How cataract occurs?
There is a lens in the eye which has its place behind the pupil. It is transparent and focuses light onto the
retina. The retina sends the image to the brain, where vision is perceived. Cataract forms when the cells and the protein of the lens begin to deteriorate. The lens gets cloudy and the light cannot be transmitted to the retina. There are many different forms and causes of cataract formation.
What is a cataract?
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. This disruption results in the loss of transparency and the resultant reduction in vision is called cataract. When the opacity is very small, it leads to blurred vision called immature cataract. When the entire lens becomes cloudy and there is loss of all functional vision, it is called mature cataract.
Cataracts vis-à-vis nuclear sclerosis
People often confuse cataract with another common problem called nuclear sclerosis. Nuclear sclerosis is a normal change that occurs in the lenses of older dogs. Nuclear sclerosis appears as a slight graying of the lens. The loss of transparency occurs because of compression of the linear fibers in the lens. It usually occurs in both eyes at the same time and occurs in older dogs. This condition does not significantly affect the vision of the dog and treatment is not recommended.
What are the causes of cataract?
There are several causes of cataracts which include:
Genetic: Cataracts in dogs are frequently inherited. Over 40 breeds of dogs are known to be predisposed to cataract e.g. Retrievers, Cocker Spaniels, Terriers, etc.
Trauma: If the lens is punctured or damaged due to automobile accident, penetration of a thorn or stick, a cat scratch, etc usually lead to cataract.
Diabetes: Diabetes mellitus (sugar diabetes) is a systemic disease where regulation of blood sugar (glucose) is not controlled. The lens requires some glucose, but when the levels are too high, cataract can form rapidly.
Old age: As animal becomes geriatric, all his body functions generally become sluggish and hence the eyesight. Age-related cataracts are usually very small and tend to progress very slowly.
Other causes: Nutritional deficiencies early in life, changes in blood calcium, exposure to certain drugs and toxins, exposure to intense microwaves, radiation therapy and electrocution may also alter both nutrition and structure of the lens, resulting in cataract.
How can cataract be diagnosed?
Diagnostic tests are needed to recognize cataracts and exclude other diseases. These tests include:
Complete eye examination: Such an examination includes fluorescein staining of the cornea, schirmer tear test, slit lamp biomicroscopy, etc.
Blood tests: Blood tests are usually done for diabetes and other systemic diseases.
Ocular ultrasound: Ocular ultrasound is performed if the retina cannot be examined because the cataract is too opaque, and if surgery is being considered.
An electroretinogram (ERG): It is also frequently performed prior to cataract surgery in order to evaluate the function of the retina. An ERG is especially important in determining underlying retinal disease masked by the cataracts (if the lens is too opaque for the entire retina to be examined).
How can cataract be treated?
Treatment for canine cataracts includes surgical removal of the lens, which may include one or more of the following:
Cataract surgery : Phacoemulsification is the most common technique used in both humans and animals to remove a cataract. Once the pupils have been dilated and animal is under general anesthesia, a small incision is made through the cornea. The lens is housed in a small bag called the lens capsule. A small tear is made in the front capsule and a circular piece of the capsule is removed. The phacoemulsification instrument uses ultrasonic waves to break apart the lens and then suck it out. Most of the lenses are removed by phacoemulsification, and then the lens capsule (the “bag”) is cleaned of any remaining lens material. Frequently an intraocular lens implant (a prosthetic lens) is then placed into the lens capsule. The lens capsule acts as a bag to hold the implant in place. There are lens implants for both dogs and cats, and these prosthetic lenses return the vision as close to normal as possible.
Extracapsular lens extraction: This is another cataract removal technique. It is used either when a phacoemulsification machine is not available or when a cataract is so hard or old that the phacoemulsification instrument isn’t powerful enough to break up and remove the lens. The surgical procedure requires making a larger incision through the cornea and a larger hole in the lens capsule so that the lens can be removed from the bag as a whole. A lens implant can still frequently be inserted during this type of procedure.
Intracapsular lens extraction: This is another surgical method that involves making a large incision through the cornea and removing the whole lens in its capsule. This procedure is generally used when a cataractous lens has shifted out of position and is no longer held firmly in place inside the eye. Because the lens capsule has been removed, if a lens implant is going to be used, it has to be sewn into place because there is no capsular bag left to hold it in the center of the eye.
Post-operative care of cataract surgery
Regardless of which type of procedure is used to remove a cataractous lens, there are many postoperative medications and important home care instructions to be followed after surgery.
After cataract surgery, the first one to two weeks are the most labour-intensive. The dog must be kept quiet and calm. Usually an Elizabethan collar is used to keep the dog from rubbing or traumatizing the eye. This collar should stay on at all times. Playing, barking and jumping should be discouraged and all pressure around the head should be minimized. Several topical (drops) and oral medications may be used after surgery, such as:
- anti-inflammatory drops
- dilating drops
- antibiotic ophthalmic drops
- oral anti-inflammatory drugs
- oral antibiotics
(Dr S S Patil is PhD scholar at Centre of Advanced Studies in Animal Nutrition, Indian Veterinary Research Institute, Izatnagar; Dr KB Kore is PhD scholar at Division of Animal Nutrition, IVRI, Izatnagar; and Dr PP Mirajkar is MVSc scholar at Division of LES, IVRI, Izatnagar)