The lens inside your eye
Just behind the pupil sits the eye’s natural lens. It is about the size of a lentil: roughly 10 millimetres across and 4 millimetres thick, held in place by fine fibres all around its edge. It is made of protein fibres packed so neatly that light passes straight through.
The lens works with the cornea, the clear dome at the front of the eye, to focus light onto the retina at the back. The retina turns that light into signals, and the optic nerve carries them to the brain.
What goes wrong
The lens keeps growing slowly throughout life. Over the years it turns slightly yellow and less flexible. Usually from about the age of 60, its proteins begin to clump and lose their clarity. That clouding is a cataract.
Light that should pass straight through is now scattered or blocked. Vision becomes foggy, colours look faded, and lights glare. The change is gradual, and it usually affects both eyes, one more than the other.
Age is the most common cause. Cataracts can also come sooner with diabetes, long-term use of steroid medicines, an injury to the eye, some other eye conditions and, rarely, inherited causes.
The signs
Cataracts develop slowly and differently in each person, so the signs vary. Common ones are:
- Cloudy or foggy vision. Many people describe a haze or film they cannot blink away.
- Glare. Sunlight or oncoming headlights at night become dazzling.
- Trouble with detail. Faces, small print, steps and kerbs become harder to make out.
- Frequent changes of glasses. A new prescription helps for a while, then stops helping.
Blurred vision has many causes, so these signs alone do not prove you have a cataract. Only an eye examination can tell.
How a cataract is assessed
A full eye examination confirms the cataract and checks for anything else that could be affecting your sight. The doctor looks at the cataract under a microscope, examines the retina and optic nerve through a widened pupil, and may order further tests.
The letter chart alone is not a good measure of how much a cataract affects you. How you manage in real life, in real lighting, matters more.
When is surgery the right choice?
There is no medicine or eye drop that has been shown to clear a cataract or slow it down. Surgery is the only treatment.
A simple cataract does not need urgent removal, and it does not usually harm the eye while you wait. The right time for surgery depends on how much the cataract gets in the way of your life: driving, reading, watching television, working or moving safely around the house. If you manage well, you can wait and be checked regularly. If it troubles you, surgery can help even when the letter chart looks reasonable.
In a few cases, a doctor may advise earlier surgery, for example if the cataract raises the risk of glaucoma or stops them from examining the back of the eye.
Read on: How cataract surgery works.
This guide explains; it does not diagnose. Your doctor’s advice about your own eyes comes first.