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A scotoma is an area of obscurity in the visual field. In a simple example, someone might notice a black spot at the corner of the eye which impedes peripheral vision. This condition cannot be treated in most cases, but it is still important to seek care from an ophthalmologist if a scotoma appears, because it can indicate a serious problem. Treatment can be used to prevent the spot from growing larger, and to address the underlying issue which led to the development of the scotoma.
These marks can appear anywhere in the visual field. Along the periphery of the vision, they can be irritating but not debilitating. However, scotomas in the middle of the visual field can cause serious problems. People may have trouble with basic tasks like reading or driving, for example, because of the obscurity. Usually the spot only appears in one eye, but this disorder has been seen in both eyes in some cases.
Many different conditions can lead to this disorder, including damage to the brain caused by a stroke, tumor, or traumatic brain injury. Demyelinating diseases like multiple sclerosis, along with damage to the optic nerve, can also lead to a scotoma, as can vascular blockages, damage to the retina, and the introduction of toxins to the body. People usually notice the visual obscurity because it changes their vision markedly, even if it's only in the corner of the eye.
When a scotoma is diagnosed, a doctor can figure out what the underlying cause is, and develop a treatment. Sometimes, the treatment will resolve the spot, assuming that the eye was not permanently damaged. In other instances, the condition will remain, but getting the underlying cause under control is still important for general health, and to prevent further damage.
Some people find that eye exercises help them to cope with the condition, by getting them used to using their eyes in unusual ways to compensate for the vision loss. Some companies even market vision treatment systems which will supposedly cure scotomas. These companies have usually not been evaluated by regulatory agencies, and their products are of questionable utility.
A special kind of mark known as a scintillating scotoma is associated with migraine headaches. In this case, the mark flashes, rather than simply being dark, and it may drift around the eye, develop a zig-zag pattern, or create arcs of light. The appearance of visual field disturbances can serve as a warning to a migraine sufferer that a headache is on the way.
@umbra21 - Technically everyone has a scotoma as well. It's known as the "blind spot" and it's the part of the eye where the nerve connecting it to the brain is attached and there are no receptive cells (I think).
But, since you've got it there all the time, your brain manages to convince you that it doesn't exist.
I remember in school they showed us a few tricks for revealing your blind spot. I don't remember them now, but I'm sure people could find them online if they wanted to.
A visual scotoma isn't the same thing as the little wiggly things you might sometimes see across your vision, particularly if you have unfocused your eyes.
I asked an optician about those once and he told me that they were left over from when you were a baby, and are just bits of cell floating around the eye and casting shadows onto the retina.
The only reason you are able to see them, when they are quite tiny, is because they are right on top of your eye.
Plus they move when you try to look at them, because they move with your eye.
So, yeah they aren't scary like a scotoma, which could be a serious medical problem. Eye floaters are something almost everyone has to some degree.