The lens of the eye sits behind the pupil. It is the size of a jelly-tot sweet and enlarges slowly over a lifetime, pushing the iris (the coloured part) gradually forward, closer to the cornea (the clear window at the front). There is normally sufficient space inside the eye to accommodate the enlarging lens, so that the iris never contacts the cornea and the ring of trabecular meshwork at the angle between the two remains exposed to drain fluid from the eye.
In eyes that are naturally smaller, the enlarging lens may cause the periphery of the iris to come into contact with the cornea, thereby obstructing the trabecular meshwork and causing eye pressure to rise. This process is called angle closure. It occurs intermittently at first, usually when the pupil is larger, as in dim light. In time, the periphery of the iris may begin to adhere to the meshwork or to the cornea, resulting in a permanent, asymptomatic rise of eye pressure or intermittent spikes of pressure for a couple of hours with brow ache, haloes and blurred vision. Rarely, angle closure occurs acutely, with extremely high pressure, severe eye pain, blurred vision and vomiting.
Without treatment, there is a risk that eye pressure will increase and damage the optic nerve and vision, a condition known as angle closure glaucoma. Low risk eyes may only need periodic monitoring. Those at greater risk may benefit from treatment to separate the iris and cornea and restore normal fluid outflow – either eye drops to tighten the iris, a laser iridotomy procedure to create a small hole in the periphery iris or, lens surgery to replace the natural lens with a thinner lens implant.