The Vitreous Humor
|Anatomy, Physiology and
||Ted M. Montgomery,
The vitreous humor is a clear gel which occupies the posterior compartment of the eye, located between the crystalline lens and the retina and occupying about 80% of the volume of the eyeball. Light initially entering the eye through the cornea, pupil, and lens, is transmitted through the vitreous to the retina.
Vitreous humor has the following composition:
The “hyaloid artery” (a branch of the primitive dorsal ophthalmic artery) grows, in the fetus, outward from the optic cup of the optic nerve into the vitreous cavity. It extend forward to the crystalline lens to aid its development.
The hyaloid artery regresses during the last trimester of fetal formation, leaving behind the “Cloquet’s canal” through the vitreous. Sometimes, the hyaloid artery remains after birth and is viewable by a doctor looking into the eye as a “persistent hyaloid artery,” but it rarely is noticeable to the person who has it.
Sometimes, remains of the lenticular attachment of the hyaloid artery can be found on the back surface of the crystalline lens. These tiny remnants appear gray or white and are located just inferior and nasal to the posterior pole of the lens. These “Mittendorf’s dots,” which are congenital (present at birth), continue throughout life. They have no effect on vision.
With age, the vitreous humor changes from a gel to a liquid. As it does so, the vitreous mass gradually shrinks and collapses, separating and falling away from the retina. This is called a “posterior vitreous detachment” (PVD) and is a normal occurrence between ages 40 and 70.
Commonly, a person having experienced a PVD will report seeing flashing lights and/or floaters in his or her field of vision. The flashes of light occur when the vitreous tugs on the sensory layer of the retina, as the vitreous is detaching. The floaters—which are cells or debris released, when the vitreous detaches—can appear as little dots, circles, lines, cobwebs, clouds, or a puff of smoke.
Floaters can be apparent especially when looking at a bright background, as the light entering the eye casts shadows of the floaters onto the retina. Sometimes a large, single floater actually can obstruct print that is being read. The observance of flashes and floaters can last two or more weeks. Episodes lasting even as long as six months can occur.
It is said that the percent chance of having a vitreous detachment is at least the same as one’s age. However, a PVD may occur earlier than normal in moderately to extremely nearsighted people, as well as in people who have had cataract surgery. A dilated eye exam should be performed to make sure the symptoms are not due to a retinal detachment, which is a much more serious and potentially sight-threatening condition.
As a posterior vitreous detachment (PVD) occurs—that is, as the vitreous fluid separates from the retina—organic debris or particles known as “floaters” are released. Another name for floaters is “muscae volitantes” (flying flies).
Most floaters are merely compressed cells or strands of the vitreous gel which have clumped together so that they are less transparent than the rest of the vitreous. Some floaters are remnants of the hyaloid artery, which usually disintegrates before birth. These types of floaters are harmless.
Floaters sometimes interfere with vision, often during reading, and they can be quite annoying. If a floater appears directly in the line of sight, the best thing to do is to move the eye from side to side or up and down. Doing so can create a current within the internal fluids to move the floater temporarily away from the line of sight.
If a floater is suspended in a portion of vitreous humor which is very viscous, it can be particularly persistent and bothersome. Unfortunately, in most instances, there is nothing to do but learn to tolerate the floater’s presence. Surgical removal, which is risky, is considered only in the most extreme cases.
Usually, the vitreous makes a clean break as it pulls away from the retina. Rarely, however, the vitreous will adhere tightly onto the retina in certain places; and a small, often horseshoe-shaped rip in the retina can result from persistent tugging and tearing by the vitreous. Unless the retinal tear is repaired, fluid can seep through this hole into or underneath the retina and cause a retinal detachment, a very serious, sight-threatening condition.
As the vitreous membrane tugs on the retina, at points where the two structures remain attached, the tension can cause “flashing” sensations, because some of the retinal nerves are stimulated. Occasional flashes of light usually are nothing to be concerned about, unless they increase in frequency and/or occur in conjunction with a sudden onset of a large number of floaters. In this case, it may be that a retinal detachment has occurred.
Not uncommonly, tiny spherical or disc-shaped, “soapy” globs, can be located in the vitreous of one eye or occasionally in both eyes. This is known as “asteroid hyalosis.” When present, these calcium-containing lipid complexes usually are suspended throughout the vitreous.
Usually, these “asteroids” are not observable by a person who has them, and they normally do not cause any decrease in vision, since light generally passes through them unaffected. Rarely, however, if the asteroids coalesce on the visual axis, especially at or near the nodal point immediately behind the lens, there can be a profound decrease in vision. In such a case, removal of the asteroids, via a vitrectomy, is an option to restore vision.
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Copyright © 1998– by Ted M. Montgomery, O.D. Most rights reserved.