From Dr. Lori Tindel-Kahn, Westmed Ophthalmologist:
The sudden onset of seeing “spots or cobwebs” in the eyes or “lightning flashes” out of the corner of the eye–lasting a split second–is one of the most common concerns that patients bring to ophthalmologists in the office, and even to the emergency room. This can be very startling for patients, and some think they are having a stroke. Often in the ER, patients will get CT scans to rule out stroke. The condition of spots and flashes in the field of vision, however, is very common and benign and usually presents to patients in their early 60s, and sometimes earlier in nearsighted patients.
We all have fluid in our eyes–the vitreous gel–that makes up most of the volume of the eyeball. When we are young, that gel is very sticky and adherent to the retina. As we age, the vitreous gel starts to shrink and condense and pull away from the retina. The pulling on the retina sends a signal to the brain that we interpret as a flash of light. It can look like a lightning bolt, or “seeing stars,” but only for an instant. When the vitreous pulls away, there can be condensations of the gel that float freely inside the eye, hence the name “floaters”. When light comes in through our pupils, it casts a shadow on these clumps of vitreous gel, and that is what we are seeing as spider webs, squiggly lines, dots, or mosquitoes.
When the entire vitreous separates, it is called a posterior vitreous detachment. This is often associated with one big dark spot centrally that moves with the eye movements. The spots generally settle over time, from gravity, so even though those vitreous condensations are still in the eye, they are no longer in the visual axis and visually disabling. The flashes usually subside after complete separation as well.
Posterior vitreous detachments do not require any treatment. It is important to get checked when you develop symptoms, however, as in a small minority of cases, when the gel pulls away, it can tear a retinal blood vessel. This can cause bleeding inside the eye, and it can pull with enough force to tear a piece of retina. Retinal tears do require treatment. The warning signs of a retinal tear include a new shower of floaters, swirls of dots, inside the eyes, more intense frequent flashes of light, and shadows blurring out fields of vision.
If you should develop split-second flashes associated with floaters, it is a good idea to have your retina checked to rule out retinal tears that may require treatment. The flashes should be monitored as well for duration, since flashes that are continuous and strobe-like with zigzag patterns may be due to migraine conditions (such as “migraine without headache”) and not due to a serious retinal condition. Your ophthalmologist will be able to help distinguish between the two and ensure that no retinal abnormalities are present.
You can find a list of Westmed ophthalmologists and contact information on this page.