I have an elderly but fit Labrador and I am a bit concerned that he may be going blind.

The pupils of his eyes are quite blue, which is different from before. He seems to get about OK but I realise that some blind dogs can use their nose very well!

Do you think he has cataracts?

GUNDOG HEALTH

Neil McIntosh

In a word, no! The lens is the transparent structure that sits in the middle of the eye and focuses light onto the retina at the back of the eye.

The retina is composed of light sensitive cells, which transmit down the optic nerve to the brain.

Cataract occurs when the lens becomes cloudy and opaque. As a result, no light is able to reach the retina and blindness occurs.

Affected dogs always have a wide-open pupil as the brain does its best to let as much light in as possible.

What you are seeing in your old dog is different. In young animals, the fibres in the lens are arranged in straight lines (like Polaroid sunglasses) so you can see through them.

In older individuals, the fibres age and become wavy (like mirrored sunglasses).

As a result, you see a bluish reflection of light off the wavy fibres, making you think about cataract.

Don?t worry, he is seeing out okay! The condition is called nuclear sclerosis and is quite common.

(It is also why most of you over 45 years old will be wearing glasses to read this!)

Compare the picture of the yellow lab (which has cataract, an opaque lens and a dilated pupil) with the black lab (light is reflecting off the lens with a constricted pupil).