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A Straight Dope Classic from Cecil's storehouse of human knowledge
23-May-1986
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
Dear Mike:
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
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