All animal eyes have a lens inside them. This lens helps the eye focus. It is a large structure shaped like two saucers placed together which is in the centre of the eye. Unlike the hard glass lens of a camera it is rather like a clear jelly in a bag of cling film. A cataract is when this jelly goes cloudy or white either all over or in parts.
There are many causes of cataracts. These include; injuries to the eye, inflammation in the eye and diseases such as diabetes. There are also inherited types of cataract. Cataracts can also form due to diseases of other parts of the eye such as the retina.
Anatomy of the eye showing the position of the lens
There is no medical treatment known to slow the progression, prevent the formation or reverse the changes of cataracts. Like the egg , once "cooked" the white stays white. Surgery to remove the affected lens is the only treatment to provide a return to vision.
Cataract surgery is generally advised for those animals who have become blind because they have developed cataracts in both eyes. Not all animals or their eyes are suitable for surgery. In view of this an important part of the process is the cataract Assessment Consultation.
Here the eyes are examined to make sure there is no other disease process present which might affect the outcome of the surgery. The Retina (film at the back of the eye) is examined using an ultrasound probe. This enables us to "see through" the opaque lens. It is important to know whether this retina is working properly because we don't want to end up in a situation that the animal cannot see once the cataract has been removed because the retina was already damaged. Further tests to examine the retina may be advised such as electro-retinography (ERG). A high percentage of cataracts tend to be in the older pet. It is therefore useful to know how fit an animal is prior to lengthy surgery. An assessment of this fitness can be done with tests on a blood sample. These tests are also useful if diabetic cataracts are suspected.
Electroretinograph (ERG) showing good retinal function
The dog's eye reacts violently to any surgery. The dog's lens is much much bigger than a human lens and to remove it surgically would require a large hole. This is why we use a technique called phacoemulsification. Here a special probe is passed into the eye. This vibrates, breaks up the cataract and then vacuums it out. This method causes least disturbance to the eye and hopefully the least problems. Unlike the human eye when a dog is anaesthetised the cornea (clear part) disappears from view. To overcome this and allow the surgery a special anaesthetic is used. This temporally paralyses the eye muscles. However it also paralyses the breathing muscles so the anaesthetist has to connect the animal to a machine to breathe for it throughout the operation. We like your pet to return home the same day if possible as they are usually more relaxed there. Post operative care involves frequent applications of eye drops and tablets sometimes for weeks.
In dogs with the advances of techniques, specialised equipment and drugs this is 75-85% . Ten years ago it was 50-60%. Fortunately these techniques mean there are very few complications during the actual surgery. Most of the complications occur in the days after surgery. These include bleeding into the eye. This particularly occurs if the animal gets excited and raises its blood pressure. A small amount of bleeding will disappear in a few days. The problems arise with major bleeding , this may take weeks to resolve and as it clots may pull off the retina. The eye may develop glaucoma - a high pressure within the eye. This if detected may be treated before it does damage to the eye. The retina is held on the walls of the eye mainly by the fluids inside. Removing a large volume lens can allow the retina to loosen and detach. Thus post operative checks are essential to recognise and treat complications as they occur. This is usually in the form of eye drops or tablets but occasionally the animal has to be taken back to surgery to flush out a blood clot.
Removal of the lens means they have a fixed focus at about 10 feet. As animals do not have the need for fine focus such as reading this sight is usually adequate for a happy life. Synthetic lenses may be fitted in dogs to improve focus. These are now available and can be implanted if the eyes are suitable.