Definition of Cataracts
The word cataract literally means ‘to break down’. This breakdown refers to the disruption of the normal arrangement of the lens fibers or its capsule, which results in the loss of the transparency of the lens and a reduction in vision. Cataracts often appear to have a white or crushed ice appearance and are found in the lens of the eye.
A cataract is a clouding of the lens inside the eye and is the most common cause of blindness in dogs. Cataracts can be caused by injuries or diabetes, but most cataracts in dogs are inherited. Any opacity in the lens is called a cataract; very small spots do not significantly affect vision. However, most cataracts will progress, and ultimately cause blindness.
The lens is located behind the colored iris; thus when a cataract occurs, the pupil may appear white. Vision through a mature cataract is like looking through white painted glass. It is important to note that a cataract is not a growth.
How do Cataracts Form?
Despite the fact that there are several different types and causes of cataracts, they all develop in a similar fashion. The normal lens is maintained in a dehydrated state. It consists of 66% water and 33% protein. There is a sodiumwater pump system in the lens that keeps this water/protein balance in check. When the biomechanical system in the lens is damaged, this pump system begins to fail and extra water moves into the lens. In addition, the percentage of insoluble protein particles increases. These changes result in the loss of transparency and cataract formation.
What Causes Cataracts?
There are many different forms and causes of cataract formation. They affect all breeds and ages of dogs, but certain types show up more commonly in specific. Despite the fact that cataracts are far too common, there is still a lot that we do not know about canine cataracts.
Cataracts may be present at birth (congenital) or develop later and can be caused by infections (such as canine herpes virus, canine parvovirus2 and canine adenovirus1), injuries, poor nutrition (i.e. puppy milk replacers), radiation therapy or toxins.
Many cataracts will worsen to the point of blindness but certain types can remain small for the entire life of the patient.
The most common metabolic disorder resulting in cataract formation in the dog is diabetes mellitus. If diabetic dogs are followed for a year or more, almost all of them will develop cataracts. In diabetic dogs, the glucose concentrations in the lens increases. The extra glucose is converted into sorbitol, which causes an increase in the influx of water to the lens. The increase in water causes a breakdown of the lens fibers and a resulting cataract. Cataracts in diabetic dogs can develop extremely rapidly if the dog’s sugar is not regulated. This type of cataract will generally affect both eyes. Surgical removal of the lens can be successfully performed in the diabetic dog, if the animal has been regulated successfully for at least three months Hypocalcemia (low blood calcium) resulting from renal failure is another cause for cataract formation in dogs. Hypocalcemia may be brought on by poor nutrition and homemade diets are the greatest offender. The ideal dog’s diet contains a ratio of calcium to phosphorus of 1.2 to 1. Liver, for example, has a calcium to phosphorus ratio of 1 to 15. Although liver is small rations is great for dogs, fed in large percentages the calcium to phosphorus ratio is improper. Low blood levels of albumen, caused by dietary protein deficiencies and sometimes kidney diseases, will cause low calcium levels.
Puppy Milk Replacers (see article on Nutritional Cataracts)
Trauma from an automobile accident or penetration of a thorn, shotgun pellet, or other object may damage the lens and a cataract may develop. These types of cataracts usually only occur in one eye and can be treated successfully with surgical removal. Electric shock (a puppy bites an electric cord) can result in anterior subcapsular cataracts.
Disophenol, given for the treatment of hookworms has been shown to cause cataracts but are usually reversible after the drug is discontinued.
Hypocalcemia (low blood calcium) resulting from renal failure is another cause for cataract formation in dogs. Hypocalcemia may be brought on by poor nutrition and homemade diets are the greatest offender. The ideal dog’s diet contains a ratio of calcium to phosphorus of 1.2 to 1. Liver, for example, has a calcium to phosphorus ratio of 1 to 15. Although liver is small rations is great for dogs, fed in large percentages the calcium to phosphorus ratio is improper. Low blood levels of albumen, caused by dietary protein deficiencies and sometimes kidney diseases, will cause low calcium levels.
How do you Classify Cataracts?
Cataracts are classified in multiple ways which leads to confusion.
Classification by stage of formation: the basis of stage of formation may be meaningful because this describes the progression of most cataracts regardless of cause.
Incipient when first evidence of opacification is seen. Vision is not affected. The opacity may or may not progress. (http://cvm.msu.edu/courses/AP/cataract/types/incipient.htm)
Incomplete (also called immature) lens is largely, but not completely, opaque. The patient may have some vision; atapetal reflex is visible. The lens may be slightly enlarged due to imbibition (absorption of fluid by a solid, causing swelling) of water. (http://www.animaleyecare.net/diseases/cataract.htm)
Complete (also called mature) lens is totally opaque preventing vision in that eye. The lens may be enlarged due to imbibition of water. (http://www.animaleyecare.net/diseases/cataract.htm)
Complete with shrinkage (also called hyper mature) remaining lens fibers are opaque, but there is a reduction in this material as well as water, causing a decrease in the size of the lens, usually through flattening. If the capsule is clear, the patient may be able to see through portions where little to no cortical material remains or around the lens if shrinkage results in a smaller diameter. (http://www.elminjas.com/cataract3.jpg)
Classification on basis of age : The age of onset can be important in determining whether the cataracts present are the result of a hereditary trait.
Congenital present at birth. These will usually occur in both eyes. Just because a dog is born with cataracts does not unavoidably mean they were inherited. Infections in utero or nutritional deficiencies may be the cause.
Developmental (Early Onset or Juvenile) develops sometime after birth, but generally before adulthood (2-5 years) many of the heritable and nutritional cataracts fall into this category.
Degenerative (Senile or Late Onset) cataract formation after normal development. These cataracts occur in dogs over six years of age. Nuclear sclerosis is often confused with cataracts at this age.
Classification on where they occur: “On the CERF form, cataracts are separated into those that occur in the anterior cortex (those occurring in front of the nucleus), the posterior cortex (the cortex behind the nucleus), the equatorial cortex (around the lens periphery), or within the nucleus itself. Cataracts that involve the suture lines are separated into anterior or posterior suture line cataracts. All cataracts are further described by size. Punctate indicates small, focal cataracts. Intermediate describes those involving a larger area of the lens than punctuate. Diffuse cataracts refer to those involving an entire area. Generalized cataracts involve all areas of the lens.” Thomas Miller, DVM, DAVCO
What are Ysuture Cataracts?
Granular clumping along the arms of the Ysutures. Not uncommon. It is important in that it progresses in some animals and it may represent a variable expression of a heritable cataract.
There are no medications which are effective in treating or preventing cataracts. Treatment requires surgical removal of the lens and once removed, a cataract cannot recur. Cataracts are not treated with lasers; the most common surgery to remove the lens uses phacoemulsification, ultrasonic fragmentation of cataract. For a successful outcome, the affected animal must undergo a thorough examination to determine if he/she is a good surgical candidate. Diabetic animals that are not regulated, aggressive animals that are difficult to treat daily, or animals in poor or failing health, are not good surgical candidates.
How about nonsurgical treatment?
Eye drops, etc. currently there no good nonsurgical treatments for this condition.
Cataract surgery is approximately 90-95% successful. However, this means that in 5-10% of cases, complications may prevent vision recovery. The purpose of the examinations before and after surgery is to detect and prevent these complications whenever possible. In uncomplicated cases, vision will begin to improve within a few days; after six weeks, healing is usually complete and medication is discontinued.
Signs/Symptoms my Dog may have Cataracts
Signs of vision loss are usually not detected by the owner until the cataract occupies 40-50% of the lens. Oftentimes, this needs to occur in both eyes for the dog’s vision to be negatively affected.
Management of Cataracts found in a Breeding Program
“These statements pertain to any potentially inherited cataract (i.e., cataract for which another cause is not found):
Usually the sire and dam of any litter or pups with cataracts are not bred again to each other, and the breeder needs to think long and hard about whether to breed them again at all. The safest advice is to retire them from breeding.
Any dog with cataracts should not be bred.
It is undecided what to do with the siblings of affected dogs because the genetics of cataract inheritance is known in so few breeds. It is a bit risky to use the siblings as breeders, but it is hard to know how risky if only one dog in the litter is affected. If multiple dogs are affected, then the safest advice is to neuter all the dogs in the litter and be careful about other (younger or older) siblings.” Dr. Rhea Morgan, DVM, Diplomate ACVO