The lens is normally transparent and devoid of 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 nucleus ("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 Cataracts

Hereditary cataracts 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 continues 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 this 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 ophthalmologist 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 Ophthalmologists/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 particular condition should be listed in spite of the absence of direct evidence of affected animals on CERF reports.

Suspicion is:

 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 another breed

Malformations Associated with Congenital Cataract Include:

 Microphthalmia may occur spontaneously in any breed. Inherited microphthalmia / cataract occurs among others in the Miniature Schnauzer, Cocker Spaniel (English), West Highland white terrier, Old English Sheepdog and Cavalier King Charles spaniel.

 Lenticonus / lentiglobus describes a weakness of the (most often) posterior lens capsule causing posterior bulging of the lens and cataract. Known as a breed-related disease in the Cavalier King Charles spaniel but may occur also in other breeds.

 PHTVL/PHPV (Persistent hyperplastic tunica vasculosa lentis / persistent hyperplastic primary vitreous). The fetal vessel system surrounding the lens are hyperplastic and do not undergo normal regression. Small changes are present as pigmented dots on the posterior lens capsule without concurrent cataract, while more severe changes may lead to secondary cataract and blindness. Described in the Dobermann and Staffordshire bull terrier, but occurs also in the giant schnauzer and the King Charles spaniel.

 PPM--persistent pupillary membranes. Breed predisposition among others in the Basenji and Chow chow, although the list can be extended depending on local variation in the gene pool. PPM is an incidental finding in many other breeds. Strands may span from iris to iris, iris to cornea and/or lens causing secondary cataract, or the malformation may present as broad sheets of tissue between iris and cornea.

Developmental (Juvenile) Cataract

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 notably, and may progress only to a limited extent. However, even posterior polar cataracts may in some cases progress to affect the whole 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 and includes:

 Alaskan malamute

 American cocker spaniel

 Belgian shepherds

 Bichon frisé

 Boston terrier (two forms, one developing early in life, one late)

 Cavalier King Charles spaniel

 English cocker spaniel

 English springer spaniel

 Lapland dog (Swedish, Finnish)

 German shepherd

 Leonberger

 Newfoundland dog

 Norwegian buhund

 Retrievers

 Rottweiler

 Samoyed

 Schnauzers (miniature, standard and giant schnauzers)

 Siberian husky

 Standard poodle (and poodle)

 Welsh springer spaniel

 Tibetan terrier

 Others (new breeds will be added to the list)

Pulverulent nuclear cataract: A special form of cataract is seen in certain breeds of dogs, including the Cocker SPANIEL, flat Coated Retriever, Leonberger and Norwegian buhund. Initial presentation is as pinpoint dots along the suture lines just posterior to the foetal nucleus. With age, the changes progress to form a ball of thread-like opacities with a candy floss appearance. The cataract changes never progress to obscure vision significantly. A dominant mode of inheritance has been suggested in the Norwegian buhund.

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 developmental 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 frisé, German shepherd, Siberian husky and Standard Poodle. In the Golden Retriever cataract has been suggested to be inherited by a dominant gene with incomplete penetrance, 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.