Sometimes, when someone has a stroke, they can lose some of their field of vision. Because strokes are usually caused by a failure of the blood supply to the brain rather than to the eye itself they tend to affect the part of the visual system that merges the images from the two eyes. The images start to be merged at a point just behind the nose, where the two optic nerves join up, and are gradually refined as the nerve tracts carry the images further back in the brain. The most detailed analysis of the images happens at the very back of the brain by which time the images have travelled, being merged and refined as they go, from the very front of the brain to the very back. Consequently the pattern and the amount of the visual field of each eye that has been lost give the neurologists a very good idea of where in the brain the stroke has occurred. This also applies to any other space-
occupying lesion in the brain space such as a tumour. The most common serious visual field loss takes away the vision from one entire side of the visual field. This isn’t like closing one eye, where the other eye can cover a large part of what is missing, but everything from a vertical line through the point you look at out to the extreme periphery on one side. What makes this more difficult to deal with is that the field loss isn’t ‘seen’ as a black or even a blank area but is simply missing. Not only is it missing but the sufferer will have no way of telling that it is missing at all and their brain will often make up what it thinks should be there and try to fill in the detail. I have seen patients that, knowing about this loss, try to compensate for it and those who are completely oblivious to it. One lady I saw on a visit to her home told me that if she went to cross a road she would look to the right (her blind side) screwing her head round to make sure that she saw the road properly. Then she would look left and check the road was clear that way. If she then glanced right again she would ‘see’ a clear road even if there was a bus there. Consequently she very rarely went out. Another lady, who was somewhat more immobile, was driving her family to distraction because she wouldn’t eat. They would place a tray with a meal on it on the left side of where she sat in front of the TV and half an hour later remove it untouched. Almost immediately afterwards she would be
complaining of being hungry. They were placing the tray exactly where she
couldn’t see and as far as she was concerned it wasn’t there at all. Placing the tray to her right solved the problem. Obviously not all strokes affect the eyesight but when they do they can be very confusing, not only to the sufferer but also to those who care for them.