Vitreous Detachment

Most of the eye's interior is filled with vitreous, a gel-like substance that helps the eye maintain a round shape. There are millions of fine fibers intertwined within the vitreous that are attached to the surface of the retina, the eye's light-sensitive tissue. As we age, the vitreous slowly shrinks, and these fine fibers pull on the retinal surface. Usually the fibers break, allowing the vitreous to separate and shrink from the retina. This is a vitreous detachment. In most cases, a vitreous detachment is not sight-threatening and requires no treatment.

As the vitreous shrinks, it becomes somewhat stringy, and the strands can cast tiny shadows on the retina that you may notice as floaters, which appear as little "cobwebs" or specks that seem to float about in your field of vision. If you try to look at these shadows they appear to quickly dart out of the way. One symptom of a vitreous detachment is a small but sudden increase in the number of new floaters. This increase in floaters may be accompanied by flashes of light (lightning streaks) in your peripheral, or side, vision. In most cases, either you will not notice a vitreous detachment, or you will find it merely annoying because of the increase in floaters.

The vitreous gel is 99% water and 1% solid elements. Of the solid portion, there are collagen filaments and hyaluronic acid molecules. The ability of hyaluronic acid molecules to retain water molecules is an important factor in maintaining the gel consistency of vitreous. With age, there is a depolymerisation of hyaluronic acid, causing these molecules to release their water and form lacunae i.e. pockets of liquefied vitreous. The collagen 'filaments' aggregate to form larger 'fibrils', causing further collapse of the vitreous gel structure. This process is known as vitreous degeneration and 'syneresis'. The collagen fibrils may 'float' within the liquid vitreous pockets, giving the patient a sensation of floaters. The same process that causes floaters may cause flashes of light. When the vitreous pulls on the retina - to which it is attached - the photoreceptors are mechanically stimulated. The retinal cells are incapable of perceiving pain, pressure, or temperature. The only stimulus that the retina responds to is 'light'. So when the retinal photoreceptors experience mechanical stimulation because of the vitreous pull, they send a signal to the brain in the form of disorganized light, which is perceived by the brain as a 'flash'.

Eventually, with the accumulation of enough lacunae (liquified vitreous pockets), the vitreous framework collapses and the vitreous completely separates from the retina. This process is called posterior vitreous detachment. Tissue may tear from an area adjacent to or from the optic nerve head due to an acute posterior vitreous detachment. This tissue (called Weiss ring) is usually visible as a large floater. Posterior vitreous detachment occurs in less that 10% people under 50 years of age but in more than 60% people who are over 70 years of age. It is more common for people who are nearsighted or who have had an eye injury or have undergone eye surgery or who have had inflammation inside the eye. B-scan ultrasound examination is the only method that can definitively diagnose posterior vitreous detachment.

The reason why you are advised not to ignore symptoms of sudden increase in the number of floaters or flashes of light especially if accompanied by subjective reduction in vision (cloud or curtain in vision) is that these symptoms signify an acute posterior vitreous detachment. There could be an associated retinal tear which can lead to a retinal detachment. Therefore you should have an immediate eye examination to rule out any retinal involvement.