Vitreous gel, also called vitreous humor, is a clear gel which fills most of the interior of the eyeball. One of the main functions of this gel is simply to enable the eyeball to hold its spherical shape, as without the gel the eyeball would collapse. The gel also helps hold the retina in place against the interior wall of the eyeball. Diseases which affect the vitreous gel can cause partial or full loss of vision.
The vitreous gel is colorless, clear, and up to four times more viscous than water. This gelatinous liquid fills almost the entire eyeball; the only structures inside the eye which are not filled with the gel are the lens at the front of the eye, and the retinal lining at the back. The humor is produced by cells present in the retina of the eye, and despite its high viscosity, the gel is up to 99% water.
Along with the water which comprises most of the vitreous humor, the gel also contains several types of sugars and salts, collagen fibers, amino acids, and proteins. A small number of cells are present in the gel. These are phagocytes, a type of cell that ingests waste matter all over the body. In the eye, these cells help ensure the visual field remains clear.
Although phagocytes are present in the eye removing cellular debris, the vitreous gel does not undergo any process of circulation. The gel is not replaced or replenished via any circulatory system; instead it is largely stagnant. This is one reason why diseases of the vitreous humor can have such a debilitating effect on vision.
One of the most common diseases of the eye caused by a disorder of the vitreous humor is called posterior vitreous detachment. This condition develops with age, and is common in people between the ages of 40 and 70. Posterior vitreous detachment occurs because the vitreous gel changes slowly over time, becoming less dense and more liquid; the mass of vitreous humor begins to shrink, and falls away from the retina. One of the most common symptoms of this disease is the appearance of vitreous floaters, tiny specks of black which appear in the field of vision. Flashes of light are another common symptom.
People with type 1 or type 2 diabetes are at risk of developing another kind of vitreous degeneration disease called retinal detachment. This disease develops as a result of damage to retinal blood vessels caused by high glucose levels, but also causes the appearance of floaters and flashes, as with posterior vitreous detachment. Destruction of blood vessels prompts the growth of new vessels, some of which may develop in the vitreous humor. This can cause the appearance of large black patches in the field of vision, as well as increasingly blurred vision.
Retinal detachment may be treated with laser photocoagulation, a treatment which improves vision by cauterizing the new blood vessels which develop. Another treatment is vitrectomy, which involves the removal of the vitreous gel. The gel is replaced with a sterile saline solution, thus improving vision due to the removal of blood vessels from the vitreous humor.