Pupillary block is a type of angle closure glaucoma. As we know that the iris lies infront of the lens and usually there is no contact between the back surface of the iris and the anterior surface of the natural lens.
The aqueous humor will flow from the posterior chamber of the eye through this space between the iris and the lens to the anterior chamber.
For many reasons such as small axial length of the eye and large lens mass, there will be direct contact between the iris and the lens and the space between them will be obliterated leading to high aqueous pressure in the posterior chamber which will push the iris forward, causing the peripheral part of the iris to bulge anteriorly and it will cover over the trabecular meshwork causing high IOP.
Sometimes this condition can be intermittent and it will be relieved either by the pupil gets miosed or constricted as in sleeping or with medications or when production of aqueous humor decreases as in sleeping also.
Blocking of the pupil also occurs in normal physiological conditions that cause the pupil to dilute in relatively small eyes. These conditions are emotional stress and dim light rooms.
Complete blocking of the pupil occurs when there are 360 degree adhesion around the pupillary margin between the iris and lens and there is no space for the aqueous to pass through from the posterior to the anterior chamber.
The intraocular pressure will increase dramatically with severe pain and redness and the only treatment for it is by iridectomy with Antiglaucoma medications.