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Vitreous detachment is a process in which the vitreous separates from the retina with or without collapsing in on itself. The vitreous is a jelly-like body that fills the posterior chamber of the eye, giving the eyeball its round shape and keeping the retina in place against the back of the eye.
It is made up of millions of tiny collagen fibrils along with ground substance mucopolysaccharides such as hyaluronic acid, which form a gel. The vitreous is mostly water, which makes up 98% of it. The collagen strands connect to the superficial layers of the retina especially around the macula, the retinal vessels or sites at the retinal periphery.
With age, the vitreoretinal adhesion weakens. The vitreous also undergoes liquefaction or syneresis. Pockets of fluid form within the vitreous. The redistribution of the water in the vitreous causes it to change from gel form to a more fluid mass which then collapses forwards and contracts. This causes it to pull away from the retinal internal limiting membrane. When the vitreous separates from the retina, a posterior vitreous detachment occurs. In most cases, this is a completely benign process. When the vitreous finally separates from the optic nerve head, a Weiss ring is observed in the vitreous. This is formed by glial remnants torn away from the edge of the nerve ring and is circular in form. The occurrence of posterior vitreous detachment in one eye is usually followed by its occurrence in the other eye within a period of 6-24 months.
In a few patients, there are significant disparities between the rate of release of vitreoretinal attachment and the shrinkage of the vitreous mass. This leads to the adherent fibrils pulling the retina forward from the retinal pigment epithelium underneath which nourishes and renews its photosensitive pigments. Excessive vitreous traction or a weakened retina may cause a retinal break or tear, which, in some patients, progresses to a retinal detachment. This is a condition which threatens sight. The same process happens in patients who have scarring of the retina or persistent vitreous cortical remnants on the retinal surface. As these organize, they exercise significant traction on the retina.
Posterior vitreous detachment itself is typically benign. However, there are some symptoms which may occur in such a case. These usually herald retinal detachment but before the actual event.
One is the occurrence of a new shower of floaters. Also called muscae volitantes, these are specks of darkness floating in front of the eye but moving in the direction of the eyeball gaze. These are also described as cobwebs or threads. They are actually shadows cast on the retina by the fluid pockets, cell clumps, or organized blood (from torn or avulsed vessels in the retina), or from collagen strands or other residue of vitreous degeneration in the vitreous. They are normally harmless but their sudden appearance should signal the need for evaluation of the eye to rule out a retinal hole or retinal detachment. This is especially true when the new floaters are too many to count.
Another abnormal phenomenon is the sudden onset of the occurrence of flashes of light, usually in the peripheral part of the field. They may be elicited in some cases when the patient changes the direction of gaze. They are monocular as opposed to the binocular photopsia, which is sometimes part of the aura of a migraine. These flashes are more obvious in the dark. They are caused by the pull of the detaching vitreous on the peripheral retina. In some patients, there is difficulty in judging which eye is affected by the photopsia.
A third characteristic but little-noted symptom of posterior vitreous detachment that has resulted in retinal detachment is the presence of a defect in the visual field. When a patient has a history of acute flashes of light or floaters, 14 out of 100 will have a tear in the retina. Even if the detachment is asymptomatic, 3 – 4% of patients will have a retinal tear which develops or is detected over the course of the next six months.
Most symptomatic patients with posterior vitreous detachment are likely to have retinal tears. Anywhere between 8 and 26 percent of such patients will develop retinal detachment.