The retina is a layer of tissue that collects and processes the light that enters the eye. Sometimes a tear occurs in the retina, allowing fluid to build up under the retina, eventually causing the retina to pull loose, or detach.
In most situations, the vitreous gel that is inside the back of the eye causes the retina to tear and detach. When we are born, the vitreous is a relatively thick gel. When we reach adulthood, the vitreous gel begins to develop pockets of fluid inside the gel. The gel then collapses, forming strands and spots that we call “floaters”. These floaters are perceived as being floating dark or grey shadows in our vision, and are more likely to be seen when looking at a uniformly lit field, such as a white desk, a brightly lit computer monitor or television, or a white cloud.
Floaters by themselves are nuisances, but nothing more. However, in the process of loosening up, the vitreous sometimes pulls on the retina. When this occurs, white flashes of light in the vision might occur. Sometimes the vitreous pulls on the retina enough to cause a tear to develop in the retina. When a tear in the retina occurs, we sometimes see a sudden, large cloud of floaters.
Tears in the retina can progress to a detachment of the retina. In these situations, a dark “shadow” or “curtain” may be seen. These situations should be considered as emergencies, and they occur it is important that you be examined by an ophthalmologist as soon as possible.
Tears in the retina can be treated with laser or freezing to help prevent a retinal detachment. If a retinal detachment should occur, there is usually an excellent chance that it can be repaired. But it is always better to treat retinal tears before a detachment develops.
For more information, click here: http://www.geteyesmart.org/eyesmart/diseases/detached-torn-retina.cfm