Post by muskyhusky on Oct 16, 2008 23:17:08 GMT -6
The normal lens of the eye is a focussing device. It is completely clear and is suspended in position by special tissue fibers (called “zonules”) just inside the pupil.
The lens focusses an image on the retina in the back of the eye in a process called “accommodation.” The focussing power of the dog’s lens is at least three times weaker than that of a human and a cat’s lens is at best half the focussing power as a
human’s.
(It is helpful to remember that dogs and cats have a sense of smell at least 1000 times more accurate than ours and this is their primary means of perceiving the world.)
Despite its clarity, the lens is in fact made of tissue fibers. As the animal ages, the lens cannot change it’s size and grow larger; instead, it becomes more compact with fibers. This condition is called “nuclear sclerosis” and is responsible for the cloudy-eyed appearance of older dogs but these lenses are still clear and the dog can still see through them; these are not cataracts. The lens is enclosed in a capsule which, if disrupted, allows the immune system to see the lens proteins for the first time, recognize them as “foreign,” and attack. The resulting inflammation is painful and can be damaging to the eye.
A cataract is an opacity in the lens.
Cataracts can be congenital, age-related, of genetic origin (the most common cause), caused by trauma, by dietary deficiency (some kitten milk replacement formulas have been implicated), by electric shock, or by toxin. The patient with a cataract is not able to see through the opacity. If the entire lens is involved, the eye will be blind.
A SPECIAL NOTE ON DIABETES MELLITUS IN DOGS:
Many things can cause the lens to develop a cataract. A special cause is Diabetes Mellitus. In this condition the blood sugar soars as does the sugar level of the eye fluids. The fluid of the eye’s anterior chamber (see illustration above) is the fluid that normally nurtures the lens but in the diabetic pet the lens can only utilize so much sugar. Excess absorbed sugar is transformed into sorbitol within the lens which unfortunately draws water into the lens causing an irreversible cataract in each eye. Cataracts are unavoidable in diabetic dogs no matter how good the insulin regulation is; diabetic cats have alternative sugar metabolism in the eye and do not get cataracts.
WHAT ELSE COULD IT BE?
Many owners are not really able to tell which portion of the eye looks cloudy. Cloudiness on the cornea, as caused by other eye diseases, can be mistaken for a cataract by an inexperienced owner. Also, in the dog, the lens will become cloudy with age as more and more fibers are laid down as described above. Nuclear sclerosis, as described, can mimic the appearance of a cataract yet the eye with this condition can see and is not diseased. It is a good idea to have your veterinarian examine your pet if you think he or she has a cataract as you could be mistaken.
WHY IS HAVING A CATARACT BAD?
The area of the lens involved by the cataract amounts to a spot that the cannot be seen through. If the cataract involves too much of the lens, the animal may be blind in that eye and, of course, there could be cataracts in both eyes which means the pet could be rendered completely blind.
A cataract can “luxate” which means that it can slip from the tissue strands that hold it in place. The cataractous lens can thus end up floating around in the eye where it can cause damage. If it settles so as to block the natural fluid drainage of the eye, glaucoma (a build up in eye pressure) can result, leading to pain and permanent blindness. A cataract can also cause glaucoma when it absorbs fluid and swells so as to partially obstruct fluid drainage from the eye.
Cataracts can begin to dissolve after they have been present long enough. This sounds like it could be a good thing but in fact, this is a highly inflammatory process. The deep inflammation in the eye creates a condition called “uveitis” which is in itself painful and can lead to glaucoma.
A small cataract that does not restrict vision is probably not significant. A more complete cataract may warrant treatment. Cataracts have different behavior depending their origin. If a cataract is of a type that can be expected to progress rapidly (such as the hereditary cataracts of young thingyer spaniels) it may be of benefit to pursue treatment when the cataract is smaller and softer, as surgery will be easier.
WHAT TREATMENT IS AVAILABLE?
Cataract treatment generally involves surgical removal or physical dissolution of the cataract under anesthesia. This is invasive and expensive and is not considered unless it can restore vision. A complete examination of the eye is performed by a veterinary ophthalmologist. If a cataract is present, it is not possible to see the retina through it; a test called an “Electroretinogram” is done to determine if the eye has a functional retina and could benefit from cataract surgery. Ultrasound of the eye can be used to look for retinal detachments. If the eye has a blinded retina, there is no point to subjecting the patient to surgery.
Obviously, the patient must be in good general health to undergo surgery; diabetics must be well regulated before cataract surgery. Also, it should be obvious that in order for a patient to be a good candidate for surgery, the patient must have a temperament conducive to the administration of eye drops at home. Pets with cataracts in one eye that can see in the other eye generally do not need surgery.
If the patient proves to be an appropriate surgical candidate, the lens can be softened and sucked away using a technique called “phacoemulsification.” If the patient is relatively young, the lens is relatively soft and phacoemulsification is the first choice procedure.
Older patients have much harder lenses and it is more common in these patients to simply remove the lens surgically.
COMPLICATIONS: Some degree of uveitis (deep inflammation) in s unavoidable. This can cause a pupil constriction reaction which can increase the risk to scarring within the eye. Eye drops to keep the pupil dilated are usually effective in preventing this but not always. Inflammation in the eye will resolve over weeks to months after surgery. Success rate is higher for cataract surgery if minimal inflammation is present in the eye prior to surgery thus pre-operative anti-inflammatory eye drops are frequently prescribed.
Another complication involves the development of opacities on the remaining lens capsule. In humans, laser surgery is used to remove the lens capsule but in the dog, the capsule is too thick for this. Some ophthalmologists prefer to remove the capsule as a preventive measure. The portion of the capsule that is involved in this reaction is present in young dogs but not in adult dogs.
Bleeding after surgery can be an enormous complication and can easily be caused by excess barking or activity after surgery. Small bleeds are of little consequence but a large bleed could ruin vision.
OVERALL, A 90% SUCCESS RATE IS DESCRIBED AFTER CATARACT SURGERY.
An Elizabethan collar will be very important after surgery as patient rubbing or pawing at the eye could be a disaster. Topical therapy (eye drops) will be needed for several months after surgery.
BEFORE EMBARKING ON THE ADVENTURE OF
CATARACT SURGERY, BE SURE TO OBTAIN
A CLEAR EXPLANATION FROM YOUR
OPHTHALMOLOGIST OF EXACTLY
WHAT THE HOME CARE WILL INVOLVE.
LENS IMPLANTATION
A prosthetic lens may or may not be inserted after the cataractous lens has been removed. Without the prosthesis, the dog’s vision will be approximately 20/800, and objects will appear to be reversed (as in a mirror). This sounds debilitating but, again, it is important to remember that pets do not depend on their eyes in the way we do so it is hard to make a good analogy.
Pros: A prosthetic lens allows for normal restoration of vision.
Cons: The incidence of uveitis (deep eye inflammation) after surgery is increased for patients receiving lens implants. Dogs appear to have functional vision without prosthetic lenses. In one study, the complication rate was 15% after lens implantation.
WHAT IF IT GOES UNTREATED?
A cataract by itself does not necessarily require treatment. If there is no associated inflammation and no associated glaucoma and the only problem is blindness, it is perfectly reasonable to have a blind pet. Blind animals have good life quality and do well though it is important not to move furniture around or leave any hazardous clutter in the home.
For further information, the following web sites may be helpful:
www.eyevet.ca
This site has a helpful area on dealing with a blind dog.
www.eyevet.ca/cataract.html
There is also a good area on cataracts with more details on the procedures.
www.veterinaryvision.com/dvm_forum/dvm-cataracts.htm
A good description of the surgical procedures for cataracts.
www.acvo.com
The Official web site of the American College of Veterinary Ophthalmologists.
The lens focusses an image on the retina in the back of the eye in a process called “accommodation.” The focussing power of the dog’s lens is at least three times weaker than that of a human and a cat’s lens is at best half the focussing power as a
human’s.
(It is helpful to remember that dogs and cats have a sense of smell at least 1000 times more accurate than ours and this is their primary means of perceiving the world.)
Despite its clarity, the lens is in fact made of tissue fibers. As the animal ages, the lens cannot change it’s size and grow larger; instead, it becomes more compact with fibers. This condition is called “nuclear sclerosis” and is responsible for the cloudy-eyed appearance of older dogs but these lenses are still clear and the dog can still see through them; these are not cataracts. The lens is enclosed in a capsule which, if disrupted, allows the immune system to see the lens proteins for the first time, recognize them as “foreign,” and attack. The resulting inflammation is painful and can be damaging to the eye.
A cataract is an opacity in the lens.
Cataracts can be congenital, age-related, of genetic origin (the most common cause), caused by trauma, by dietary deficiency (some kitten milk replacement formulas have been implicated), by electric shock, or by toxin. The patient with a cataract is not able to see through the opacity. If the entire lens is involved, the eye will be blind.
A SPECIAL NOTE ON DIABETES MELLITUS IN DOGS:
Many things can cause the lens to develop a cataract. A special cause is Diabetes Mellitus. In this condition the blood sugar soars as does the sugar level of the eye fluids. The fluid of the eye’s anterior chamber (see illustration above) is the fluid that normally nurtures the lens but in the diabetic pet the lens can only utilize so much sugar. Excess absorbed sugar is transformed into sorbitol within the lens which unfortunately draws water into the lens causing an irreversible cataract in each eye. Cataracts are unavoidable in diabetic dogs no matter how good the insulin regulation is; diabetic cats have alternative sugar metabolism in the eye and do not get cataracts.
WHAT ELSE COULD IT BE?
Many owners are not really able to tell which portion of the eye looks cloudy. Cloudiness on the cornea, as caused by other eye diseases, can be mistaken for a cataract by an inexperienced owner. Also, in the dog, the lens will become cloudy with age as more and more fibers are laid down as described above. Nuclear sclerosis, as described, can mimic the appearance of a cataract yet the eye with this condition can see and is not diseased. It is a good idea to have your veterinarian examine your pet if you think he or she has a cataract as you could be mistaken.
WHY IS HAVING A CATARACT BAD?
The area of the lens involved by the cataract amounts to a spot that the cannot be seen through. If the cataract involves too much of the lens, the animal may be blind in that eye and, of course, there could be cataracts in both eyes which means the pet could be rendered completely blind.
A cataract can “luxate” which means that it can slip from the tissue strands that hold it in place. The cataractous lens can thus end up floating around in the eye where it can cause damage. If it settles so as to block the natural fluid drainage of the eye, glaucoma (a build up in eye pressure) can result, leading to pain and permanent blindness. A cataract can also cause glaucoma when it absorbs fluid and swells so as to partially obstruct fluid drainage from the eye.
Cataracts can begin to dissolve after they have been present long enough. This sounds like it could be a good thing but in fact, this is a highly inflammatory process. The deep inflammation in the eye creates a condition called “uveitis” which is in itself painful and can lead to glaucoma.
A small cataract that does not restrict vision is probably not significant. A more complete cataract may warrant treatment. Cataracts have different behavior depending their origin. If a cataract is of a type that can be expected to progress rapidly (such as the hereditary cataracts of young thingyer spaniels) it may be of benefit to pursue treatment when the cataract is smaller and softer, as surgery will be easier.
WHAT TREATMENT IS AVAILABLE?
Cataract treatment generally involves surgical removal or physical dissolution of the cataract under anesthesia. This is invasive and expensive and is not considered unless it can restore vision. A complete examination of the eye is performed by a veterinary ophthalmologist. If a cataract is present, it is not possible to see the retina through it; a test called an “Electroretinogram” is done to determine if the eye has a functional retina and could benefit from cataract surgery. Ultrasound of the eye can be used to look for retinal detachments. If the eye has a blinded retina, there is no point to subjecting the patient to surgery.
Obviously, the patient must be in good general health to undergo surgery; diabetics must be well regulated before cataract surgery. Also, it should be obvious that in order for a patient to be a good candidate for surgery, the patient must have a temperament conducive to the administration of eye drops at home. Pets with cataracts in one eye that can see in the other eye generally do not need surgery.
If the patient proves to be an appropriate surgical candidate, the lens can be softened and sucked away using a technique called “phacoemulsification.” If the patient is relatively young, the lens is relatively soft and phacoemulsification is the first choice procedure.
Older patients have much harder lenses and it is more common in these patients to simply remove the lens surgically.
COMPLICATIONS: Some degree of uveitis (deep inflammation) in s unavoidable. This can cause a pupil constriction reaction which can increase the risk to scarring within the eye. Eye drops to keep the pupil dilated are usually effective in preventing this but not always. Inflammation in the eye will resolve over weeks to months after surgery. Success rate is higher for cataract surgery if minimal inflammation is present in the eye prior to surgery thus pre-operative anti-inflammatory eye drops are frequently prescribed.
Another complication involves the development of opacities on the remaining lens capsule. In humans, laser surgery is used to remove the lens capsule but in the dog, the capsule is too thick for this. Some ophthalmologists prefer to remove the capsule as a preventive measure. The portion of the capsule that is involved in this reaction is present in young dogs but not in adult dogs.
Bleeding after surgery can be an enormous complication and can easily be caused by excess barking or activity after surgery. Small bleeds are of little consequence but a large bleed could ruin vision.
OVERALL, A 90% SUCCESS RATE IS DESCRIBED AFTER CATARACT SURGERY.
An Elizabethan collar will be very important after surgery as patient rubbing or pawing at the eye could be a disaster. Topical therapy (eye drops) will be needed for several months after surgery.
BEFORE EMBARKING ON THE ADVENTURE OF
CATARACT SURGERY, BE SURE TO OBTAIN
A CLEAR EXPLANATION FROM YOUR
OPHTHALMOLOGIST OF EXACTLY
WHAT THE HOME CARE WILL INVOLVE.
LENS IMPLANTATION
A prosthetic lens may or may not be inserted after the cataractous lens has been removed. Without the prosthesis, the dog’s vision will be approximately 20/800, and objects will appear to be reversed (as in a mirror). This sounds debilitating but, again, it is important to remember that pets do not depend on their eyes in the way we do so it is hard to make a good analogy.
Pros: A prosthetic lens allows for normal restoration of vision.
Cons: The incidence of uveitis (deep eye inflammation) after surgery is increased for patients receiving lens implants. Dogs appear to have functional vision without prosthetic lenses. In one study, the complication rate was 15% after lens implantation.
WHAT IF IT GOES UNTREATED?
A cataract by itself does not necessarily require treatment. If there is no associated inflammation and no associated glaucoma and the only problem is blindness, it is perfectly reasonable to have a blind pet. Blind animals have good life quality and do well though it is important not to move furniture around or leave any hazardous clutter in the home.
For further information, the following web sites may be helpful:
www.eyevet.ca
This site has a helpful area on dealing with a blind dog.
www.eyevet.ca/cataract.html
There is also a good area on cataracts with more details on the procedures.
www.veterinaryvision.com/dvm_forum/dvm-cataracts.htm
A good description of the surgical procedures for cataracts.
www.acvo.com
The Official web site of the American College of Veterinary Ophthalmologists.