Floater

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For other uses, see Floater (disambiguation).

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Floaters, or muscae volitantes (Latin: "flying flies"), are entoptic phenomena characterized by shadow-like shapes which appear singly or together with several others in one's field of vision. They can take the form of spots, threads, or fragments of cobwebs, that float slowly before one's eyes.

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Description

Floaters are suspended in the vitreous humour, the thick fluid or gel that fills the eye. Thus, they generally follow the rapid motions of the eye, while drifting slowly within the fluid. Floaters located slightly to the side of one's direction of gaze can be particularly annoying. When they are first noticed, the natural reaction is to attempt to look directly at them. However, attempting to shift one's gaze toward them can be difficult since floaters follow the motion of the eye, remaining to the side of the direction of gaze. Floaters are, in fact, visible only because they do not remain perfectly fixed within the eye. Although the blood vessels of the eye also obstruct light, they are invisible under normal circumstances (and thus not annoying) because they are fixed in location relative to the retina, and the brain "tunes out" stabilized images (see neural adaptation). This does not occur with floaters and they remain visible, and, in some cases when large and numerous, very annoying.

Floaters are particularly noticeable when lying on one's back and gazing at blue sky. Despite the name "floaters", many of these specks have a tendency to sink toward the bottom of the eyeball, in whichever way the eyeball is oriented; the supine position tends to concentrate them near the fovea, which is the center of gaze, while the textureless and evenly lit sky forms an ideal background against which to view them.

Floaters are not uncommon, and don't cause problems for most sufferers. However, floaters are more than a nuisance and a distraction to those with severe cases, especially if the spots seem to constantly drift through the field of vision. The shapes are shadows projected onto the retina by tiny structures of protein or other cell debris discarded over the years and trapped in the vitreous humour. It should also be noted that they can even be seen when the eyes are closed on especially bright days, when sufficient light penetrates the eyelids to cast the shadows. It is not, however, only elderly people who suffer from floaters; they can certainly become a problem to younger people, especially if they are myopic. They are also common after cataract operations or after trauma. In some cases, floaters are congenital.

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Causes

There are various causes for the appearance of floaters, of which the most common are described here. Basically, any way by which material enters the vitreous humour is a cause for floaters.

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Vitreous syneresis

The most common cause of floaters is shrinkage of the vitreous humour: this gel-like substance consists of 99% water and 1% solid elements. The solid portion consists of a network of collagen and hyaluronic acid, with the latter retaining water molecules. Depolymerisation of this network makes the hyaluronic acid release its trapped water, thereby liquefying the gel. The collagen breaks down into fibrils, which ultimately are the floaters that plague the patient. Floaters caused in this way tend to be few in number and of a linear form.

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Posterior vitreous detachments and retinal detachments

In time, the liquefied vitreous body loses support and its framework contracts. This leads to posterior vitreous detachment, in which the vitreous body is released from the sensory retina. During this detachment, the shrinking vitreous can stimulate the retina mechanically, causing the patient to see random flashes across his visual field, sometimes referred to as "flashers." The ultimate release of the vitreous sometimes makes a large floater appear, usually in the shape of a ring. As a complication, part of the retina might be torn off by the departing vitreous body, in a process known as retinal detachment. This will often leak blood into the vitreous, which is seen by the patient as a sudden appearance of numerous small dots, moving across the whole field of vision. Retinal detachment requires immediate medical attention, as it can easily cause blindness. Both the appearance of flashes and the sudden onset of numerous small floaters warrant an ophthalmological investigation.

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Regression of the hyaloid artery

The hyaloid artery, an artery running through the vitreous humour during the fetal stage of development, regresses in the third trimester of pregnancy. Its disintegration can sometimes leave cell matter.

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Other common causes

Other causes for floaters include active toxoplasmosis, cystoid macular oedema and asteroid hyalosis. The latter is an anomaly of the vitreous humour, where by calcium clumps attach themselves to the collagen network. The bodies that are formed in this way move slightly with eye movement, but then return to their fixed position.

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Tear film debris

Sometimes the appearance of floaters has to be attributed to dark specks in the tear film of the eye. Technically, these are not floaters, but they do look the same from the viewpoint of the patient. People with blepharitis or a dysfunctional meibomian gland are especially prone to this cause, but ocular allergies or even the wearing of contact lenses can cause the problem. To differentiate between material in the vitreous humour of the eye and debris in the tear film, one can look at the effect of blinking: debris in the tear film will move quickly with a blink, while floaters are largely unresponsive to it. Tear film debris is diagnosed by eliminating the possibility of true floaters and macular degeneration.

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Treatment

Normally, there is no treatment indicated. Vitrectomy operations to remove them are normally advised against as they are risky and may cause more severe problems or even blindness. Floaters may become less annoying as sufferers grow accustomed to them, even to the extent that they may no longer notice them.

Another treatment is laser vitreolysis. In this procedure a YAG laser is focused onto the floater and in a quick burst vaporizes the structure into, presumably, a less dense and not as noticeable consistency. This procedure can be time consuming and there is no consensus as to how completely effective it is. One study found laser vitreolysis "to be a safe but only moderately effective primary treatment conferring clinical benefit in one third of patients" PMID 11913884 .

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Observing floaters

Floaters are often readily observed by a doctor with the use of an ophthalmoscope or slit lamp. However, if the floater is a small piece of debris and near the retina they may not be able to observe it even if it appears large to the sufferer.

Increasing background illumination or using a pinhole to effectively decrease pupil diameter may allow a person to obtain a better view of his or her own floaters. The head may be tilted in such a way that one of the floaters drifts towards the central axis of the eye. In the sharpened image the fibrous elements are more conspicuous. (If the pinhole is kept moving slowly in small circles, the same technique evokes an interesting entoptic effect known as the vascular figure, which is a view of the blood vessels within one's own eye.)

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Quotations

A common experience... is for a person who has some ocular trouble that impairs his vision to become suddenly aware of the so-called mouches volantes in his visual field, although the causes of this phenomenon have been there in the vitreous humour all his life. Yet now he will be firmly persuaded that these corpuscles have developed as the result of his ocular ailment, although the truth simply is that, owing to his ailment, the patient has been paying more attention to visual phenomena.
— H. von Helmholtz, Handbuch der Physiologischen Optik, published as "Helmholtz's Treatise on Physiological Optics, Translated from the Third German Edition," ed. James P. C. Southall, 1925, The Optical Society of America. v. III, pp. 6-7
They could have done the same thing, alone, in the back yard, seeing the shapes swimming in the sky. I forget how old I was when I asked somebody about it, and I was told that those wonderful gliding changing spots were imperfections in the fluid of my eye-ball, that what I was seeing was in my eye. In your eye! For so long, for a child's years, the sky was full of wonder, these shapes were in the sky, the sky was full of transparent things that swooped and swam. They were almost invisible, and, I thought, almost bodiless, they were there, but you could go right through them, they were animals that lived in the air. You see, we didn't go around talking about things like this. It's only now, when I am grown up and know everything, that I talk about this.
— Robert Paul Smith, 1957, Where Did You Go? Out. What Did You Do? Nothing. Norton, New York.
At first the amoebae look like muscae volitantes, those curled moving spots you seem to see in your eyes when you stare at a distant wall. Then I see the amoebae as drops of water congealed, blusish, translucent, like chips of sky in the bowl.
— Annie Dillard, 1974, "Pilgrim at Tinker Creek." Bantam: Toronto.
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See also

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External links

it:Miodesopsie nl:Glasvochttroebeling ja:飛蚊症 pt:Moscas Volantes sv:Mouches volantes zh:飛蚊症

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