A cataract is a clouding of the lens in the eye. This prevents light from passing through the lens and reaching the back of the eye, interfering with vision.
Cataracts most commonly develop as people get older - these are called age-related cataracts.
Congenital cataracts are present from birth and are usually the result of exposure to infection, radiation or drugs before birth, or because of a genetic problem.
Left untreated, congenital cataracts can result in permanent visual loss.
Other causes of cataracts include illnesses such as diabetes, certain drug treatments, long-standing inflammation and injury.
Age-related cataracts develop gradually
The symptoms of age-related cataracts develop gradually, getting worse over many years. Blurred and cloudy vision is common. It may be difficult to see in dim light and bright lights may appear more glaring. Colours may appear different to how they once did and there may be a yellow tinge to vision.
The older someone is, the more likely they are to suffer from age-related cataracts. It's estimated that around 71 per cent of people over the age of 85 years have a cataract bad enough to affect their sight. Women are more commonly affected than men.
Good management of conditions such as diabetes reduces the risk of cataracts developing.
It isn't possible to prevent age-related cataracts, but regular eye tests can help to detect problems early.
The most effective treatment is to remove the affected lens surgically and replace it with a new one.