The back of the eye is filled with a gel called the vitreous. Over time, the gel becomes less gel-like and waterier and it starts to peel off the back of the retina, which is directly behind it. In the process, it can break off little pieces of itself which look like floaters in the vision. They can look like bugs, hairs, amoebas, or curved, squiggly lines. They are more prominent when looking at a bright white background like a computer or cell phone screen. The vitreous is attached to the retina at certain locations—the optic nerve, the central part of the retina (macula), and the periphery. When the vitreous starts breaking down and begins pulling forward, it tugs on the retina and electric signals are produced causing flashing lights. This process is called a vitreous detachment and usually affects patient in their 50-70s. It can affect younger patients who are nearsighted or have experienced trauma to their eyes.
Usually the vitreous eventually peels off the retina and the flashing lights stop. It may take a few months. Sometimes the vitreous tugs so hard that it rips off a piece of the retina, causing a retinal tear. This can be vision-threatening as fluid can go into the hole and cause the retina to detach. Symptoms of this include a sudden increase in floaters, flashing lights or a dark curtain or veil coming over the vision in the case of a retinal detachment. This would require seeing a retina specialist for treatment and could require surgery.