The eye is a hollow spherical structure. Internally, a delicate sheet of nerve tissue called the retina lines the wall of the eye like wallpaper. The central cavity of the eye is filled with a sticky gel called the vitreous. The vitreous gel is 99% water and 1% solid elements. Of the solid portion, there are filaments that help to support the shape and tenacity of the gel, similar to steel bars reinforcing concrete. At the back of the eye, the filaments attach the vitreous to the optic nerve and blood vessels. Over time, the vitreous gel tends to liquefy in its center, creating liquid pockets surrounded by the more formed gel. This process is known as vitreous degeneration or “syneresis.”
Now, the filaments in these pockets are free to clump together. The clumps may ‘float’ within the liquid vitreous pockets, giving the patient a sensation of floaters. Floaters appear as dots, spots, or curly lines that appear suspended in front of you and move as your eye moves. The floaters are most noticeable when viewed in bright illumination because they cast a shadow on the retina.
Eventually, with enough degeneration within the vitreous gel, the filamentous scaffold collapses and the vitreous separates from the retina. This event is called a posterior vitreous detachment (PVD) and is a normal event occurring in most people sometime between 40-70 years of age. A PVD will often occur at an earlier age in people who are nearsighted or have undergone cataract surgery.
Flashes of light are also a common symptom of a PVD. When the vitreous pulls on the retina – to which it is attached – the photoreceptors are mechanically stimulated. The retinal cells are incapable of perceiving pain, pressure, or temperature. The only stimulus that the retina responds to is ‘light’. So when the retinal photoreceptors experience mechanical stimulation because of this vitreous pull, they send a signal to the brain in the form of disorganized light, which is perceived by the brain as a flash.
If a retinal blood vessel is broken from the pulling a vitreous hemorrhage can occur. A small amount of blood may be seen as a shower of spots. Larger hemorrhages can cause large dark blobs in the visual field or a overall decrease in vision. When hemorrhage occurs, retinal tears and or retinal detachment are very frequent (90%).
If the vitreous gel is abnormally adherent to the retina or the retina is weak in a certain area a retinal tear can occur as the gel separates and pulls away from the retina. Once a retinal tear develops there is a significant risk of the liquid vitreous going through the break and detaching the retina from the back wall of the eye. If a retinal tear can be discovered before a retinal detachment develops it can be treated with the laser to seal the break and prevent a retinal detachment. Finding a retinal break can be difficult and requires a very complete examination of the edge of the retina (the retinal periphery). Fortunately the great majority of PVDs do not cause a retinal tear and not all retinal breaks will lead to a detachment. In general, though, if a tear is associated with a symptomatic PVD, the tear is at high risk for leading to a retinal detachment and should be treated. If you have new symptoms of a PVD (new floaters, flashes of light, shower of spots or a curtain or shade in your vision) it is important to have a prompt and thorough examination of the retina.
Retinal detachment should it occur, requires surgical correction. There are a variety of procedures that might be used, depending upon the nature of the specific patient’s problem. These surgeries are highly successful at reattaching the retina, and usually succeed at preserving useful vision.