From time to time nearly everyone experiences floaters -- small dots, lines, clouds, or "cobwebs" across the visual field. This phenomenon is caused by shadows cast on the retina by microscopic structures within the vitreous humor (a thick, gel-like substance that fills the back of the eyeball behind the lens). In most cases, flashes and floaters are harmless, but they may they indicate that the retina (the innermost layer of the eye that consists of light-sensitive nerve tissue) is tearing or in danger of detaching from the underlying layers of the eyeball. Detached retina may be a medical emergency that can result in blindness.

As you edge over age 40, the vitreous -- the clear gel-like substance inside your eyes -- begins to liquify and shrink. Within the gel are millions of fibers attached to the retina, the light-sensitive nerve tissue lining the interior of the eye. As the gel shrinks, the fibers break, allowing the vitreous to peel away from the retina, a process called posterior vitreous detachment (PVD).

Most of the time, the development of a PVD doesn't cause any problems for eye health, and typically no treatment is needed. When a PVD occurs you may develop floaters that move or swim as you move your eyes.

Many older people get floaters without PVD. But their sudden appearance or an increase in the number of typical floaters may announce PVD, as may flashes of light. These arise as the separating vitreous tugs on the retina, stimulating cells that send a message of light to the brain. In some cases, this pulling may break a blood vessel on the retina, resulting in what's called a vitreous hemorrhage.

To check for PVD, an eye doctor will dilate your pupil with eye drops and use a special lens to see the vitreous and the retina. If you do have PVD, often the physician will see a ring of the vitreous fibers floating in the vitreous cavity (a Weiss ring). This represents the site where the vitreous was formerly attached to the retina.

In 10-15% of people who have PVD with symptoms, some of the vitreous fibers pull so hard as they naturally separate from the retina that they create a tear or hole, especially in the thin retinal peripheral tissue. Vitreous fluid then has the opportunity to leak through the tear, which can cause the retina to detach from the back wall of the eye. This is a sight-threatening situation: If the tear is not repaired, the retina can detach completely.

Bottom line: Although you can't prevent or treat PVD, you can try to prevent its repercussions by alerting your doctor at the first sign of floaters, flashes of light, or any change in side vision. It is usually possible to repair a retinal tear, but a tear can also evolve quickly to retinal detachment. So don't wait to see whether floaters discontinue when you suddenly begin to notice lots of them. Even if no retinal tear appears, your doctor can help you stay alert for that possibility.