Blindness

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The eye acts functions like a camera. Light entering the eye is focused onto the light sensitive area (retina) by the lens. The iris automatically adjusts the amount of light entering the eye. Nerves from the eye transmit the information to the brain which converts it to an image. Another part of the brain automatically adjusts the focusing and iris size.

Blindness can occur due to damage anywhere along this pathway. The surface of the eye (cornea) can become damaged by trauma, a foreign body or disease. Any damage to the cornea results in swelling. This gives the cornea an opaque bluish colour which prevents light entering the eye. This opacity may be reversible with the correct treatment.

Ulceration of the cornea occurs when the surface layer of the eye is worn away. This is generally very painful and the eye will water profusely and the dog will keep the eyelids shut.

Chronic trauma to the cornea can result from long standing irritation from “in-growing” eye lashes (distichiasis) or from interning of the lids (entropion). This can also be the result of a relative lack of tear production called “dry eye”. The repeated trauma to the surface of the cornea results in the deposition of black pigment on the surface of the eye which will prevent light entering the eye.

Opacity of the lens is called a cataract. This may be due to hereditary conditions, be secondary to generalised disease such as sugar diabetes, or be a function of old age. Generally this if not painful, and the degree of visual impairment will depend on the extent and maturity of the cataract.

Damage to the retina can result from hereditary problems in some breeds (Progressive retinal atrophy). Swelling of the eyeball resulting from glaucoma (see picture on right) will also result in retinal damage. Tumours or swellings that occur behind the eye may not only cause protrusion of the eye ball but may also damage the nerve as it travels from the eye to the brain.

What your veterinarian will want to know

Your veterinarian will want to know how long you have noticed the problem. Are the changes subtle or does the dog frequently bump into objects? Has the blindness occurred suddenly or gradually become noticeable over a long time? Is the problem worse in bright light or at dusk? Is the condition painful? Has there been a discharge from the eye? Is the problem affecting one or both eyes?

The breed and age of the dog is important in considering hereditary conditions. Are there any other symptoms that may suggest some underlying generalised condition – such as sugar diabetes?

What your veterinarian may do

Following a full clinical examination, your veterinarian will want to make a detailed study of both eyes using special ophthalmic instruments, checking each of the separate structures in turn. He may put some local anaesthetic in the eye to check for foreign bodies under the lids. In checking the cornea, he may add a dye to the eye which helps show the presence of ulceration. Special paper strips can be used to measure the tear production.

The examination will involve checking the iris. Is the pupil widely dilated and does it constrict when a light is shone directly into the eye.

Examination of the retina may require a darkened room and the use of drops which dilate the pupil.