In healthy eyes the lens is clear and transparent in order to transmit and focus the light into the retina at the rear of the eye. When the eye becomes cloudy as a result of disease, ageing and injury to the lens, the dog can suffer vision related problems.

One of the most common forms of cloudy lenses is cataracts, which are mostly genetically related, which cause a cloudy film to gradually cover the lens and obscure vision. The age of the dog and how severe the cataracts are depends largely on the breed of the dog. Cataracts can also develop after injury to the eye, as a complication from diseases or as a result of poor nutrition. Cataracts can also occur, in rarer cases, as a response to medication, toxin, or environmental factors. Depending on the dog’s age and health, most cataracts can be successfully removed with surgery.

In older dogs, nuclear sclerosis or lenticular sclerosis causes the nucleus of lens to become denser and the ageing fibres to be compressed by newer fibres causing a blue-grey haze over the lens. In most cases, however, the vision is not impaired for the dog.

In healthy eyes, the lens is clear, with a green glow that comes from the eye when illuminated for examination. The eye will appear cloudy if cataracts or nuclear sclerosis are present. Diagnosis of cloudy eyes will be done using an ERG (electroretinogram) to measure the electrical activity of the retina in response to flashes of light.