Behind the lens, and in front of the retina, is a space that’s mostly filled with a transparent gel called the vitreous. The vitreous is contained within a membrane (called the posterior hyaloid membrane). But as people age, the vitreous starts to liquify and shrink, and eventually it the vitreous and its membrane can start to peel away from the retina – a process that’s called posterior vitreous detachment (PVD).

Sometimes, parts of the vitreous membrane sticks to the retina, rather than coming loose. This can exert pulling forces on the retina (called vitreomacular traction). This can lead to sudden flashes of light or a dramatic increase in the number of floaters.

However, in some cases, the pulling forces from the shrinking vitreous can tear the retina, letting fluid get behind the retina, and causing a retinal detachment. This is an ophthalmological emergency and must be treated quickly, often by performing a vitrectomy (removal of the vitreous).