How does pupillary block glaucoma prevent aqueous humor from draining?

In pupillary block glaucoma, the lens seals against the iris, creating a physical barrier that prevents aqueous humor from flowing through the pupil from the posterior chamber into the anterior chamber.1

As fluid continues to be produced by the ciliary body, this blockage causes pressure to build up behind the iris. This increased posterior pressure pushes the root of the iris forward, forcing it against the wall of the eyeball at the periphery of the anterior chamber. This action closes the drainage angle, obstructing the outflow of aqueous humor through the trabecular meshwork and leading to a rapid rise in intraocular pressure. If left untreated, this high pressure can cause severe pain, vision deterioration, and permanent damage to the optic nerve.21