The Retina and Optic Nerve

By | November 12, 2015

Tapetal coloration of the fundus of the puppy and kitten is usually gray or blue at 6 to 8 weeks of age, gradually acquiring its adult coloration by 4 to 7 months of age when the tapetum matures. Myelination of the optic disc may also be incomplete in the puppy and kitten, giving the impression of a small, well-defined nerve head that takes on a more fluffy appearance as adult myelination occurs.Both congenital and acquired disorders of the retina and optic nerve are recognized in the young dog and cat. These may be inherited, as with collie eye anomaly, or secondary to postnatal influences, as occurs with canine distemper-induced retinitis. Congenital abnormalities can be diagnosed as early as 6 weeks of age, when the posterior segment is clearly observed. The more common congenital abnormalities of the canine fundus are summarized in Table Congenital Abnormalities of the Canine Fundus. Acquired abnormalities develop with advancing age and in this discussion are limited to those in dogs and cats younger than 6 months of age.

Table Congenital Abnormalities of the Canine Fundus (diagnosed as early as 6-8 wk of age).

DISORDER BREED CHARACTERISTIC FEATURES
Collie eye anomaly Collie, Shetland sheepdog, border collie, Australian shepherd Chorioretinal hypoplasia, optic disc coloboma, retinal detachment
Retinal dysplasia Springer spaniel, Labrador retriever Multifocal retinal folds, retinal detachment
Cocker spaniel, beagle Multifocal retinal folds
Hemeralopia Alaskan malamute Day blindness, normal fundus
Night blindness Tibetan terrier, briard Night blindness, normal fundus
Optic nerve hypoplasia Any breed Visual deficit, small optic disc

Congenital Anomalies of the Posterior Segment

Collie Eye Anomaly. An autosomal recessive trait affecting about 80% of the breed, collie eye anomaly is characterized by an array of posterior segment abnormalities. Both smooth-coated and rough-coated collies are affected, regardless of color pattern. The disorder is usually nonprogressive and can be easily diagnosed in the 6- to 8-week-old puppy. Bilateral ophthalmoscopic lesions, in order of increasing severity, include choroidal hypoplasia, optic nerve and scleral colobomas, and retinal detachment. Affected eyes may demonstrate one or more of these lesions, although rarely symmetrically. Choroidal hypoplasia is a common finding in affected animals and appears to have minimal effect on functional vision. The lesion appears as a focal abnormality temporal to the optic disc, usually at the junction of the tapetal and nontapetal fundus (). Mildly affected puppies may demonstrate choroidal hypoplasia at 4 to 6 weeks of age, only to have the lesion masked by pigmentation as the eyes mature.

A coloboma occurs in about 20% to 30% of the breed. When the optic disc is affected, its surface appears excavated; overlying retinal vessels change course abruptly (). Scleral colobomas (also known as ectasias or staphylo-mas) produce similar excavations in the fibrous tunic and alterations in the course of the overlying vessels (). Visual field defects probably accompany large colobomas, but vision is functionally unimpaired. About 5% to 10% of affected animals experience retinal detachments. These may be congenital or may occur in dogs up to 1 year of age with extensive colobomas.

Because of the high prevalence of collie eye anomaly, breeders have continued to breed affected animals. Owners should be warned that the breeding of mildly affected collies may produce offspring with more serious ocular defects than their parents. The Shedand sheepdog, border collie, and Australian shepherd also have a congenital ocular anomaly similar to that of the smooth-coated and rough-coated collie. The prevalence in the Shetland sheepdog is less than 20%, and its mode of inheritance is not known.

Microphthalmia with Colobomas. Large equatorial staphylomas occur in Australian shepherds with multiple ocular anomalies. These areas of thin, stretched sclera are up to 20 diopters deep. Associated ocular defects include retinal detachment, retinal dysplasia, pupillary abnormalities, and cataracts. The disorder is inherited as an autosomal recessive trait with incomplete penetrance. Affected dogs are homozygous merles and have a predominantly white haircoat. A similar syndrome occurs in merled Shetland sheepdogs, merled rough-coated collies, and harlequin Great Danes.

Retinal Folds. Retinal folds predominantly in the nontapetal portion of the fimdus are seen frequently in young collies and occasionally in other breeds of dog. They appear as vermiform streaks because of their rod-like shape and pale color (). Retinal folds should be differentiated from the retinal disorganization that occurs in retinal dysplasia.

Heritable Retinal Dysplasia. Retinal dysplasia is characterized by folds in the outer retinal layers and by retinal rosettes, in which variably differentiated retinal cells are arranged around a central lumen. Severe forms of retinal dysplasia may demonstrate retinal detachment due to subretinal fluid accumulation. The diagnosis is easily made in the 6- to 8-week-old puppy or kitten. Many breeds of dog have retinal dysplasia alone or in association with other congenital ocular defects. Retinal dysplasia is uncommon in the cat.

English springer spaniels inherit retinal dysplasia as an autosomal recessive trait. Affected animals exhibit bilaterally nonprogressive, multifocal lesions that appear as round or linear areas of discoloration in the tapetal fundus (). As the tapetum matures, the margins of the dysplastic foci appear hyperreflective, while their centers are darkly pigmented. Vision is usually spared. Blindness results when retinal tears or detachment accompany severe disorganization of the retina.

Several types of retinal dysplasia occur in Labrador retrievers. One form closely resembles that of the English springer spaniel, with multifocal folds and rosettes within the tapetal portion of the fundus. Another form is characterized by totally detached dysplastic retinas and blindness (). Both forms are recessively transmitted. Severe retinal dysplasia also occurs in association with short-limbed dwarfism. The gene has recessive effects on the skeleton and incompletely dominant effects on the eye. Affected animals demonstrate retinal detachments, corneal opacification, cataracts, and vitreal degeneration.

In vitreoretinal dysplasia of the Bedlington terrier, most affected dogs have a complete retinal detachment and are blind at initial examination. Sealyham terriers also present with totally detached dysplastic retinas and blindness, as well as microphthalmia. Multifocal retinal dysplasia is detected in American cocker spaniels. In the tapetal fundus, dysplastic foci appear as small linear or branching folds that are less reflective than adjacent unaffected tissue. In the nontapetal fundus, the folds appear white or gray in comparison with the normally pig-mented background. Blindness or other apparent visual deficits have not been observed in these affected animals. A recessive mode of inheritance is likely. Beagles have multifocal retinal dysplasia that is similar to that of the American cocker spaniel. Other dog breeds in which retinal dysplasia are reported either singly or in association with multiple ocular defects include the Akita, Australian shepherd, Doberman pinscher, Old English sheepdog, Rottweiler, and Yorkshire terrier.

Nonheritable Retinal Dysplasia. The retina continues to mature until 6 to 8 weeks of age in both the puppy and kitten. Infectious agents incriminated in the dog to cause retinal dysplasia are canine herpesvirus, canine parvovirus-2, and canine adenovirus-1. In the kitten, irregular areas of atrophy in the tapetal fundus and pigment mottling in the nontapetal area follow in utero or postnatal feline parvovirus infection. Feline leukemia virus infection may also induce retinal dysplasia.

Tapetal Hypoplasia. Beagles with no visible tapetum and a uniform reddish brown fundus reflex are occasionally seen. The lack of tapetal development is inherited as an autosomal recessive trait. Lack of visible tapetum may occur infrequently in other breeds of dog and appears to have no discernible effect on vision. Cats affected with Chediak-Higashi syndrome may also have a reddish fundic reflection due to decreased tapetal development and fundic hypopigmentation.

Congenital Night Blindness. Night blindness is first evident by 6 weeks of age in the briard and Tibetan terrier. The fundus of the briard appears normal, whereas low-level illumination shows an increased tapetal granularity in the Tibetan terrier. Electroretinographic (ERG) studies confirm the presence of congenital retinal dysfunction. The inheritance pattern is not known, but an autosomal recessive mode is suspected.

Hemeralopia. Day blindness occurs in Alaskan malamutes, with onset of behavioral changes as early as 8 to 20 weeks of age. Hemeralopia also occurs in the miniature poodle by 3 months of age. Dogs are visually impaired in daylight but function well at night and on overcast days. The fundus appears normal in affected animals. Diagnosis is confirmed on ERG evidence of abnormal cone response.

Feline Retinal Degeneration. Hereditary retinal degeneration is uncommon in the cat. Diffuse outer segment retinal atrophy occurs in Persian kittens with signs of tapetal hyperreflectivity and retinal vessel attenuation. An autosomal recessive trait is suspected. Photoreceptor degeneration also occurs in domestic short-haired cats with visual impairment as early as 14 weeks of age and fundic signs similar to those in Persian cats. A dominant inheritance pattern is suspected.

GM, Gangliosidosis. Fundic lesions occur in 3-month-old kittens with lysosomal storage disease, although the progressive ataxia and tremors demonstrated by affected mixed-breed and Siamese cats are far more impressive. Accumulations of glycolipid within retinal ganglion cells appear as small dark spots in the tapetum and pale gray spots in the nontapetal fundus. The disorder is transmitted as an autosomal recessive trait in the Siamese cat.

Optic Nerve Hypoplasia. Unilateral or bilateral optic nerve hypoplasia occurs infrequently in various breeds of dog, including the beagle, dachshund, collie, Russian wolfhound, German shepherd dog, Great Pyrenees, and Saint Bernard. Optic nerve hypoplasia is inherited in the miniature poodle. Unilateral lesions are often incidental findings because the dog compensates with the nonaffected eye. Poor vision is the usual presenting complaint in bilaterally affected animals, although owners may fail to recognize the vision problem when the animal is with its siblings. Affected eyes exhibit sluggish to absent direct pupillary light reflexes. Resting pupil size may be larger than normal. The affected optic disc is often less than half its normal size, its center is depressed, and the periphery is pigmented (). The retinal vasculature is usually normal. Complete aplasia of the optic nerve is uncommon.

Optic Nerve Colobomas. Colobomas of the optic disc are most often associated with collie eye anomaly. Colobomas also occur as one of a multitude of ocular anomalies found in the Australian shepherd and basenji and occasionally in the cocker spaniel. Choroidal and optic nerve colobomas are occasionally seen in domestic cats, including the domestic short-haired and Persian cats.

Acquired Disorders of the Posterior Segment

Progressive Retinal Atrophy. Progressive retinal atrophy (PRA) is an inherited disorder affecting the retinal photoreceptor layer. Early-age onset PRA occurs in the collie, Irish setter, and Norwegian elkhound and possibly also in the miniature schnauzer, Tibetan terrier, Cardigan Welsh corgi, and miniature longhaired dachshund. Dogs with PRA first demonstrate their visual deficit in dim lighting. Progressive loss of day vision occurs, followed by total blindness. The pupils dilate as PRA advances, and the owners may comment on the bright green or yellow tapetal reflection from the eyes. Total blindness usually occurs in 6 to 18 months after the onset of night blindness. If the dog’s environment is changed suddenly, owners may mistake the dog’s disorientation for acute vision loss. Progressive retinal atrophy is characterized by bilateral attenuation of the retinal vessels, involving first the peripheral arterioles and later the retinal veins. Altered tapetal reflectivity also occurs; as the retina thins and absorbs less light, the tapetum appears increasingly granular and hyperreflective (). Optic disc pallor and total absence of retinal vessels typify the most advanced cases of PRA.

Several types of PRA occur in the dog. The PRA in the collie, Irish setter, and Cardigan Welsh corgi is characterized as rod-cone dysplasia. Dysplastic abnormalities are evident as early as 24 days of age. Affected animals may demonstrate night blindness by 6 weeks, although the first detectable fundic evidence occurs at 14 to 16 weeks. Total blindness occurs by 1 year of age. The PRA is inherited as an autosomal recessive trait. A blood-based DNA test is available to identify the gene mutation responsible for rod-cone dysplasia in the Irish setter and Cardigan Welsh corgi. Two progressive retinop-athies occur in the Norwegian elkhound: rod dysplasia and early retinal degeneration. Dogs affected with rod dysplasia show night blindness at 6 months of age but may retain some vision until 3 to 5 years of age. An ERG may detect rod dysplasia as early as 6 weeks of age, but only subtle changes in tapetal granularity are seen in the 6-month-old dog. Easily detectable fundic signs occur at 1 to 3 years of age. The signs of early retinal degeneration occur much earlier than those of rod dysplasia. Affected puppies are night blind by 6 weeks of age and totally blind by 1 year of age. Fundic changes are detectable at 6 months of age and advanced by 1 year. The mode of inheritance is autosomal recessive. A progressive retinal degeneration that differs from classic PRA occurs in the borzoi and papillon as early as 6 months of age. Initial lesions may be unilateral, with focal areas of hyperreflectivity in the extreme peripheral tapetum. These areas eventually coalesce into a diffuse retinal degeneration.

Hereditary retinal degeneration is uncommon in young cats. Signs of tapetal hyperreflectivity and retinal vessel attenuation are similar to those in affected dogs. An early-age onset rod-cone dysplasia occurs in the Abyssinian cat, inherited as an autosomal dominant trait. An increase in pupil size between 2 and 3 weeks of age is followed by changes in tapetal reflectivity by 8 weeks of age. Progressive vascular attenuation results in an avascular retina by 1 year of age. The ERG is unrecordable as early as 17 days of age.

Nonheritable Retinopathies. Primary retinitis is uncommon in the young dog and cat. More often, evidence of active retinitis accompanies choroidal disease and consists of cellular infiltration, exudation, edema, or hemorrhage. The retinitis appears as ill-defined areas of diminished tapetal reflectivity or nontapetal discoloration (). Inflammatory exudate from the choroid or the retina may separate the inner neural retina from the underlying pigment epithelium, causing a retinal detachment. Inactive retinitis lesions are well marginated, appearing hyperreflective in the tapetal region and depigmented in the nontapetal area (). Pigment clumping may be seen at the center of inactive inflammatory foci. Infectious causes of chorioretinitis in the young dog include canine distemper, toxoplasmosis, bacterial septicemia, and intraocular larval migrans by Toxocara cants. Causes in the young cat include toxoplasmosis, lymphosarcoma, feline infectious peritonitis, and ophthalmomyiasis. Noninfectious causes range from toxins such as ethylene glycol to embryonic neuroepithelial tumors such as medulloepithelioma. Treatment is generally directed at the underlying systemic disease, remembering that topical medication does not reach therapeutic levels in the retina or the choroid.

Posterior segment hemorrhage may occur as a nonspecific component of chorioretinitis or may accompany one of several bleeding disorders, including anemia, immunemediated disease, thrombocytopenia, or polycythemia. Retinal vascular congestion may occur with congenital heart defects, such as atrioventricular septal defects and tetralogy of Fallot, and secondary to systemic hypertension. Retinal hemorrhage and venous engorgement may occur in a cat with thiamine deficiency. Classically, taurine deficiency is seen in cats maintained on a taurine-poor diet. Retinal changes usually require several months to develop, but lesions have been identified within 18 weeks of beginning a taurine-poor diet. A bilaterally symmetric, hyperreflective ellipse develops temporal to the optic disc. This progresses to a horizontal band-shaped lesion across the posterior pole (). Progression results in generalized retinal atrophy and irreversible blindness.

Optic Neuritis. Optic neuritis is characterized by blindness during active inflammation. Bilateral cases show fixed, dilated pupils. The affected optic disc appears enlarged and hyperemic, with indistinct margins, peripapillary retinal elevation, and adjacent hemorrhage (). The causes of optic neuritis are varied and in the young dog or cat usually include infectious diseases and traumatic insult. Treatment is directed at the underlying cause. Systemic corticosteroids are indicated to minimize the amount of nerve damage. Oral prednisolone (1 to 2 mg/kg) is administered daily for 14 days, then gradually decreased in dosage over 2 to 3 weeks until an alternate-day maintenance dose is reached. In general, if medical therapy is successful, improvement will be seen within 7 to 10 days. Optic atrophy and permanent blindness are common sequelae.

Optic Nerve Edema. Papilledema may mimic optic neuritis on fundic examination; vision and pupillary light responses are usually intact. Papilledema may be associated with increased cerebrospinal fluid pressure in congenital hydrocephalus or may be secondary to orbital inflammation. The treatment of papilledema is directed at the primary cause and, in general, has a better prognosis than does optic neuritis.

Optic Nerve Atrophy. Progressive degeneration of the optic nerve may follow optic neuritis, chronic papilledema, traumatic insult, advanced retinal degeneration, glaucoma, or demyelinating disease. The optic disc appears depressed, shrunken, and pale, and there is diminished blood supply (). The affected eye is blind. Optic atrophy is an irreversible process that does not respond to therapy.

 

Selections from the book: “Veterinary pediatrics: dogs and cats from birth to six months”. Johnny D. Hoskins. (2001)

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