Usually caused by an illness affecting the whole body; both eyes are usually involved. The symptoms are generally the same for all conditions, but they vary in degree. They include declining sharpness of vision, (partial) loss of field of vision, lack of sensitivity to light, possibly to the extent of night blindness, and sometimes oversensiti-vity to light stimuli. Sometimes objects are perceived incorrectly. Pain is rare. The conditions can usually be detected by examination of the eye with a special instrument (ophthalmoscope). The commonest source of difficulty is a disorder of the blood vessels in or of the blood supply to the retina. Other possibilities are inflammation, degenerative disorders, detachment of the retina, and tumours. Disorders occur in the wall of the small arteries in high blood pressure, diabetes mellitus, and severe cases of pre-eclampsia. This shows in the retina as a thickening of the arterial wall, which impairs circulation and induces the secretion of moisture from the blood vessels. Haemorrhage can also occur in the retina and new blood vessels can form; these conditions decrease vision. Severe disorders can often be prevented by good treatment of the underlying condition, although some damage to eyesight may be inevitable. Careful checking of the retina and laser therapy to close blood vessels can be important in limiting any damage.