In a healthy eye, fluid flows from behind the iris through the pupil (the black center) and then drains out. This flow helps keep eye pressure at a normal level.
When fluid gets trapped behind the iris and can't drain, pressure builds in the eye. High pressure can damage the optic nerve, which is what helps you see. If not treated, this pressure can lead to permanent vision loss. Creating a small hole in the iris allows fluid to bypass the blockage and drain out of the eye. This lowers eye pressure and helps protect vision.
This procedure is also done to treat an acute angle-closure attack, which can be painful and cause vision loss.
It is also done to prevent problems in people at risk, such as people with narrow drainage angles. Some people with narrow drainage angles may not need treatment right away. The decision to treat depends on your risk level and your doctor's advice.