Q. Grandpa saw flashes and floaters, and the doctor said he should have come in much sooner. What's the scoop?

A. The clear jelly that fills the inside of our eyes, called the vitreous, sometimes shrinks with age, pulling away from the delicate layer of light-sensitive cells — the retina — that coats the back of the eye. If that's all that happens, you have so-called posterior vitreous detachment with sudden, painless onset of flashes, floaters or both. Trouble is that the jelly sometimes pulls the retina along with it, shearing it away from underlying tissues, causing either a retinal tear or detachment. When this happens, there's reduced vision and it's a medical emergency, as permanent loss of vision can occur. If caught early enough, outpatient treatment can sometimes be done. Small holes and tears can be treated with painless laser surgery or a freeze treatment called cryopexy. Late cases need to be treated in the operating room and may not always restore vision. Once the retina is reattached to the underlying tissues, vision improves.

Floaters are dark shapes, often cell- or strand-like, that may move or keep their position with eye movement. They are noticed best against a solid color, like a wall or the sky. Floaters you've had for years are common, harmless and caused by clumps of vitreous protein and collagen. But sudden showers of floaters or any new floaters warn of dangerous vitreous or retinal detachment. Flashes are repeated, brief flashes of white light in the peripheral vision of one eye due to traction on the retina from the shrinking vitreous jelly. Sometimes the aura of migraine headaches can be mistaken for the flashes of vitreous detachment. But migraine auras usually consist of light patterns or jagged lines that are colored and often "march" through the visual fields of both eyes, whereas the flashes of vitreous detachment are white and occur in the periphery or edges of just one eye.

In summary, posterior vitreous detachment is the most common cause of a sudden increase of flashes and/or floaters in one eye, and can lead to retinal damage with a painless decrease in vision that only a doctor may be able to detect. It may also be like a curtain in front of the eye or like smoky, cloudy vision. Central vision may be OK, so there's often a dangerous delay in seeing the doctor if only peripheral vision is reduced. But we need to repair the retina before the macula — the part responsible for central vision, like reading — becomes detached, so don't delay.

Dr. Landis Lum is a family-practice physician for Kaiser Permanente and an associate clinical professor at the University of Hawai'i John A. Burns School of Medicine. Write: Prescriptions, Island Life, The Honolulu Advertiser, 605 Kapi'olani Blvd., Honolulu, HI 96813; or e-mail islandlife@honoluluadvertiser.com.