The popular idea of a cataract is that it is a film which grows over the eye. In fact, a cataract is any opacity in the normally clear protein of the lens which lies behind the iris. The problem can be likened to white of egg when fresh this is clear protein, but when damaged (by heat, in the case of the egg) it becomes progressively more opaque. The same thing happens in a cooked fish lens. Worldwide, cataract is the most common cause of poor vision and is estimated to have blinded 20 million people who do not have access to surgery.

There are few well defined causes of cataract though many factors may contribute. Age is the most important and dehydration another, particularly in a hot climate. There is little evidence for light induced damage, though ionizing radiation can produce opacities. Long term systemic corticosteroid treatment can hasten the process. Congenital rubella or rare metabolic disorders can cause cataract in childhood and diabetics develop opacities earlier than age matched controls. Trauma to the eyeor inflammation inside it may hasten the ageing process.

The patient with cataract complains usually of blurring or misting of vision which is poorly corrected or even made worse with a pin hole. Symptoms are often worse in bright light (especially sunlight) as the opacity both scatters light and is more noticeable with a small pupil if it is central in the lens. The opacity may be visible to a bright light shone onto the pupil but early change can be detected as it causes blurring of the retinal view with the direct ophthalmoscope which cannot be corrected by altering the focusing lenses. With the ophthalmoscope backed off a little and the red reflex brought into focus (together with the pupil margin), the opacity shows as darker spokes, shadows or granular dots against the reflected light, like obscured glass. The pattern is seen better after dilating the pupil.

There are no known preventive measures or medical treatments for cataract. Long term aspirin has been debated as a retardant, but the results of studies are conflicting. The surgical management has been revolutionized by the development of artificial plastic lens implants, usually inserted into the same position that the natural lens occupied, behind the iris. Non-specialists often wonder when to refer a patient with lens opacity for surgery. Any patient who is troubled by the symptoms of cataract might be helped by surgery. Younger patients may be particularly bothered by glare from early cataract when driving at night. The elderly are particularly suitable for a lens implant, but this may be feasible in most younger patients. Surgery may be postponed until likely benefit exceeds risk. Children with cataract should be referred early for investigation and consideration for surgery.
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