Chapter 138 Diseases of the Retina, Choroid, and Optic Nerve
EVALUATION OF THE POSTERIOR SEGMENT
Ophthalmoscopy is the technique used to examine the fundus (see Chapter 131). The technique is ideally performed after dilating the pupil (except in an animal with ocular hypertension, glaucoma, or lens luxation) with a short-acting parasympatholytic drug such as tropicamide (Mydriacyl 0.5%, Alcon).
• Direct ophthalmoscopy with a hand-held ophthalmoscope is commonly used in practice. Unfortunately, this technique yields limited information about the fundic appearance even in the hands of experienced ophthalmologists. Small lesions and variations in contrast between normal and abnormal areas of the fundus are often overlooked when utilizing the direct technique.
• Indirect ophthalmoscopy, utilizing a 20D planoconvex lens held 2 to 3 inches from the animal’s eye and illuminated with a penlight held at arm’s length from the animal, is more useful as a diagnostic technique.
• Ocular ultrasonography and color-flow Doppler ultrasonography are utilized to evaluate the structures of the posterior segment when opacities of the ocular media prevent an ophthalmoscopic examination.
• Electroretinography employs a light stimulus of varying intensity and frequency to elicit a retinal response from rod and/or cone photoreceptors. This test evaluates the functional integrity of the retina and is a procedure that requires referral to a specialist. Neuroophthalmology is discussed in Chapter 141.
DISEASES OF THE VITREOUS HUMOR
Etiology
Persistent Hyperplastic Primary Vitreous (PHPV)/Persistent Tunica Vasculosa Lentis
• Persistent hyperplastic primary vitreous (PHPV)/persistent tunica vasculosa lentis is inherited in some breeds, notably the Doberman pinscher. The hyaloid artery is retained and is hyperplastic and associated with vessels that ramify over the posterior lens surface. Blood vessels and associated posterior lens capsular pigmentation and opacities (cataracts) or intralenticular hemorrhage is observed clinically. Color-flow Doppler ultrasonography may enable diagnosis and aid in predicting prognosis in cases of PHPV where the posterior segment cannot easily be seen by ophthalmoscopy. Surgical therapy yields poor results. Do not use these animals for breeding.
Asteroid Hyalosis
• Asteroid hyalosis is a unilateral or bilateral degenerative change associated with aging in dogs. Calcium-lipid complexes are deposited on the vitreous protein matrix. These appear as multiple white sparkles that oscillate slightly with eye movements. Asteroid hyalosis does not significantly impair vision in most dogs although advanced lenticular nuclear sclerosis and severe asteroid hyalosis in old dogs may reduce vision to some extent. No treatment is indicated.
Cholesterosis Bulbi
• Cholesterosis bulbi or synchysis scintillans is a far less common entity in which white cholesterol crystals are deposited in the vitreous secondary to intraocular inflammation. The vitreous often will become liquefied (syneresis) as a result of posterior segment inflammation. The cholesterol deposits sink to the bottom of the vitreous cavity. When the eye moves, the deposits swirl in the liquid vitreous before settling again under gravity. The initial cause of ocular inflammation and vitreous liquefaction may often have subsided by the time cholesterosis bulbi is detected. Vision is not affected by the cholesterosis bulbi per se, although it may be compromised by the underlying inflammatory disease.
Exudate and Cellular Infiltrate
• Exudate and cellular infiltrate appear as hazy, white, or gray material in the vitreous or on the posterior capsule of the lens. Pigment may also be deposited in the vitreous. Cellular infiltrate may become dense enough to obscure a view of the retina. Investigate for anterior uveitis or chorioretinitis (see later section).
Diagnosis
• Ophthalmoscopic examination with a focal light source (penlight) or an ophthalmoscope will reveal the nature of the opacities (asteroid hyalosis, hemorrhage, pigment).
• Submit blood for a clotting profile when vitreous hemorrhage occurs unassociated with ocular trauma or intraocular surgery.
• Perform physical examination, complete blood count, serum biochemistry, and serology evaluations for infectious inflammatory disease (see Chorioretinitis) in cases in which vitreous exudate or cellular infiltrate is seen.
Treatment
• Vitreous hemorrhage resorbs slowly over the course of several weeks. Treat the coagulopathy systemically when possible.
• Exudate and infiltrate decrease by treating the underlying inflammatory disease with systemic antibacterial, antifungal, and nonsteroidal anti-inflammatory drugs (NSAIDs), as indicated. Corticosteroids can be used in dogs and cats once infectious etiologies are ruled out (prednisone or prednisolone, 1–2 mg/kg, q12–24h PO). NSAIDs (aspirin, 25 mg/kg q12h PO in dogs only) often improve ocular inflammatory disease.
CONGENITAL DISEASES OF CHORIORETINA AND OPTIC NERVE
Etiology
Collie Eye Anomaly
• Collie eye anomaly (CEA) is a disorder that occurs commonly in rough and smooth collies and less often in Shetland sheepdogs and some other breeds (border collies, Australian shepherds, Lancashire heelers). CEA affects the retinal the choroid and retinal pigment epithelium and, in severe cases, the retina and optic nerve. The disease is usually asymmetric in the two eyes.
• The most characteristic lesion is choroidal hypoplasia (an area of thin, poorly vascularized choroid), which appears as a pale area of fundus temporal to the optic nerve. The lesion is one of depigmentation of the retinal pigment epithelium and choroid and focal absence of tapetum exposing hypoplastic choroidal blood vessels overlying the sclera. Occasionally, hazy white corneal opacities accompany the chorioretinal lesions.
• In severe cases, colobomas (gaps or holes due to incomplete development) may occur in the optic nerve or choroid. These are often circular or oval lesions which may appear out of focus compared with the surrounding structures when seen by ophthalmoscopy. Retinal detachment or intraocular hemorrhage is found in the most severe cases.
• It has long been believed that the disease has a simple autosomal recessive means of inheritance, although recent studies of large numbers of dogs in Scandinavia have suggested that the inheritance is either polygenic or that choroidal hypoplasia and coloboma are inherited as separate traits. However, a gene for this disease has been located on canine chromosome number 37 and the recessive disease-causing mutation has been identified. Affected dogs are homozygous recessive—that is, both copies of the gene are mutant. All dogs that are homozygous recessive affected will show at least the mild form of the disease (choroidal hypoplasia) and may have more severe manifestations (coloboma, retinal detachment). Genetic linkage testing (from blood sample) can distinguish normal, carrier, and affected dogs (Optigen, LLC, Ithaca, NY; see www.optigen.com).
• Advise breeders to breed together genetically normal dogs (difficult in some breeds due to high incidence of affected animals) or carriers with subsequent testing to identify and eliminate affected dogs from the breeding program.
Retinal or Vitreoretinal Dysplasia
• Retinal or vitreoretinal dysplasia occurs in cats and dogs. The disease may be inherited (often recessively) in the dog or may be a sequela to intrauterine infection (canine herpesvirus, feline panleukopenia).
• In the dog, inherited retinal dysplasia may be manifested as bilaterally asymmetric, small focal folds in the outer layers of the retina that do not result in retinal elevation. These folds are observed in the cocker spaniel and the Labrador retriever. The folds may appear as small dark dots, streaks, or circles in the tapetal fundus and gray or dark spots or streaks in the non-tapetal fundus. In some breeds (Sealyham and Bedlington terriers, Labrador retrievers, springer spaniel), the lesion is severe, and the dog is blinded by the retina lying unattached in abnormally formed vitreous.
Optic Nerve Hypoplasia
• Optic nerve hypoplasia, if bilateral, may be noted by breeders or owners in animals a few weeks old. The disease may be inherited in certain breeds, including the miniature poodle and dachshund. Affected eyes lack pupillary light reflexes and are blind. Ophthalmoscopy reveals a small optic nerve head with normal retinal vasculature.
Diagnosis
Congenital abnormalities of the fundus are most readily diagnosed by the ophthalmoscopic appearance at 4 to 8 weeks of age. Some dogs have been seen to develop circular (or geographic) folds and dysplastic lesions at several months of age. Perform a second examination at 6 to 12 months of age in susceptible breeds. Genetic testing is available for dogs that might be affected with collie eye anomaly and can distinguish normal, carrier, and affected animals (Optigen, LLC, Ithaca, NY; see www.optigen.com).
ACQUIRED DISEASE OF THE CHORIORETINA AND OPTIC NERVE
Chorioretinal Degeneration
Etiology
Progressive Retinal Atrophy (PRA)
• Progressive retinal atrophy includes several genetic retinal diseases in which the ultimate outcome is degeneration and loss of retinal structure with varying degrees of blindness. The different forms of PRA are all progressive and bilaterally symmetric. The disease occurs commonly in many purebred and mixed-breed dogs. The inheritance in most instances in the dog is autosomal recessive. Recessive sex-linked PRA is seen in the Siberian husky and Samoyed. PRA also occurs in various breeds of cat, although it has been best characterized in the Abyssinian.
• PRA may primarily affect the photoreceptors (rods/cones) or the adjacent layer of retinal pigment epithelium. Pigment epithelial dystrophy, or central PRA, is extremely rare in the United States and apparently on the decline in other parts of the world, including Europe. PRA affecting the rods or cones can be distinguished by the age of onset. In early-onset PRA (rod, rod-cone dysplasias), affected dogs show signs of night blindness during the first year of life.
• Affected breeds include the collie, Irish setter, miniature schnauzer, elkhound, and miniature dachshund. In late-onset PRA (progressive rod-cone degeneration), night blindness may be noticed at any age from 1 year on. Breeds affected include the miniature poodle, cocker spaniel, and Labrador retriever.
• The typical ophthalmoscopic appearance of diffuse retinal degeneration is an increased reflectance of light from the tapetal fundus (tapetal hyper-reflectivity). Retinal blood vessels are narrowed or lost, and depigmentation and patchy hyperpigmentation in the non-tapetal fundus may be seen. In advanced cases, atrophy of the optic nerve occurs.
• The various forms of PRA are gradually being identified at the molecular genetic level. In 2005 the DNA markers near the gene for late-onset progressive rod-cone degeneration (prcd-PRA) on chromosome 9 had been identified in a dozen canine breeds. Separate genes for early-onset rod-cone type dysplasia had been identified in a smaller number of breeds. Tests based on these genetic associations may either allow very accurate testing (gene mutation–based tests) or relatively accurate testing (linkage-based tests) of blood samples from dogs that are either normal, carriers, or affected with the disease (Optigen, LLC, Ithaca, NY; see www.optigen.com for details of interpretation of the various tests).
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