Detachment of part of the retina at the back of the eye from the vascular membrane beneath it. Usually a tear occurs in the retina, through which aqueous humour can pass between retina and vascular membrane. Detached retina causes spontaneous, partial and painless loss of field of vision (hemianopia). A retinal tear usually occurs in later life in people with severe myopia (60 per cent of cases). In 35 per cent of the cases, old age alone is the cause. In cases of severe short-sightedness the eyeball is malformed, and this slight stretching means that part of the retina is less well supplied with blood, and tears more easily. Old age brings abnormalities of the blood vessels, which can reduce blood supply to the retina; it becomes thinner, and can tear more easily. Sometimes the condition is caused by eye injury or a blow to the head, but in such cases too the retina is likely to have already been prone to damage in the first place. The condition causes sudden painless impairment of vision; a veil or shadow can sometimes be seen as well, often in the region of the nose. In some cases flashes of light may be seen just before the retina becomes detached. Treatment must begin as a matter of urgency, because there is danger of detachment of the entire retina, causing blindness. Treatment is by bed rest, sometimes in a certain position in order to try to settle the retina in its original position. Laser therapy is also used to attempt to weld the retina to the vascular membrane by the formation of small scars. The eye must be kept completely still in the period immediately after the operation. In 80 per cent of cases vision is wholly or partly restored, but there is a slight chance that the retina might become detached again.