to rule out this disease in individuals less than 2 years old; electroretinography is necessary. See paper by Parshall et al. for more information (Parshall, et al.).

Other breeds of dogs and other species also may have forms of progressive retinal degeneration which do not fit the neat pattern described above. All, however, result in photoreceptor loss and blindness and all should be considered heritable.

Breeds affected

'Miniature' and 'toy' poodle, Irish setter, collie, Norwegian elkhound, Labrador retriever, Gordon setter, Samoyed, Siberian husky, Doberman pinscher, English and American cocker spaniel, Borzoi, miniature schnauzer, dachshund (Curtis and Barnett), Tibetan terrier (Millichamp, et al.), greyhound (Slatter, et al.) and numerous other canine breeds. It also is found sporadically in dogs of mixed breeding.

Progressive retinal degeneration also occurs in cats (West-Hyde and Buyukmihci) (especially the Persian and Abyssinian (Narfström) in whom it is considered heritable), and sporadically in the other domesticated species (Buyukmihci; Curtis, et al.).

Heritable retinal disease is an important problem in rodents such as rats and mice (Dowling).

Clinical signs and ophthalmoscopic findings

Nyctalopia: This group of diseases characteristically begins with night blindness (nyctalopia). The age at which this occurs depends upon the nature of the photoreceptor degeneration. In rod-cone dysplasia and rod dysplasia, loss of night vision may be evident as early as 6 weeks of age. In individuals having rod-cone degeneration, night vision loss may not become evident until they are a year or more old.

When nyctalopia is first demonstrable, the ocular fundi of the affected individual will appear normal. The degenerative changes are too subtle to see.

Total blindness: In rod-cone dysplasia, some cones appear to develop sufficiently to allow vision for many months and affected individuals may not become completely blind until they are 6 months old or older. With the other forms of progressive retinal degeneration, blindness may not occur until the individual is 3 years or older. In some cases, complete blindness may not occur although the individual may have severely limited vision.

By the time blindness occurs, there will be advanced changes in the ocular fundi which will be easy to visualize ophthalmoscopically.

Ocular fundus changes: Similar in most breeds regardless of age of onset; in almost all cases, the degenerative changes begin in the central region of the retina (near the optic disk) and proceed peripherally.

Early - increased granularity to tapetal region of the posterior pole (may have a ground-glass appearance), thinning of peripheral retinal vessels. Should be able to demonstrate nyctalopia at this time.

Intermediate - increased tapetal reflectivity, more pronounced retinal vessel attenuation, 'ridging' (an appearance of radial folds possibly due to uneven thinning of outer retinal layers), retinal epithelial disturbance (hypo- and hypermelanotic with clumping of melanin). Vision disturbance should be obvious.

Late - marked tapetal hyperreflectivity, only larger retinal vessels are identifiable (others may appear as 'ghost' vessels - no blood column), marked retinal epithelial disturbance, optic nerve may appear degenerated (optic disk is pale and appears avascular). Patient blind or almost blind.

Pupillary responses to light: As more of the retina becomes nonfunctional, the pupillary responses to light become more sluggish and eventually the pupils may become widely dilated and only slightly responsive to light.

Do not, however, use pupillary responses as a means of diagnosing this disease; ophthalmoscopic findings and visual disturbance are the only reliable indicators.

Cataract: Although difficult to determine if interrelated, cataract is present in many affected individuals, especially poodles. This is important to keep in mind when evaluating a patient who has cataracts and is showing visual difficulty.

Diagnosis

History: The human guardian often will state the patient has been reluctant to go out at night or stays close to her or him while outside at night.

Vision function test (obstacle course): This is useful and should routinely be done if vision disturbance is suspected. Set up an obstacle course of some sort and have the patient traverse it first with lights on (photopic conditions). Then rearrange the course and have the patient re-traverse it under red light (scotopic conditions). Remember to allow sufficient time for dark adaptation (when you can see under the darkened conditions, the patient usually should be dark adapted enough for testing). Individuals with early progressive retinal degeneration should show a marked difference in their behavior. Later, they will have equal difficulty under photopic or scotopic conditions.

Ophthalmoscopic changes: See above.

Electroretinography: In early cases (even before nyctalopia is apparent), or in cases where the ocular fundus is obscured by cataract (or other opacification of the ocular media), the retina can be evaluated by electroretinography (Acland). Affected individuals show decreased amplitudes and increased implicit times for the various wave forms; there also is a delay in or loss of electrophysiological dark adaptation.

The electroretinogram especially is useful in a patient who has complete cataracts and is of a breed which is predisposed to progressive retinal degeneration. Before cataract extraction is done in this individual, an electroretinogram would be in order to be sure there is normal retinal function.

Treatment

There is no treatment for progressive retinal degeneration.

Selective breeding is the only control. Affected individuals or those with affected relatives should not be used for breeding.

Central progressive retinal degeneration

Unlike generalized progressive retinal degeneration, central progressive retinal degeneration initially involves the retinal epithelium (Aguirre and Laties; Bedford). It is called 'central' because the first ophthalmoscopic changes are visible in the central (posterior) region of the retina. From there, the degenerative process proceeds peripherally. The photoreceptor cells degenerate as the retinal epithelial cells become severely affected. This is a rare condition in this country. It also is called central progressive retinal atrophy.

Pathogenesis

The retinal epithelial cells accumulate a light brown pigment (lipofuscin) and become hypertrophied to form individual large cells or multicellular clumps of pigmented cells. The sensory retina is normal early in the disease, but soon the photoreceptor outer segments overlying the abnormal epithelial cells degenerate followed by degeneration of all outer retinal layers.

The process begins in the tapetal region lateral to and above the level of the optic disk, spreads to involve the entire tapetal region and eventually involves the peripheral part of the retina.

Experimental deficiency of vitamin E can lead to a similar disease (Riis, et al.) and many people believe that central progressive retinal degeneration is the result of nutritional problems perhaps associated with genetic predisposition.

Breeds affected

Labrador and golden retriever, Shetland sheepdog, border collie and others including mixed breeds. Primarily working breed dogs are affected.

Not recognized in most other nonhuman species; there are similar conditions in humans.

May be heritable, but the exact mode is not known.

Clinical signs and ophthalmoscopic findings

Visually guided behavior: Initially both day and night vision are clinically normal. Central vision is affected first and, as it becomes worse, only moving objects are seen (this is because peripheral fields are normal and movement stimulates peripheral fields).

Daytime vision may be more hampered initially because the pupil is smaller under these conditions and results in illumination of primarily the central part of the retina which is not functioning properly. As the disease progresses, vision deteriorates and total blindness may occur late in the disease.

Ophthalmoscopic findings:

Early - characteristic light brown pigment clumps are present lateral to and slightly above optic disk. This generally begins at 2-3 years of age.

Intermediate - increase in the number of pigment clumps, which spread into peripheral retina.

Late - thinning of retinal vessels (usually not to degree seen in generalized progressive retinal degeneration), tapetal hyperreflectivity, and loss of the pigment clumps seen initially.

Diagnosis

Remember that this is a rare condition and the likelihood that you will see a patient with it in this country is small.

History: Patient chases moving objects, but loses them when they become motionless.

Ophthalmoscopic changes: See above.

Visual behavior: Try rolling objects in front of the patient in an attempt to demonstrate the previously mentioned visual deficit.

Treatment

There is no treatment for central progressive retinal degeneration.

In early cases, pharmacologic dilatation of the pupils may provide some improvement in vision by allowing more peripheral retinal stimulation.

Control is by selective breeding or proper nourishment. Because there is a strong possibility that there is a genetic component to this disease, affected individuals or their relatives should not be used for breeding.

Cone degeneration (hemeralopia, day blindness)

This is a rare disease of dogs.

Pathogenesis

In the breeds studied, cones deteriorate starting by at least seven weeks of age with total loss of cones by 2-4 years of age. The cones are nonfunctional during this time even if structurally present. There may be an inability to regenerate visual pigment at an adequate concentration for day vision.

Breeds affected

Primarily the Alaskan malamute (Rubin, a, b) and poodle, in which the disease is a simple autosomal recessive trait.

Clinical signs and ophthalmoscopic findings

Affected individuals are day blind, but have normal vision under dim light conditions.

There are no ophthalmoscopic abnormalities because cones are in small numbers and their loss is not ophthalmoscopically detectable .

Pupillary responses to light are normal.

Diagnosis

Demonstration of day blindness - the patient sees when the illumination is dim, but becomes blind or has difficulty navigating when the lights are on.

Electroretinography is diagnostic because it shows absence of cone input into the response. Specifically, affected individuals cannot respond to high intensity, flickering light.

Treatment

There is no treatment for cone degeneration. As with other heritable retinal disease, selective breeding is the only control.

Feline central retinal degeneration

This originally was thought to be a specific disease of unknown cause which was only moderately progressive or non-progressive (Bellhorn, et al.). Affected individuals show no visual deficits, but have ophthalmoscopic abnormalities. Ophthalmoscopically, the lesion can range in size from a small, circular zone of retinal degeneration in the area centralis, to a larger, elliptical lesion with prominent medial extensions or satellites. Rarely, there may be large, band-shaped degeneration of the central retina. These lesions appear variably as hyperreflective or dark depending on the angle of incident light.

The electroretinogram in these patients demonstrates a generalized cone abnormality.

It has been shown that in cats fed a diet consisting of only dog-food, or where the only protein is casein, a similar disease (ophthalmoscopically) is produced (Aguirre). However, in these individuals, continuation of the diet causes the central lesion to progress eventually to complete retinal degeneration (with an appearance similar to that in the heritable forms). The specific problem is taurine deficiency in these diets. Taurine is an amino sulfonic acid which is necessary, at least in the cat, for normal photoreceptor function and structure.

An important question is whether cats with central retinal degeneration were actually nutritionally deficient at one time, but recovered before retinal damage had progressed too far, or whether central retinal degeneration and nutritionally induced retinal degeneration in the cat are separate diseases.

There are several points to consider concerning this issue:

Feline central retinal degeneration has not yet been shown to be heritable.

Improper nourishment of the cat is a significant cause of retinal degeneration whose progress can be stopped by returning to normal diet.

Cats should not be fed an all dog-food diet.

Inflammatory disease of the retina and choroid

Inflammatory diseases affecting the choroid usually have retinal involvement also and vice versa.

Although there are some specific diseases in which there is primary retinitis or choroiditis, secondary changes usually occur in the initially uninvolved tissue making it difficult to be sure which came first. When describing the inflammatory process, one usually places the primary target first. That is, chorioretinitis describes primarily choroiditis with secondary retinitis and retinochoroiditis indicates that retinitis is primary.

Many of the inflammatory diseases affecting these tissues are secondary to systemic disease.

This section will describe the general principles of inflammatory disease of the retina and choroid.

Clinical signs

Usually none unless both eyes are involved severely enough to cause blindness. However, if the inflammation is a manifestation of systemic disease, the patient may be showing other signs. In addition, if the choroidal inflammation extends far enough anteriorly to also cause anterior uveal inflammation, signs typical of uveitis may be present.

Ophthalmoscopic appearance

Depends on stage of the disease when examined.

Active inflammation: Retinal edema - affected regions appear dull, usually gray with blurred borders.

Exudation and cellular infiltration - may appear as gray or white zones, usually with indistinct borders . May extend into the vitreous and obscure visualization of other structures. Exudation into the choroid may break through the choriocapillaris and retinal epithelium to cause elevation of the sensory retina; this may become extensive enough to cause complete retinal separation.

Granulomatous inflammation usually has a characteristic appearance which somewhat depends on the region of the fundus. In the tapetal region, a choroidal granulomatous infiltrate often will destroy the overlying tapetum and the choroidal melanin becomes visible with the ophthalmoscope . The overlying retina will be blurred due to fluid and cellular infiltrate. The lesion, therefore, appears as a somewhat circular blurred zone with a dark brown center. In the nontapetal region, the granulomas may appear as zones of lighter pigmentation because the inflammatory cell infiltrate will dilute the color of the melanin in the choroid and retinal epithelium. Thus, you will see zones of light brown in the background of the patient's normal nontapetal fundus color . If the infiltrate is extensive and concentrated, the major color imparted will be from the inflammatory cells. Thus, you may see fluffy, white, cotton-like masses, particularly if they have invaded the retina .

Blood vessels may appear congested or may be cuffed by inflammatory cells. This cuffing obscures vessel detail.

Retinal bleeding may occur. The appearance of the blood indicates the level at which it is located:

Fan-shaped or streak-like - within nerve fiber layer.

Punctate circular (also called 'blot and dot') - within inner or outer plexiform layer.

Teardrop or crescent-shaped - so-called preretinal (between inner limiting membrane and nerve fiber layer).

Dull red with retina elevated - either beneath sensory retina or beneath retinal epithelium.

Blood in vitreous body - may gravitate to ventral part of fundus or remain stationary; obscures visualization of other structures and provides an intense red ocular fundus reflection.

Chronic inflammation: Edema and exudates - the amount depends on whether the inflammation has lessened or increased. Generally there is more organization of the inflammatory process so that the inflamed zones have more distinct borders.

Retinal folds may occur as organization continues (MacMillan). Contracting vitreous strands may cause separation of the retina in addition to folding.

Blood - usually resorbs in 1-2 weeks except from vitreous where resorption may take months.

Pigmentary changes - the retinal epithelial cells may degenerate, or may lose or increase their melanin content or they may become hypertrophic, hyperplastic or atrophic. These changes appear as hypomelanosis, hypermelanosis or mottling of the ocular fundus.

Inactive lesions: The appearance depends on the location (tapetal or non-tapetal) and the degree of cellular change which occurred.

Tapetal lesions - where the retina has been thinned, tapetal hyperreflectivity occurs. If there is gliosis, then the zone appears dull gray, but differs from active lesions in that the borders are distinct. Often there will be aberrant melanosis of the retinal epithelial cells so that these lesions may have melanin surrounding or within them .

Nontapetal lesions - most commonly appear as sharply demarcated hypomelanotic zones. Depending on retinal epithelial reaction, some lesions may appear hypermelanotic or a mixture of the two . In ocular fundi normally having very little melanin (e.g., in individuals with blue irides), hypermelanosis is a common reaction.

Blood vessels - may appear as white lines due to sclerosis. Inactive lesions generally are less vascular than normal.

Nongranulomatous versus granulomatous

It is helpful to differentiate between these two types of inflammation because specific diagnosis is more likely with granulomatous types. Unfortunately, most of the agents causing granulomatous retinal and choroidal disease are poorly manageable and the prognosis must remain unfavorable for retention of vision.

Treatment

If the cause can be determined, specific therapy should be instituted.

If a specific cause cannot be determined, therapy must be aimed at reducing the damage done by inflammation alone as well as treating the most likely cause of the problem. For the latter, considerations such as granulomatous versus nongranulomatous, the region of the country, your level of experience and so forth will be instrumental in making a decision (read: guess).

Anti-inflammatory: Systemic corticosteroids at fairly high concentrations for 2-3 weeks. This must be balanced against the possible adverse side effects on the immune system in patients with an infectious process. Although there are no specific guidelines here, sometimes the inflammation is a more serious problem for the eye than the possible adverse effects. In these cases, the use of steroids may be highly desirable. In other cases, it may be that the immunosuppression caused by steroids may cause the disease to progress. The use of nonsteroidal anti-inflammatory agents might be considered, but as of the time of writing this the complications from their use in dogs or cats, for example, were more serious than those for steroids. So, do you use steroids? I suggest you try them on a patient by patient basis and monitor the situation closely.

Antimicrobial: Broad spectrum antibiotics could be used systemically for 1-2 weeks. However, this is not warranted unless you have good reason to suspect, but cannot prove, an infection which would be susceptible to the antibiotic being used.

If you suspect granulomatous inflammation, it is reasonable to assume that it may be fungal in origin. In this case, it may be appropriate to start right off with systemic antifungal medication, even if you do not know which organism is present. Antibiotics having only an antibacterial spectrum would not be indicated in this case.

Prognosis

Must always remain guarded, but depends on cause.

Comments

Regardless of the cause of retinal or choroidal inflammation, the ophthalmoscopic signs usually are similar. You cannot determine the cause simply on the basis of fundus changes.

Many people have a tendency to attribute hyperreflective or other inactive lesions of the dog fundus to distemper. This is inaccurate at best. If you survey populations of any species, numerous inactive retinal lesions can be found. You can only speculate as to their cause.

Any time active retinitis or choroiditis is diagnosed, a complete physical examination is mandatory to rule out systemic disease.

Retinal separation

The clinical condition termed retinal detachment usually is a retinal separation anatomically. The term separation is preferable because the site of discontinuity is between the retinal epithelial cells and the photoreceptor outer segments (the retina does not detach from itself). If the retinal epithelium were to lose continuity with the choriocapillaris, this would be a true detachment; this is a rare occurrence.

Retinal separation usually is secondary to intraocular inflammatory disease, especially choroidal. However, it can occur in many other disease states or alone (idiopathic). Because retinal separation usually is secondary to other ocular or systemic disease, it should not be considered a specific entity in itself.

Some of the conditions in which retinal separation may include: Collie eye anomaly; retinal dysplasia; any abnormal ocular development; trauma to the head; exudation from choroid through retinal epithelium due to inflammation or bleeding; renal disease; hypertension; and vitreous inflammation with formation of contraction bands.

Clinical signs

If bilateral and complete, the patient will be blind; if unilateral, the patient will be blind on affected side.

The retina may be in apposition to the lens thus creating a leukocoria .

If the sensory retina still is attached at the optic disk and ora serrata, the separation is described as funnel-shaped or 'morning glory.'

If the sensory retina is torn from the optic disk or ora serrata, it is said to be dialyzed as well as separated.

If incomplete (terms used are bullous or flat separations), the degree of vision loss from a focal lesion is insufficient to result in signs we could recognize. It is important, therefore, to always examine the ocular fundus because such a lesion may suggest the presence of serious, but presently ill-defined disease.

Ophthalmoscopic findings

With complete separation, the retina easily is visible through the pupil, using a penlight after pupillary dilatation.

Incomplete separations appear as zones out-of-focus with their surroundings; if a vessel courses through the affected zone, it will appear elevated.

Tears may be associated with the separated tissue or there may be disinsertion from the strong attachments, called dialysis. In the latter, if the disinsertion is at the ora serrate, it is termed peripheral dialysis ; if at the optic disk, it is termed central dialysis.

Pupillary responses to light - may be normal, but there often is dilatation with incomplete response.

Diagnosis

Ophthalmoscopic findings are diagnostic.

Treatment and prognosis

Determine specific cause and treat accordingly if practical (e.g., retinal separation in a collie is not practical to treat).

Patient should have minimal exercise; sedate if necessary to keep calm.

Prognosis depends on cause and length of time retina has been separated (after a couple of weeks, degeneration in the outer retinal layers may not be reversible).

In addition to treatment of an underlying cause, try the following:

High concentrations of systemic steroids for at least 7-10 days.

Diuretics which are carbonic anhydrase inhibitors, such as dichlorphenamide, for at least 7 days. This recommendation is based upon some experimental work, but there is no clinical proof that it is beneficial.

The prognosis in purely idiopathic cases often is better because the retinal layers may reunite, but this depends on length of time of separation.

Peripheral cystoid retinal degeneration

From middle age onwards, the peripheral retina (adjacent to the ora ciliaris retina) begins to develop cysts of various sizes and other degenerative changes (Bellhorn and Haring). Through a widely dilated pupil, these can be seen as gray cystic zones and alteration of the melanin background. These are not considered pathological; they are an expected occurrence with aging.

Bracken induced retinal degeneration in sheep

This disease, also known as 'bright blindness,' has been reported in Great Britain and is due to ingestion of bracken (Pteris aquilina) (Barnett, et al.).

The affected sheep show no signs other than progressive retinal degeneration which clinically is similar to that seen in other species.

Stationary night blindness in the Appaloosa horse

The syndrome appears to be a stationary (vision does not deteriorate further) lack of night vision, and is diagnosed by having the patient traverse an obstacle course under dim light conditions (Rebhun, et al.). The ocular fundus appears normal and the retina is histologically normal. The reason for this abnormality is unknown, but it is heritable and affected horses or their relatives should not be used for breeding.

Light induced retinal degeneration

Ambient light, even at low intensities, can cause irreversible outer retinal degeneration in many species (Lanum). This is most pronounced in albino rodents such as the rat. These animals need cyclic light during a 24 hour day with approximately equal time on and off. Even one 24 hour period of continuous light will cause some damage; after several weeks of this, complete outer retinal degeneration occurs.

An important aspect of this phenomenon lies in the evaluation of drugs in toxicity trials or any experiment in which deleterious effects on the eyes are possible or being determined. In these cases one must be sure to provide lighting conditions satisfactory for the species involved.

This phenomenon has been observed using various light sources in rats, mice, rabbits, pigs, pigeons, primates and dogs (Buyukmihci). Bear in mind that this phenomenon requires manipulation of the individual's environment and is not necessarily a clinical problem when proper conditions exist for the species in question.

Proliferative optic neuropathy in horses

Occasionally, white or gray masses are seen on or near the optic disk in middle-aged or older horses (Vestre, et al.). They are vascularized and are sometimes multilobular. These are incidental findings. Vision is not affected clinically and the lesions appear to be stable so that even if some form of treatment could be devised, it would not be necessary.

The cause is not known, but some appear histologically similar to xanthelasmas seen in humans (due to lipid 'storage' disease).

Sudden acquired retinal degeneration (silent retina syndrome)

This is a condition recognized in middle-aged and older dogs. Affected individuals have sudden onset of blindness with no discernible ocular changes.

The following is a list of clinical signs or findings shared by many patients with this condition:

1. Tendency to be overweight
2. Polyphagia
3. No breed predisposition
4. Abnormal electroretinogram
5. Polyuria and polydipsia
6. Mid-dilated or widely dilated pupils with sluggish, incomplete responses to light
7. Normal appearance to ocular fundus

Histologically, these dogs have diffuse necrosis of the photoreceptor outer segments. Eventually, over a period of weeks to months, the photoreceptors are lost and the ocular fundus shows the changes typical of diffuse retinal degeneration as seen in the heritable forms of retinal degeneration.

No cause has been found for this condition. No treatment is available (van der Woerdt, et al.). If you see a dog whose signs fit this syndrome, you should refer the patient to an ophthalmologist for electroretinography and further work up.

Retinal neoplasia

Primary neoplasms of the retinas are rare and only one is of significance. This is retinoblastoma, which does not occur naturally in nonhuman animals (or at least has not been documented). It is of embryonic origin and usually occurs within the first few years of life in humans in whom it is highly malignant and often fatal.

Secondary neoplasms occur due to metastasis from other sites and include such entities as lymphosarcoma and reticulosis.

Diseases of the vitreous humor

As previously mentioned, most diseases of the vitreous humor are secondary to or inseparably a part of retinal, choroidal or optic nerve disease.

There are no known primary neoplasms of the vitreous body.

The following describes a few entities which you may encounter during routine ophthalmoscopic examination. Alone they do not produce clinical signs and no attempts at treatment should be made. If secondary to other disease, the primary disease should be treated.

Vitreous floaters

White or gray opacities of various sizes, usually in the form of strands. These are not uncommon in most domestic species and may increase in quantity as the individual ages. They move with ocular movements.

These must be differentiated from inflammatory exudates which represent retinal or posterior uveal inflammation (these appear darker and 'dirty' as opposed to the 'clean' appearance of benign floaters).

Asteroid hyalosis

This is a degenerative disease, generally of elderly individuals (usually dogs, horses or humans) and usually is unilateral (Rubin). The vitreous is in its gel state and contains numerous tiny, round, opaque white bodies which reflect light and appear as stars in the sky (hence its name). The opacities appear to vibrate with ocular movement.

These bodies are calcium-lipid complexes which generally do not interfere clinically with sight nor are they necessarily associated with other ocular diseases.

Syneresis

This is the liquefaction of the vitreous body and can be spontaneous (degenerative) or due to lens motion (in luxations or subluxations), ocular posterior segment inflammation or following vitreous hemorrhage. When the vitreous liquefies, aqueous humor replaces it.

Signs depend on the cause (see specific disease descriptions in other sections of these notes), but with a liquid vitreous, retinal separation always is a possible complication.

Synchysis scintillans

This occurs when the liquefied vitreous contains crystalline deposits (usually cholesterol, therefore it sometimes is referred to as cholesterolosis bulbi). It usually is the consequence of other pathologic processes, such as mentioned under syneresis.

The deposits are stirred up by ocular movement and settle downward when the eye stops moving (falling snow effect).

Footnotes:

Retinal folds and probability of heritability: Although non-heritable retinal folds may be present occasionally in any breed of dog, their presence in individuals of certain breeds such as the Labrador retriever or English springer spaniel must be taken as evidence that those individuals are affected by the heritable form of the disease.