Canine Ocular Fundus and Optic Nerve

13
Canine Ocular Fundus and Optic Nerve

Diseases of the ocular fundus include those that affect the retina, choroid, and optic nerve head (also referred to as the posterior segment). They are congenital or developmental in origin, or inflammatory, traumatic, neoplasitic, or degenerative. For examination purposes, the canine ocular fundus is divided into tapetal fundus, nontapetal fundus, optic nerve head, and the retinal vasculature. A complete fundic examination evaluates the size and shape of the optic nerve head, the perfusion of the retinal arterioles and venules, and the state and position of the retina. There are many variations in the normal ocular fundus that will become apparent on ophthalmoscopy. It is important to perform routine ophthalmoscopy on all patients so that these variations can be documented, which will facilitate the early detection of developing pathology.

Recent advances in the development of specific DNA mutation tests for retinal diseases in purebred dogs have markedly increased our knowledge of these diseases and our ability to detect carrier and affected dogs prior to the onset of vision impairment.

Normal Ocular Fundus and Variations

The canine ocular fundus has considerable variations, and this often confuses the novice attempting to learn ophthalmoscopy at the same time as diagnosing fundus disease. Those breeds with blue irides or heterochromia iridis, and breeds with merling (or dappling) can have markedly different appearances of the ocular fundi between fellow eyes but may nevertheless be normal in both eyes (Figure 13.1).

Fundus photograph of a 10‐week‐old puppy. — **Figure 13.1** There are considerable normal variations of the ocular fundus and optic nerve head or disc variations among more than 250 recognized dog breeds. Coat color and degree of pigmentation have a significant impact on the appearance of the fundus. (A) Ocular fundus of a 10‐week‐old puppy. The tapetal fundus is blue and will continue to change color until eye development is complete (about 16 weeks of age). (B) The most common ocular fundus appearance in the dog. The dorsal triangular tapetal fundus is yellow–green; the nontapetal fundus is very dark brown or nearly black; the optic disc is at or slightly below the junction of the tapetal and nontapetal fundi; and the retinal blood vessels emerge from the disc’s surface and periphery. Position of the optic disc varies by breed and may be present totally within the tapetal fundus, the nontapetal fundus, or at their junction. (C) Completely pigmented ocular fundus, without any observable tapetal fundus in a mature Boston Terrier. A light orange blood vessel adjacent to the dorsal retinal blood vessels is within the chorioid. (D) A sub‐albinoid ocular fundus in a dog with heterochromia iridis. No tapetum is observable. Sometimes some pigmentation will be present in the nontapetal fundus. Because of the paucity of pigmentation, the deep choroidal larger diameter blood vessels can be clearly visualized.

Fundus photograph displaying dorsal triangular tapetal fundus, nontapetal fundus, and optic disc at or slightly below the junction of the tapetal and nontapetal fundi of a dog. — **Figure 13.1** There are considerable normal variations of the ocular fundus and optic nerve head or disc variations among more than 250 recognized dog breeds. Coat color and degree of pigmentation have a significant impact on the appearance of the fundus. (A) Ocular fundus of a 10‐week‐old puppy. The tapetal fundus is blue and will continue to change color until eye development is complete (about 16 weeks of age). (B) The most common ocular fundus appearance in the dog. The dorsal triangular tapetal fundus is yellow–green; the nontapetal fundus is very dark brown or nearly black; the optic disc is at or slightly below the junction of the tapetal and nontapetal fundi; and the retinal blood vessels emerge from the disc’s surface and periphery. Position of the optic disc varies by breed and may be present totally within the tapetal fundus, the nontapetal fundus, or at their junction. (C) Completely pigmented ocular fundus, without any observable tapetal fundus in a mature Boston Terrier. A light orange blood vessel adjacent to the dorsal retinal blood vessels is within the chorioid. (D) A sub‐albinoid ocular fundus in a dog with heterochromia iridis. No tapetum is observable. Sometimes some pigmentation will be present in the nontapetal fundus. Because of the paucity of pigmentation, the deep choroidal larger diameter blood vessels can be clearly visualized.

The tapetum develops during the first 16 weeks of life through a series of color changes (black or gray to lavender or purple to blue followed by the adult coloration of yellow–orange or yellow–blue–green). The tapetum is a roughly triangular area in the dorsal ocular fundus that is thought to assist vision in reduced illumination. The retinal pigment epithelium in the tapetal fundus has limited melanin in its cytoplasm, permitting the deeper tapetal layer (within the anterior choroid) to be visible behind the retina. In the merled and toy breeds, the tapetal fundus can be markedly reduced in size, occur as multiple tapetal “islands,” or be totally absent.

The nontapetal fundus is normally dark brown to black and derives its color from the melanin granules within the retinal pigment epithelium. When heterochromia iridis is present, the nontapetal fundus is only lightly, or partially, pigmented, permitting visualization of the deeper choroidal blood vessels (“tigroid” nontapetal fundus), or nonpigmented (allowing visualization of the deeper choroidal blood vessels against the white scleral background). The junction between the nontapetal and tapetal fundus is often not a distinct line, but a gradual transition in the nontapetal pigmentation and increased thickness and color of the tapetum. Within the tapetal fundus, slightly temporal and dorsal to the optic disc is the area centralis (a cone‐rich area).

The optic nerve head (also called the papilla or optic disc) can be located within the tapetal fundus, within the nontapetal fundus, or at the junction of the two. When the optic nerve head is in the tapetal fundus, it is often surrounded by a pigment ring. Its surface is myelinated, and from its periphery arise the primary retinal arteries and veins. The shape of the pink to white disc ranges from round to oval to quite irregular. There is often a central depression, the physiologic cup, which was formed by the prenatal atrophy of the hyaloid system.

The canine ocular fundic vasculature is holangiotic, and consists of about 15–20 arterioles, and 3–4 primary larger arteries and veins. The arterioles are lighter red while the veins carrying deoxygenated blood are darker red. Sometimes, a partial to complete venous circle is apparent on the disc’s surface.

Congenital Diseases

Congenital diseases of the canine fundus are not uncommon and occur mostly in selected breeds. Because the canine ocular fundus does not reach maturity until about 15 weeks of age, ophthalmoscopic examinations in puppies at weaning time (about 6–8 weeks of age) does not allow detection of all congenital anomalies. Subsequent re‐examination at 4–6 months of age, when congenital defects are more apparent, is advisable. The continuing development of DNA or genetic tests has been beneficial to confirm clinical diagnoses as well as detect carriers of selected diseases, to permit rationally programmed elimination of a disease. Total reliance on DNA testing is not recommended as periodic ophthalmic examinations are indispensable to detect new variations of the disease or a new mutation.

Collie Eye Anomaly

Collie eye anomaly (CEA) is one of the most investigated posterior segment anomalies in the dog. In the 1960s and early 1970s, this anomaly was also called posterior scleral ectasia or posterior staphyloma. This defect appears inherited as an autosomal recessive trait; the optic nerve defects (optic nerve colobomas or “pits”) can be transmitted as an autosomal dominant trait. Recently, a study in Sweden challenged the mode of inheritance.

This disease affects Rough and Smooth Collies (80–90% of individuals of these breeds are affected), Shetland Sheepdog (5% in the USA to 60% in the UK), Border Collie (<5%), Australian Shepherd (<5%), and, more recently, has been reported in the Nova Scotia Duck Tolling Retriever and the Lancashire Terrier.

By ophthalmoscopy, the ocular fundus disease is characterized as normal, “go normal,” or affected. The “go normals” are adult dogs that as puppies had very small focal areas of choroidal hypoplasia that eventually pigmented and now appear normal (although these dogs breed as affected animals). CEA manifests with blood vessel tortuosity, focal choroidal hypoplasia lateral to the optic disc (100%), posterior colobomas (papillary or peripapillary staphylomas or “pits”) (10–20%), retinal detachments (5%), and intraocular hemorrhage (Figure 13.2).

Fundus photograph displaying focal choroidal hypoplasia lateral to the optic disc in a dog with Collie eye anomaly. — **Figure 13.2** Collie eye anomaly (CEA). (A) The basic abnormality of Collie eye anomaly is focal choroidal hypoplasia lateral to the optic disc. (B) The more severe defect, optic nerve coloboma, affects about 20–30% of Collies. (C) Colobomas (arrow) can also occur near but distinct from the optic disc. Note the focal choroidal hypoplasia area. (D) Retinal detachments develop in about 2–5% of affected Collies, usually near the optic disc and often associated with optic disc colobomas. The detached and edematous retina is protruding forward of the optic disc. (E) Vermiform streaks are occasionally observed in young Collie puppies. These streaks represent an imbalance of growth between the retina and outer choroid and sclera. They disappear over several weeks. (F) Young (8 weeks) Collie with very small choroidal hypoplasia. This lesion is quite small and a candidate for “go‐normal.” As adults, the “go‐normal” Collies appear normal but breed as CEA affected dogs. This puppy also has early progressive retinal atrophy (rod–cone dysplasia) and is night blind.

Fundus photograph displaying a more severe defect, optic nerve coloboma in a dog’s eye. — **Figure 13.2** Collie eye anomaly (CEA). (A) The basic abnormality of Collie eye anomaly is focal choroidal hypoplasia lateral to the optic disc. (B) The more severe defect, optic nerve coloboma, affects about 20–30% of Collies. (C) Colobomas (arrow) can also occur near but distinct from the optic disc. Note the focal choroidal hypoplasia area. (D) Retinal detachments develop in about 2–5% of affected Collies, usually near the optic disc and often associated with optic disc colobomas. The detached and edematous retina is protruding forward of the optic disc. (E) Vermiform streaks are occasionally observed in young Collie puppies. These streaks represent an imbalance of growth between the retina and outer choroid and sclera. They disappear over several weeks. (F) Young (8 weeks) Collie with very small choroidal hypoplasia. This lesion is quite small and a candidate for “go‐normal.” As adults, the “go‐normal” Collies appear normal but breed as CEA affected dogs. This puppy also has early progressive retinal atrophy (rod–cone dysplasia) and is night blind.

Vermiform streaks or retinal folds can also affect the ocular fundus but appear unrelated to CEA but instead to unbalanced growth between the sclera and the inner retina. They resolve (or disappear) with maturity. They appear as gray to white irregular lines within the retina.

Recommendations for the past 45+ years have been to breed only mildly affected dogs (with only choroidal hypoplasia) and to avoid use of the more severely affected Collies (those with colobomas or staphylomas). Many progressive Collie breeders are now breeding normal eyed Collies (many are carriers) with the eventual goal of eliminating the disease from the breed. In other breeds with this defect affected dogs should be neutered, and both parents of affected offspring should not used for breeding. There is a commercially available DNA test for CEA (CEA‐CH: intronic mutation in NEHJ1) which helps detect the carriers and “go normal” dogs, to facilitate a CEA‐free breeding program (for more information see Optigen, www.optigen.com and other sites).

Retinal Dysplasia

Retinal dysplasia in many breeds seems to be increasing, and is of increasing concern for the conscientious purebred dog breeder and fancier. The causes of retinal dysplasia are genetic or viral (herpesvirus), or the result of irradiation, certain drugs, hypovitaminosis A, or intrauterine trauma (Figure 13.3). There are three categories of retinal dysplasia: (i) focal or multifocal; (ii) geographic; and (iii) the most severe, total retinal dysplasia with retinal detachment.

Fundus photograph displaying non‐pigmented areas (arrows) within the nontapetal fundus in an American Cocker Spaniel with focal retinal dysplasia. — **Figure 13.3** Retinal dysplasia occurs in several different forms in specific breeds (focal or multifocal, geographic, or total with retinal detachment), and may not impair vision. (A) Focal retinal dysplasia in the American Cocker Spaniel. The non‐pigmented areas (arrows) are located within the nontapetal fundus. Vision appears normal. (B) Geographic retinal dysplasia in a Labrador Retriever. The dysplastic area is dorsal of the disc and contains areas of hyperreflectivity and pigmentation, and can be confused with previous chorioretinitis. Vision can be normal or subtle visual impairment can be demonstrated for visually critical tasks. (C) Geographic retinal dysplasia in an English Springer Spaniel. Located in the tapetal fundus and above the optic disc, the dysplasia has both hyperreflective (small arrow) and pigmented (large arrow) areas. Often no visual impairment can be detected. (D) Total retinal dysplasia in a Bedlington Terrier. The condition affects the entire retina, and has caused complete retinal detachment (or retinal nonattachment). Note the anterior portions of the retinal detachments are immediately behind the posterior lens. Affected dogs are blind.

Fundus photograph displaying geographic retinal dysplasia in a Labrador Retriever. — **Figure 13.3** Retinal dysplasia occurs in several different forms in specific breeds (focal or multifocal, geographic, or total with retinal detachment), and may not impair vision. (A) Focal retinal dysplasia in the American Cocker Spaniel. The non‐pigmented areas (arrows) are located within the nontapetal fundus. Vision appears normal. (B) Geographic retinal dysplasia in a Labrador Retriever. The dysplastic area is dorsal of the disc and contains areas of hyperreflectivity and pigmentation, and can be confused with previous chorioretinitis. Vision can be normal or subtle visual impairment can be demonstrated for visually critical tasks. (C) Geographic retinal dysplasia in an English Springer Spaniel. Located in the tapetal fundus and above the optic disc, the dysplasia has both hyperreflective (small arrow) and pigmented (large arrow) areas. Often no visual impairment can be detected. (D) Total retinal dysplasia in a Bedlington Terrier. The condition affects the entire retina, and has caused complete retinal detachment (or retinal nonattachment). Note the anterior portions of the retinal detachments are immediately behind the posterior lens. Affected dogs are blind.

Breeds commonly affected with focal or multifocal retinal dysplasia include American Cocker Spaniel, Beagle, Labrador Retriever, Rottweiler, and Yorkshire Terrier. This form of retinal dysplasia does not produce clinical vision impairment. Both the tapetal and nontapetal fundi are affected. In the tapetal fundus, lesions appear as hyperreflective to pigmented “Y,” “X,” or irregular streaks or folds, and most frequently affect the area dorsal to the optic nerve head. In the nontapetal fundus, affected areas are gray to white and similar in shape.

Those breeds commonly affected with geographic retinal dysplasia include the Cavalier King Charles Spaniel, English Springer Spaniel, and the Labrador Retriever. Affected dogs have normal clinical vision or demonstrate vision impairment to blindness. Blind dogs can also have nystagmus, microphthalmia, intraocular hemorrhage, cataract formation, and retinal detachment. Less affected dogs show focal areas of retinal dysplasia, most frequently in the central tapetal fundus, which by ophthalmoscopy appear as large hyper‐ and hyporeflective areas with variable and often dense pigmentation. The retinal blood vessels can be attenuated in the involved areas. There is no treatment, and affected dogs should not be used as breeding animals.

Total retinal dysplasia with retinal detachment (or nonattachment) affects the Bedlington Terrier, Akita, Chow Chow, Doberman Pinscher, Labrador Retriever, Sealyham Terrier, and Samoyed. Affected puppies usually show esotropia, smaller than normal palpebral fissures, dilated pupils, smaller than normal eyes, protruding nictitating membranes, rotary nystagmus, blindness, and intraocular hemorrhage. They will often have microphthalmia and cataract formation in addition to detachments and posterior segment hemorrhage. The Labrador Retriever and Samoyed breeds, which have the more severe forms of this disease, can also have skeletal abnormalities. Affected dogs as well as the parents of affected puppies should not be used as breeding animals. DNA tests are available for many of these mutations.

Retinal Photoreceptor Dysplasia and Degeneration (Retinal Atrophy)

Progressive retinal atrophy (PRA) is a leading cause of inherited blindness in the dog. Affected dogs develop gradual visual impairment and eventual blindness. First reported in the Gordon Setter in Sweden in 1911 as retinitis pigmentosa, PRA now affects more than 60 breeds of dogs (Figure 13.4). PRA affects the photoreceptors (rods and cones) and has been divided into those conditions that occur during development (rod–cone dysplasia) and those that develop later in life (rod–cone degeneration after development of the rods and cones is complete), and those wherein the rods are predominantly affected first or those initially affecting the cones. These two distinctions, based on electroretinographic and ultrastructural studies, also help predict the time of onset. The rod–cone dysplastic types affect younger animals and the degenerative rod–cone diseases involve older dogs. Experimental studies indicate serial electroretinography can detect the disease months to years before clinical decline in vision develops.