The lens is normally transparent and devoid the blood vessels. Cataract is opacity of the lens, either due to hereditary factors, incidental malformation or because of cataractogenous factors affecting the lens. For full examination of the lens a mydriatic is needed. Tropicamide is instilled at least 20 minutes before examination to allow full pupil dilation. A simple examination of the lens can be performed with a focal light source; more sophisticated examination includes the use of a slit-lamp biomicroscope. In a normal lens the following findings should be noted. The suture lines of the lens, shaped as a Y at the anterior pole and an inverted Y at the posterior pole may be observed as faint lines. So-called "arrowhead" opacities may be seen at the peripheral tips of the suture lines, especially in young dogs. A faint opacity delineating the nucleaus ("nuclear ring") is also occasionally seen. A small hyaloid artery remnant may be seen attached to the posterior lens capsule and extending into the vitreous. Occasional pinpoint dots representing remnants of the tunica vasculosa lentis system may also be seen on the posterior lens capsule. However, there should be no concurrent sheath of fibrous tissue. On the anterior lens capsule small, pigmented dots representing remnants of the pupillary membrane are not infrequently observed. These small opacities are not connected to the iris with strings and do not cause secondary cataracts. In older animals, nuclear sclerosis is the result of lens growth throughout life, decreased water content of the lens and subsequent increased lens density. This increased density, resulting in a bluish appearing lens, while still allowing examination of the fundus, must be distinguished from cataract. Normally, no opacities apart from what is mentioned here should be seen in retroillumination.
Hereditary catataracts may be divided according to age at presentation in congenital cataract with or without other ocular malformation and developmental ("juvenile") cataract.
Hereditary cataracts have been described in many dog breeds, and the list of affected breeds continue to grow. The mode of inheritance of most of the cataracts presumed to be hereditary has not been adequately documented, however. When evaluating a dog for cataract, one must both consider the fact, as well as the fact that other cause than inheritance may cause cataract in a dog. As there will be different breeding pools in different countries, findings may differ between countries regarding hereditary cataracts. Thus, cataract may be a problem in a breed in one country, while the condition may only occasionally occur in the same breed in another country. When the opthalmologist diagnoses cataract the main issue is to determine if the cataract represents an inherited disease or not. Certain criteria exists, however, that may help in evaluating the case:
"Classical" criteria for hereditary cataracts:
Cataract has been described in the breed
Cataract changes should be localized to the expected area
Cataract changes should be bilateral (there are exceptions)
Cataract changes should progress, but progression may be slow.
Extracts from the American College of Veterinary Opthalmologists/Canine Eye Registry Foundation (ACVO/CERF) criteria for defining a disease as hereditary.
There are published reports in the literature regarding a condition in a particular breed with evidence of inheritance.
The incidence of affected animals is greater than or equal to 1% of the examined population with a minimum of five affected animals per five years period.
A specific request from a breed club that a condition be included for their breed or overwhelming opinion by a majority of the committee members that clinical experience would indicate that a particlual condition should be listed in spite of the absence of direct evidence of affected animals the CERF reports:
When the frequency is greater than in other breeds
When the frequency increases in a given breed as a whole
When the frequency is greater in related dogs within a breed
When it has a characteristic appearance and location
When it has a characteristic age of onset and of course of progression (predictable stages of development and time for each stage to develop)
When it looks identical to an entity which has been proven to be inherited in other breeds
Developmental (juvenile) cataracts
Hereditary cataract not present at birth has been described in a long list of dog breeds. Cataract changes most often develop early in life, around 12 months of age, but in some cases the lens may be normal until the dog is 4-5 years old. There is therefore no upper age limit on when to stop re-examining a dog for inherited cataract. Differential diagnoses of hereditary cataract include PRA, secondary cataracts ( diabetes mellitus, uveitis, injuries) and non-hereditary primary cataracts. When hereditary cataract is suspected in a "new" breed the breeding advice should be not to breed affected dog and examine offspring and close relatives. Developmental cataracts primarily affect the lens cortex; the nucleus is rarely involved in the initial stages of development. Cortical changes are often restricted to opacities around the posterior pole, posterior polar cataract. Small posterior polar cataract changes do not affect the dog's vision notable, and may progress only to a limited extent. However, even posterior polar cataracts may in some cases progress to affect the whhole lens. In many breeds, posterior polar cataract is the most common manifestation of hereditary cataract, but there are breed differences as to localization of initial cataract changes within the lens. Thus, initial cataract changes in the flat coated retriever may be seen in the anterior suture lines, while cataract in the Afghan hound usually starts at the equator, in the periphery of the lens. Late developing cataract in the Boston terrier presents as discrete linear or wedge-like anterior subcapsular or outer cortical opacities extending in a radial fashion from the equator to the centre of the lens. The fact that some cataract changes initially appear in the periphery emphasizes that pupil dilation is necessary for a thorough examination of the lens.
The list of breeds affected with developmental (juvenile) cataract is long.
Mode of Inheritance of hereditary cataracts:
Very few breeds have been investigated regarding modes of inheritance. An autosomal recessive model has been shown for congenital cataract in the miniature schnauzer and Staffordshire bull Terrier and develpmental cataract in the Boston Terrier. For the two latter breeds there is now a gene test available. Recessive mode of inheritance has been suggested in the Afghan hound, American Cocker Spaniel, Bichon Frise', German Shepherd, Siberian Husky, and Standard Poodle. In the Golden Retriever cataract has been suggested to be inherited by a dominant gene with incomplete penetaance, while others consider a recessive mode of inheritance more likely. One must admit that our knowledge regarding modes of inheritance is restricted. However, with identification of the dog genome and the research activity aimed at defining disease genes in the dog, one must expect more gene tests to become available in the near future.
How a Cataract forms:
Mechanisms of lens metabolism become damaged resulting in altered hydration and subsequent osmotic effects. Insoluble proteins increase while the soluble proteins decrease. Electrolyte, lipid, and glucose metabolic pathways are affected. Increased hydration results information of vacuoles and water cleft. Lens fibers undergo oxidative damage, which results in retention of sodium and calcium. The lens eventually opcaifies.
Several methods are used to classify cataracts: Age of Animal at onset
Congenital: Present at birth; often nuclear; sometimes nuclear and cortical.
Cataracts can also result from exposure to a toxin or infectious agent in utero.
Juvenile: Occurs in young animals, less than 6 years of age, usually occurs at 2 to 3 years of age. In many breeds, juvenile cataracts are hereditary.
Senile: Develops at 6 years of age and older.
Trauma: Perforating injury to the cornea and lens frequently induces cataract formation. Rarely does blunt trauma cause cataracts to form.
Nutritional: Canine milk replacers (arginine and tryptophan deficiency)
Electric Shock: Anterior subcapsular (puppy that bites an electric cord)
Radiation: Causes decreased epithelial cell mitosis (young animals are more susceptible)
Toxins: Disophenol for hookworms