What is Glaucoma?

Glaucoma is an eye disease which is caused by raised pressure within the eye.

The eyeball is filled with fluid called the aqueous humor. The fluid is constantly produced and drained from the eye and supplies nutrition for all parts of the eye. Glaucoma is caused by a decrease in the amount of fluid that flows out of the eye which results in pressure within the eye rising. A delicate balance between the production and reabsorption of aqueous humor maintains the normal intraocular pressure.

This is a serious disease and without proper treatment it can result in blindness, severe pain and the eventual loss of the eye. Glaucoma is one of the most frequent causes of blindness in adult dogs of many breeds including Cocker Spaniels.

Types of Glaucoma:

There are two main types of glaucoma, primary and secondary. In primary glaucoma, the cause of the increase in pressure is due to a narrowing of the drainage angle. It is frequently an inherited problem. Beagles, Basset Hounds, & Cocker Spaniels are prone to this type of glaucoma. In secondary glaucoma the pressure is too high because something else is wrong inside the eye, such as a lens luxation, bleeding, inflammation, or tumour.

Aqueous humor made in the ciliary body flows through the pupil into the anterior chamber. The Aqueous humor then drains into the bloodstream through the iridocorneal angle.

Signs/Symptoms:

Bloodshot (red) eye, painful eye, lids may be held shut, tearing, eye may appear cloudy or blue, sudden blindness, depression, appetite loss & bulging eye.

Treatment:

The treatment chosen (i.e. surgery &/or medical therapy) will be influenced by what the goal of therapy is: to stop pain in a blind eye or to preserve vision.

Medical treatment consists of a number of different drugs used in combination. Some are given by mouth and affect the whole body, while others are put directly into the eye and have a local effect. The drugs that work when the problem is first diagnosed may not work forever. Therefore, the intraocular pressure needs to be monitored on a regular basis so that the medication regimen can be altered to fit the needs of the patient. Unfortunately, glaucoma cannot be cured, only controlled. When medical treatment fails, surgical therapy can help prolong vision.

Medical Therapy:

The following drugs are frequently used:

Carbonic anhydrase inhibitors: Oral and topical formulations reduce the amount of fluid produced inside the eye.

Miotics (Parasympathomimetics): Used on the eye, it helps to increase the outflow of fluid from the eye. It may cause a temporary redness, burning or stinging in the eye.

Beta adrenergic blockers: Used on the eye, to reduce the amount of fluid produced inside the eye.

Surgical Treatment:

In some cases, there are surgical procedures that may provide permanent or at least long-term control. One of these procedures uses a laser to destroy the part of the eye that produces fluid and thereby reduces the pressure. Another surgery inserts a tube into the eye that shunts the fluid under the conjunctiva (pink tissue) deep in the eye socket. Neither surgery is 100% successful nor may multiple surgeries over several years be required to preserve vision.

Blind, painful eyes can be removed to eliminate discomfort for your pet and to avoid the need for medicines that are expensive and affect the whole body. This surgery is called enucleation and the eyelids are permanently sewn shut. Blind painful eyes may also have an intraocular prosthesis placed. With this surgery, the contents inside the eye are removed and sterile silicone prosthesis is placed. The eye is preserved but no longer has the pain from high pressure. Afterwards, the eye looks fairly normal, but remains blind.

Eye Testing under the KC/BVA Eye Scheme

The Kennel Club/British Veterinary Association offers a test for a predisposition to Glaucoma. The test is called Gonioscopy and it is looking for a condition known as Goniodysgenesis (the abnormal & incomplete development of the intraocular fluid egress channels inside the eye which is associated with Glaucoma).

How IOP is balanced

The fluid in the eye drains through small channels which form tunnels for the fluid to drain through, these tunnels are called 'pectinate ligaments'. If a dog has goniodysgenesis, the flow holes contained in the pectinate ligaments are compromised and the animal may have an increased glaucoma risk.

There are varying degrees of abnormality seen in the flow holes from mild abnormality to a complete occlusion (blockage) of the drainage angle. The degree of abnormality is proportional to the risk of glaucoma developing. Mildly affected dogs will not develop glaucoma & they will not pass glaucoma onto their offspring when mated to a partner with good drainage angles and pectinate ligaments.

Mode of Inheritance

Glaucoma is thought to be controlled by more than one mutant gene and with threshold characteristics (rather like hip dysplasia) and therefore there are degrees of severity to the 'defect'.

The mere fact that a dog is diagnosed with goniodysgenesis does NOT mean that they have Glaucoma or that Glaucoma is inevitable. A lot depends on the extent of the abnormality. For example, if you rate the defect on a rising scale of 1 - 4, a dog scoring 1 - 2 is not thought to be at risk of developing Glaucoma. They will "fail" the eye test but they will not develop the disease & are not thought to be at risk of passing the disease onto their offspring.

Breeding Options/Plans

Due to the pain and loss of eye sight and possibly the eye, Veterinary Ophthalmologists take Glaucoma very seriously and in an attempt to control the possible inheritance of the disease they will mark the dogs eye certificate with "affected" even when the degree of abnormality is minimal i.e. the tested dog does not have Glaucoma.

Currently (2011) the eye scheme only allows for a "unaffected" or "affected" grade, no account is taken for the degree of abnormality i.e. very mild to extreme. This aspect of the scheme is under investigation at the moment and it is possible that the scheme will change & a dog will be given a grade, advice will then be given to which grades are acceptable for breeding and which dogs clearly shouldn't be bred.

The important thing to remember is that a dog failing the test with a theoretical score of 1 - 3, if mated to a partner who has a clear certificate (i.e. 100% normal drainage angle & pectinate ligaments) is not expected to produce affected offspring. This is because the absence of abnormality in the clear partner counterbalances the slight abnormality in the "affected" partner in the same manner that mating a higher hip scoring dog to a low scoring partner will normally produce offspring free of dysplasia.

Of course when planning any mating it is the sum total of both parents that is important & all aspects should be considered and clearly a dog severely affected with goniodysgenesis or glaucoma should not be bred due to the risk of producing affected offspring.