Faculty of Veterinary Medicine and Biosciences, Norwegian University of Life Sciences, Oslo, Norway
Inherited eye diseases comprise both congenital and developmental conditions where the clinical symptoms become apparent later in life. The cut-off point for congenital eye diseases has been set to 8 weeks of age (http://www.ECVO.org).
Although not comprehensive, a few highlights of inherited eye diseases in the dog and cat are described below.
Microblepharon/blepharophimosis: Relatively small eyelids/eyelid openings. Usually in conjunction with a too small eyeball (microphthalmia/cystic eye/ anophthalmia). Usually no treatment needed, but if necessary; surgical enlargement of the eyelid openings.
Coloboma: Parts of/entire eyelid(s) not developed. Seen in all breeds of dogs and in cats, most commonly Himalayan. Usually affects lateral aspect of upper eyelid. Often leads to irritation/pain because of hairs rubbing the cornea/conjunctiva (trichiasis) in addition to repeated secondary conjunctivitis.
Dermoid: Normal tissue in an abnormal location. Usually excessive skin/ hair on the eyelids. Surgery is indicated if it causes irritation/recurrent infections/ malfunctioning of the eyelid. Surgical techniques vary with extent/location. Dermoids can also affect the conjunctiva and/or cornea.
Entropion: Inverted eyelids. Can lead to trichiasis/ irritation/ recurrent secondary infections/ corneal ulcers/ pigmentary keratitis. Can be primary (inherited) or secondary. Spastic entropion is excluded by relief of the condition after application of topical anesthetic. Several surgical techniques exist.
Ectropion: Everted eyelids. Usually affects lower eyelid. Affects mainly dogs. Surgical intervention is indicated if recurrent secondary infections/irritates the dog. Several surgical techniques described.
Euryblepharon: Too large eyelids/eyelid openings. Most common in large breed dogs with “diamond eyes”. Usually en-and ectropion simultaneously. Requires surgical intervention if recurrent secondary infections/trichaisis/corneal affection. Various surgical techniques described. Important to stabilize the lateral canthus.
Distichiasis/ectopic cilia: Distichiasis are hairs exiting the meibomian gland openings along the eyelid margins. Ectopic cilia are hairs originating from hair follicles close to the Meibomian glands, exiting the conjunctiva. These usually causes acute ocular pain/blepharospasm. The hairs are usually located in the central part of the upper eyelid.
Tear Canal and Tear Glands
Aplasia/hypoplasia of the tear canals/canuliculi: Symptoms are excessive tearing usually without any signs ocular discomfort. Secondary infections occur. Aplasia of the lower tear canal opening is the most common, sometimes accompanied with a subconjunctival swelling underneath the conjunctival surface in the anatomical area of the opening. Problems further down in the tear canals are less prevalent. Eventual treatment consists in surgical opening of the tear canals. Diagnostic imaging with or without contrast is advisable for is advisable if there is a suspicion of locations other than the openings. Dermoids/foreign bodies/neoplasias, etc., are differentials.
Aplasia/hypoplasia of the tear glands: Dogs and cats normally possess two tear glands for each eye (the orbital [principal]) and the tear gland of the third eyelid (accessory). These glands are responsible for the production of the watery part of the tear film, and aplasia/ hypoplasia will lead to no/ insufficient production, with consequent symptoms of dry eye (kerato conjunctivitis sicca [KCS]). The diagnosis is made by Schirmer tear test (STT), often accompanied by mucoid/mucopurulent discharge, corneal fibrosis/neovascularization/pigmentation, etc. Lower values than normal (15–25 mm) are indicative of KCS. Borderline measurements should be confirmed by repeating measurements at recheck. Inherited neurogenic KCS often is evident early in life, can be unilateral or bilateral, often accompanied by ipsilateral dry nostrils and/ or oral cavity. Several dog breeds are genetically predisposed to immune mediated KCS later in life. Other etiological causes (drug induced/toxic/neoplasia/endocrine/trauma/infectious, etc.) have to be ruled out before the definite diagnosis of inherited KCS can be made.
Nanophthalmia/microphthalmia/anophthalmia: Nanophthalmia is a functional eye bulb of a smaller size than normal, without any other abnormalities. A microphthalmic eye is smaller than norma l accompanied by other developmental conditions such as micro cornea, anterior segment dysgenesis, lens abnormalities in addition to retinal and optic nerve abnormalities. A microphthalmic eye can be visual or non-visual. Anophthalmia is lack of eyeball. Sometimes the remnants of an eyeball is hid den underneath the conjunctiva (cystic eye). The diagnosis of a too small eyeball or lacking eye/cystic eye can be made/confirmed by the use of ultrasound.
Corneal stromal dystrophy: Lipid and/or mineral deposits in the corneal stroma, often bilaterally. Most commonly seen in dogs. It often manifests as opaque shadow in the axial/paraxial cornea. The condition can be congenital, but usually develop later in life.
Corneal endothelial dystrophy: Reported to be inherited in the Chihuahua, Dachshund and Boston terrier. It manifests in middle aged-older individuals. The clinical signs are whitish-bluish opaque cornea, usually staring in the axial cornea. With time, it usually involves the entire cornea.
Persistent pupillary membranes (PPM) are congenital remnants of the pupillary membranes. These should normally regress by the time of the eyelid opening. Both cats and dogs can be affected. The condition can be seen as strings from iris-iris, iris-lens, iris-cornea, plaque on the corneal endothelium and/or anterior lens capsule, or combinations of the former. Iris-cornea PPM can be accompanied by opacification of the cornea. If pronounced it can compromise vision/ lead to blindness. If attached to the anterior lens capsule it can result in secondary cataract.
Iris hypoplasia/coloboma: Due to dysgenesis of the iris stroma resulting in a thinner iris than normal or holes/complete lack of iris tissue. Photophobia can be a consequence in pronounced cases.
Microphakia: Is a congenital condition where the lens is smaller than normal. It can be accompanied by other abnormalities such as cataract and microphthalmia. As the lens is crucial for normal development of the eye, congenital aphakia is only seen in association with cystic eyes.
Lens coloboma: Lens coloboma is a consequence of focal lack zonula zinni attaching to the lens, resulting in a flattening of the lens curvature.
Lenticonus: A bulging of the lens surface, most commonly the posterior aspect of the lens, but on rare occasions it can affect the anterior aspect of the lens.
Cataract: An opacity of the lens. It can be either congenital or developmental. Congenital cataracts usually involves the lens nucleus, but can evolve to affect the cortex. Developmental cataracts usually involves the lens cortex initially but can evolve to affect the nucleus and/or the entire lens (total cataract). Developmental inherited cataracts can develop later in life varying with breed and form of cataract. It can develop to complete cataract. Other etiologies for cataract such as metabolic, trauma, toxic etc have to be excluded before the diagnosis of inherited cataract is made.
Persistent hyperplastic tunica vasculosa lentis/persistent hyperplastic primary vitreus (PHTVL/PHPV): A congenital condition in which there is an abnormal regression of the vascular system nourishing the lens during its development. This regression should be complete by the opening of the eyelids. The condition is reported to be inherited breeds such as pinscher breeds, the Staffords hire bullterrier amongst others. The severity of the conditions varies from pigmented spots on the posterior lens capsule to lenticonus/intra-lenticular bleeding/ plaque on the posterior lens capsule and cataract.
The Retina and the Optic Nerve Head
Retinal dysplasia (RD): Due to uneven growth of the two ectodermal layers of the retina (neuro retina and the retinal pigment epithelium). Focal/multifocal RD is observed as elevated greenish streak-like areas in the retina. Geographic RD is a horseshoe shaped-round greenish elevations in the retina, most commonly located along the dorsal retinal vessel. In total retinal dysplasia the retina has detached leading to blindness. Histologically RD can be observed as rosettes. RD is reported inherited in several dog breeds including retrievers and spaniels. The funduscopic signs usually become less pronounced with age, although in the English springer spaniels progression to retinal detachment has been reported.
Collie eye anomaly (CEA): Mainly seen in collies and collie-related breeds. The condition comprises optic nerve head coloboma, choroid hypoplasia/chorio retinal dysplasia (CH) and complications such as retinal detachment/intraocular hemorrhage. The optic nerve colobomas, may involve parts of or the entire optic nerve head. Extensive colobomas may be associated with secondary retinal detachments. Choroidal hypoplasia is an abnormal development of the choroidal vessels, usually seen as distorted/absence of vessels lateral to the ONH in the tapetal areas. In non-merle dogs, these areas are usually covered by pigment (“Go normal”). CEA is reported to be a polygenic disease, and a commercial gene test exists for the CH part of the CEA-complex.
Rod-cone dysplasias/dystrophies (RCD): The photoreceptor dysplasias usually present early in life, often as early as 5–6 weeks on electroretinographical (ERG) exam ination. In the progressive retinal atrophies (PRA), the photo receptors develop normally initially, but degenerate later in life. Early forms of PRAs are evident from about 2 years of age, whereas the late forms may become evident as late as ten years of age or even later. Most common is rod dystrophy with subsequent affection of the cones. Initial signs are nyctalopia, tunnel vision, insecurity under dark conditions etc. The endresult is complete blindness. Funduscopically there is an initial thinning of the retinal vessels, with subsequent tapetal hyper-reflectivity and pallor of the optic nerve head. Secondary cataracts are not uncommon. The cone-rod dystrophy is characterized by an initial affection of the cones with a subsequent affection of the rods. Initial symptoms are photophobia, blindness in bright light, miotic pupils, etc. Later in the course of the disease, the rods are affected and the animal becomes completely blind. Funduscopic changes are similar to those of rod-cone dystrophies. In cone dystrophies the clinical symptoms are similar to the initial symptoms seen in cone-rod dystrophies, but rod-function usually remains normal. Funduscopically there are no obvious changes . ERG can be used to confirm the diagnosis of rod-cone, cone-rod and cone dystrophies often before clinical signs are evident. Differential diagnoses are chorioretinis, post inflammatory, sudden acquired retinal degeneration (SARDS), toxic retinopathy, etc. Several gene-tests are available for specific photoreceptor dysplasias/degenerations (www.optigen.com, www.aht.org [VIN editor: URL not accessible as of 5-18-2018]).
Optic nerve head (ONH) aplasia/hypoplasia/micropapilla: ONH smaller than normal or absent. Animals with ONH-aplasia are blind from birth, whereas animals with hypoplasia have visual affection or are blind. A sequelae is retinal detachment. In animals with micropapilla there is no evidence of abnormal vision. Differential diagnosis is ONH-atrophy.
1. Gelatt et al. Veterinary Ophthalmology. 5th ed. Wiley Blackwell.
2. Maggs et al. Slatter’s Fundamentals of Veterinary Ophthalmology. 4th ed. Saunders