What are those threads that float in your field of vision?

Dear Cecil: Sometimes when I’m lying on my back looking at the sky or the ceiling or some other light-colored background, I swear I can see specks and what looks like little threads floating by. They seem to move when I move my eyes, leading me to believe they’re actually on my eyes. Is there some optical phenomenon that allows us to focus that close? Is there a name for this effect? Mike P., Dallas

Cecil replies:

Those little specks and threads aren’t on your eyes, you silly, they’re IN your eyes. Doctors call them floaters, muscae volitantes (Latin for “fluttering flies”), or, if they’re in a prosaic mood, spots. The specks are variously described as particles, soot, spiders, cobwebs, worms, dark streaks, or rings. Just about everybody experiences them, although they’re most common in people who are nearsighted. Usually — but not always — they’re harmless.

The little threads are believed to be the sad remnants of the hyaloid artery, which nourishes the lens and other parts of the eye during fetal development and then withers away. During its brief life the artery floats in the vitreous humor, the goo that fills the eyeball behind the lens. Running from the lens to the back of the eye where the optic nerve comes in, it reaches the high point of its existence around the third month of development, then starts to atrophy. By the seventh month blood stops flowing through the artery and it gradually disintegrates. Most of the debris disappears by the time you’re born, but some of it remains on the scene indefinitely.

As you get older, the number of floaters in your eyes tends to increase due to the formation of fibrous clumps and membranes in the vitreous fluid. If things really start to slide, the vitreous material may even pull away from the inside of the eyeball, in which case what you’re seeing may be crudniks stuck to the back side of your eyeball jelly. Disgusting, sure, but more or less normal, they tell me. Your vision remains unimpaired.

But floaters aren’t always benign. Sometimes they’re errant blood cells resulting from hemorrhage of the delicate vessels inside the eye. This can be caused by a good whack to the head or by a variety of ailments. A sudden shower of spots, for instance, often accompanied by flashes of light, can signal that you’re about to suffer a detached retina.

Floaters can also be debris resulting from an eye infection — or worse. I note here in my ophthalmology handbook, which needless to say I keep with me always, that sometimes floaters can be “intraocular parasites” — meaning that what look like flies may actually be flies, after a manner of speaking. Fortunately, these are rare.

Assuming your floaters aren’t caused by some ongoing disease or other problem, they’ll generally go away or at least settle out of your line of vision eventually. If not, and if your sight is seriously impaired as a result, the vitreous fluid can be surgically drained and replaced with an inert substitute. This is called a vitrectomy, and it’s a bit delicate. If you’re checking out surgeons and the guy says, “Sure, I’ll take a stab,” he is not, in my book, the ideal candidate for the job.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.