Vitreous detachment
Vitreous detachment describes the condition in which the eye's vitreous gel separates from the retina. The vitreous gel is composed of 99% water and 1% solid elements. Within the solid portion are hyaluronic acid molecules and millions of fine collagen fibers which are intertwined and attached to the surface of the retina, the eye's light-sensitive tissue. With age, depolymerization of the hyaluronic acid occurs, causing these molecules to release 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 or 'syneresis' and results in a shrinkage of the vitreous. As the vitreous shrinks, the collagen fibers pull on the retinal surface. Usually the fibers break, allowing the vitreous to separate and shrink from the retina. 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 a posterior vitreous detachment (PVD). In most cases, a vitreous detachment is not sight-threatening and requires no treatment.
One symptom of a vitreous detachment is a small but sudden increase in the number of new floaters. As the vitreous shrinks, the collagen fibers can aggregate and cast tiny shadows on the retina that you may notice as floaters. This increase in floaters may be accompanied by flashes of light (lightning streaks) in your peripheral, or side, vision. 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'. In most cases, either you will not notice a vitreous detachment, or you will find it merely annoying because of the increase in floaters.
Posterior vitreous detachment is less common in people under 50 years of age but occurs in about 75% of people over 65 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.
References:
National Eye Institute - Facts About Vitreous Detachment
Royal National Institute of Blind People - Posterior Vitreous Detachment
